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GRB 221009A is the brightest gamma-ray burst (GRB) observed to date, and its afterglow has been characterized with unprecedented detail at TeV energies by LHAASO. Quite puzzlingly, it is also the most energetic GRB known. Among the questions posed by this mysterious source, however, the sheer energetics are hardly the most intriguing; an unprecedented, narrow, luminous emission line at around 10 MeV has been uncovered by a detailed spectral analysis of Fermi/GBM data immediately following the brightest peak in the GRB prompt emission and the peak of the TeV afterglow. As noted in the discovery article, the temporal evolution of the line properties can be explained as being due to high-latitude emission from a geometrically thin, relativistically expanding shell where annihilation of a large number of electron─positron pairs took place. We show that this interpretation yields stringent constraints on the properties of such a shell, which point to a process that happens at radii typical of external shocks. We then demonstrate that the shell could have been the blastwave associated with the GRB precursor, with the line arising after pair loading of the blastwave as it was illuminated by the bright and hard radiation of the GRB main event. The scenario, which also explains the abrupt initial rise of the LHAASO afterglow, requires the progenitor of the GRB to have been surrounded by a circumstellar medium (CSM) extending out to a few 10<SUP>15</SUP> cm, with a density n<SUB>ext</SUB> ∼ 10<SUP>8</SUP> − 10<SUP>9</SUP> cm<SUP>−3</SUP> reminiscent of those found from studies of type IIn supernovae. The consequences of such a CSM on the dynamics and emission of the external shock are yet to be fully explored. If future, more detailed work confirms the compatibility of the GRB 221009A afterglow emission with our scenario, this will provide a precious clue to the nature of the progenitor of this peculiar GRB, which could also be present in other bursts that feature a long quiescence followed by a bright emission episode with a hard spectrum.

Short gamma-ray bursts (sGRBs) are widely believed to originate from mergers of compact binaries, including binary neutron stars and neutron star─black hole systems. However, the observed sGRB population is strongly shaped by selection effects associated with detector sensitivity, triggering criteria, and jet orientation, which bias the inferred luminosity, redshift, and spectral distributions. We develop a forward-modeling framework to constrain the intrinsic properties and cosmic formation rate of sGRBs by simulating their detection with two independent instruments operating in different energy bands: the Fermi Gamma-ray Burst Monitor (GBM) and the Swift Burst Alert Telescope (BAT). At the level of the raw observations, the two samples exhibit apparent differences in their brightness and spectral distributions. We generate synthetic sGRB populations using parameterized luminosity functions, spectral models, and redshift evolution tied to the cosmic star formation history convolved with a power-law delay-time distribution. Each burst is passed through realistic, instrument-specific detector responses, background conditions, and onboard trigger algorithms, and is then spectrally fit to recover observable quantities. We find that a single intrinsic population model nevertheless provides a statistically consistent description of both datasets, reproducing the observed distributions of low-energy spectral index, peak energy, and fluence. The analysis reveals strong selection biases that favor low-redshift detections and truncate the observed distributions of peak energy, fluence, and duration, while only weakly affecting the low-energy spectral index. Correcting for jet beaming, we infer local sGRB formation rates of ∼258 and 562 yr<SUP>−1</SUP> Gpc<SUP>−3</SUP> from Swift/BAT and Fermi/GBM, broadly consistent with binary neutron-star merger rates inferred from gravitational-wave observations.

GRB 221009A is the brightest gamma-ray burst observed to date. We analyze its precursor and Phase I of the main burst, using Fermi/Gamma-ray Burst Monitor data and aiming to investigate the jet composition during these two phases of the prompt emission. We find that spectral analyses reveal thermal emission in both phases. Within the framework of the hybrid jet model of Gao &amp; Zhang, the dimensionless entropy η and magnetization parameter (1 + σ<SUB>0</SUB>) significantly exceed unity, indicating a substantial contribution from the Poynting-flux component in addition to the thermal fireball component in the prompt emission jet. For an engine radius of ∼10<SUP>7</SUP> cm, the time-resolved magnetization parameter <inline-formula> <mml:math><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>σ</mml:mi></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>r</mml:mi></mml:mrow><mml:mrow><mml:mn>15</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:math> </inline-formula> is essentially unity in both phases, suggesting that the nonthermal emission may originate from internal shock (IS). Furthermore, the Lorentz factor Γ<SUB>ph</SUB> and photospheric radius r<SUB>ph</SUB> derived at this engine radius are also consistent with the values and evolutionary behavior expected from the fireball model. In the case r<SUB>0</SUB> ∼ 10<SUP>8</SUP> cm, the precursor suggests that the nonthermal radiation may still arise from IS, whereas portions of the main burst are likely produced by ICMART events. Nonetheless, independent of whether IS or ICMART dominates, the jet consistently carries a significant Poynting flux. Additionally, both phases exhibit broadly similar correlations of kT<SUB>obs</SUB> and E<SUB>p</SUB> with F<SUB>obs</SUB>, as well as a positive Γ<SUB>ph</SUB>─L<SUB>γ,iso</SUB> relation. These results suggest that the precursor and main burst Phase I share common properties and likely originate from the same physical source.

Short-duration gamma-ray burst (sGRB) afterglows that follow binary neutron star (BNS)─gravitational wave events are essential for understanding the tension between the observed sGRB rate and BNS merger rate, heightened by the recent conclusion of aLIGO O4 with no new confirmed BNS detections. Using a probabilistic sGRB world model derived from a source BNS merger population, we simulate afterglow emission with AfterglowPy to investigate detection prospects of afterglows in the new era of optical surveys and probe their multimessenger implications. The predicted sGRB/BNS association is strongly dependent on sGRB beaming, which may be constrained by orphan afterglows (OAs), which arise from events with no prompt gamma-ray detection. We conclude that the Vera C. Rubin Observatory's Large Synoptic Survey Telescope (LSST) may detect an afterglow sample sufficient in constraining sGRB jetting, with an estimated <inline-formula> <mml:math><mml:mn>5</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mrow><mml:mn>3</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.2</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>1.7</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula> on-axis afterglow and <inline-formula> <mml:math><mml:mn>1</mml:mn><mml:msubsup><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>3</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula> OA detections per year; while the deep sensitivity of the Roman Space Telescope appears promising for probing the faint end of afterglow events in targeted follow-up strategies. The detection of afterglows in upcoming LIGO runs is possible but challenging given that we predict fewer than ≍1.4 LSST or Roman discoverable events per year within the projected aLIGO O5 BNS range across all considered jet models and observing scenarios. We update previous sGRB─BNS rate predictions, finding that continued nondetection of a BNS in O5 would require revisiting key assumptions underlying sGRB─BNS models.

The recent candidate gravitational-wave (GW) alert from a compact object merger involving at least one subsolar mass (SSM) object has prompted questions about their origins. S251112cm is reported by LIGO/Virgo with a false-alarm rate of 1 per 6.2 yr, nearby luminosity distance of 93 ± 27 Mpc, and probability of containing an SSM object of 100%. Such a system, if astrophysical, likely did not involve the supersolar neutron stars or black holes invoked to explain kilonovae. One must then also invoke hitherto unobserved and speculative models to produce SSM mergers that may have electromagnetic (EM) counterparts. We introduce a framework that vets and scores candidate counterparts to SSM GW events to inform follow-up in search of any among the zoo of potential EM transients: kilonovae, kilonovae-within-supernovae, superkilonovae, or active galactic nucleus (AGN) flares from binary black hole mergers. We use a suite of telescopes to perform tiling, galaxy-targeted observations, and photometric/spectroscopic follow-up of promising candidates. In near-real time, we ingest candidates reported by the community, including some of the first observations reported by the Vera C. Rubin Observatory. We vet and score a total of 456 candidates, including 67 from Rubin, but find no likely counterpart. We nonetheless highlight candidates that demonstrate the ability of our framework to distinguish between different transient types and describe strategies to maximize the chances of detecting a counterpart to the next SSM event. Our framework will be implemented in the forthcoming Multimessenger Tool for Rapid Object Vetting and Examination.

We present a timing analysis of the black hole transient Swift J1727.8─1613 during its intermediate state. We use coordinated broad-band X-ray observations from Insight─HXMT (2─150 keV), together with optical data from ULTRACAM (<inline-formula><tex-math>$g_s$</tex-math></inline-formula> and <inline-formula><tex-math>$i_s$</tex-math></inline-formula> bands) and near-infrared data from HAWK-I (<inline-formula><tex-math>$K_s$</tex-math></inline-formula> band), obtained on 2023 September 9. As shown by previous studies, the Fourier power spectrum shows a strong quasi-periodic oscillation (QPO) in the <inline-formula><tex-math>$K_s$</tex-math></inline-formula>, <inline-formula><tex-math>$i_s$</tex-math></inline-formula>, and X-ray bands. Cross-correlation analysis reveals a complex coupling between the optical/near-infrared (OIR) and X-ray emission, including a delayed optical anticorrelation, a strong infrared correlation, and a pronounced dependence of these features on X-ray energy, suggesting multiple Comptonization regions. In contrast, the lag properties do not change at the QPO frequency, displaying an OIR lag of <inline-formula><tex-math>$\sim$</tex-math></inline-formula>60─80 ms up to 150 keV. We discuss these results in the context of small-scale jet and hot accretion flow scenarios.

The longest detected gamma-ray burst, GRB250702B, exhibited seven hours of prompt $γ$-ray emission, preceded by a soft X-ray precursor ($\sim1$ day earlier) and followed by a weeks-long fading X-ray tail. Lacking an established progenitor for all three phases, we propose that this ultra-long GRB (ULGRB) is powered by a jetted micro-tidal disruption event (micro-TDE), in which a spinning stellar-mass black hole (BH) disrupts a Sun-like star and launches a relativistic jet via the Blandford-Znajek mechanism. Micro-TDE debris disks have hours-to-days viscous timescales, naturally explaining ULGRB durations. Using 3D hydrodynamic AREPO simulations of a $1\,M_\odot$ star disrupted by a $10\,M_\odot$ BH, we show that within $\sim1$ day the debris forms a quasi-steady envelope with a low-density polar funnel ($ρ\propto r^{-2}$, half-opening angle $\approx15^\circ$). Applying an analytic jet-stability framework to these profiles, we find that the $r^{-2}$ funnel keeps the jet below the kink-instability threshold, enabling stable propagation and breakout for jet powers, $L_{\rm jet}\gtrsim10^{47}$ erg s$^{-1}$. We attribute the X-ray precursor to pre-disk stream-fed accretion; the prompt GRB to a tightly beamed jet ($θ_{\rm b}\lesssim1^\circ$, $L_{γ,\rm iso}\sim10^{51}$ erg s$^{-1}$) escaping the funnel, launched by a rapidly spinning BH ($a_\bullet\sim0.9$); and the weeks-long X-ray decline to disk-wind mass loss ($L_{\rm jet}\propto t^{-2}$) combined with jet widening ($θ_{\rm b}\propto t$, initially steepening the decay to $L_{\rm X,iso}\propto L_{\rm jet}/θ_{\rm b}^{2}\propto t^{-4}$). Our model reproduces the multi-phase evolution of GRB250702B and establishes jetted micro-TDEs as a physically motivated ULGRB engine.

Burst oscillations observed during thermonuclear X-ray bursts arise from asymmetric brightness patterns on the neutron star surface and provide a direct probe of the neutron star spin frequency. We present a detailed timing analysis of the neutron star low-mass X-ray binary 4U 1323-62 using 2024 observations with XMM-Newton and NuSTAR observatories. We identify nine thermonuclear X-ray bursts in the XMM-Newton/EPIC-pn data, along with eclipsing dips in the light curve. One of the XMM-Newton bursts exhibits a rare doublet structure. In addition, NuSTAR detects six bursts, four of which occur simultaneously with those observed with XMM-Newton. We identify a possible burst oscillation signal at $\sim$611.5 Hz in the XMM-Newton data. The strongest oscillation, detected during the primary burst of the doublet burst, reaches a maximum $Z_1^{2}$ power of $\sim35$. An analytical estimate accounting for the searched frequency and time intervals gives a significance of $\sim3.0σ$, whereas independent Monte Carlo simulations yield a more robust global significance of only $\sim2.4σ$. We therefore interpret the signal as a tentative detection of burst oscillation. The folded pulse profile in the 0.5-10 keV band is well described by a sinusoid, with a fractional rms amplitude of $\sim30\pm6$%. The oscillation frequency corresponds to a neutron-star spin period of $\sim$1.635 ms, suggesting that 4U 1323-62 may harbor a rapidly rotating millisecond neutron star.

The polarization signatures of Gamma-ray Burst (GRB) afterglows serve as a powerful diagnostic tool for studying their environments and jet physics. This work systematically investigates the X-ray (2--8~keV) polarization properties of standard GRB afterglows and assesses their detectability with the Polarimetry Focusing Array aboard the enhanced X-ray Timing and Polarimetry (eXTP) satellite. A Morris global sensitivity analysis is first conducted to identify the dominant parameters, which are then assigned observationally motivated probability distributions. In particular, the isotropic energy, half-opening angle, and initial Lorentz factor are sampled jointly via a Gaussian copula to reproduce the empirical Ghirlanda and Liang correlations. Monte Carlo simulations of $10^{3}$ afterglows are performed and validated against the observed 10~keV flux distributions of a selected Fermi--Swift sample (K--S $p = 0.29$ at $10^{3}~\mathrm{s}$ and $p = 0.18$ at $10^{4}~\mathrm{s}$). The simulations yield an overall polarization event rate of $\lesssim 1.5\%$ for standard GRB X-ray afterglows with eXTP/PFA, reflecting the intrinsically low polarization produced by a random magnetic field confined to the shock plane. The optimal detection window occurs near the jet break at late times, when the PD peaks. For exceptionally luminous events such as GRB~221009A, however, the PD remains above the MDP over the full interval $10^{3}$--$10^{6}~\mathrm{s}$, demonstrating that eXTP/PFA can capture nearly the entire polarization evolution for such rare, bright bursts.

The origin of the plateau phase in gamma-ray burst (GRB) afterglows remains under debate, with the energy injection model being one of the most competitive explanations. If the plateau is truly driven by energy injection, the average X-ray and optical luminosities during the plateau phase, $L_{\rm X, plat, ave}$ and $L_{\rm opt, plat, ave}$, should naturally be correlated. Moreover, under this scenario, the scaling relations between the luminosities during the plateau and the normal decay phase are expected to be consistent since they share the same origin, i.e., synchrotron radiation from the external forward shock. Therefore, simultaneous multi-band observations are essential to verify this mechanism. In this work, we select a sample of 47 GRBs with simultaneous plateaus in both bands. We calculate their time-averaged isotropic luminosities for the plateau and the subsequent normal decay phases. We find a moderate positive correlation $\log L_{\rm X, plat, ave}=m\log L_{\rm opt, plat, ave}+c$ with a slope $m = 0.86 \pm 0.11$ for the plateau phase. This correlation supports the energy injection origin and offers a promising diagnostic approach to test the model. Furthermore, we obtain a similar slope $m = 1.05 \pm 0.05$ for the normal decay phase, which reinforces the idea that both phases share the same physical origin. Notably, the post-plateau data exhibit a systematic downward shift in luminosity, which may indicate the cessation of the central engine.

The angular structure and temporal variability of neutrino emission from binary neutron star mergers are characterized using fully general-relativistic simulations with energy-integrated M1 neutrino transport across a representative set of equations of state, total masses, and mass ratios. The angle-dependent neutrino energy flux is extracted on a spherical surface outside the remnant and decomposed into spherical harmonics to quantify its multipolar content and evolution. Following the initial post-merger transient, the neutrino radiation flux approaches an axisymmetric configuration dominated by a strong quadrupolar component, producing persistent polar flux enhancement and equatorial suppression due to torus shadowing. The dipolar contribution remains subdominant, indicating the absence of sustained one-sided emission. The degree of anisotropy increases with mass asymmetry and for softer equations of state, reflecting the compactness and morphology of the remnant--disk system. Superimposed on this time-averaged geometry, coherent azimuthal modulations associated with the $m=1$ mode are identified. Fourier analysis reveals a characteristic frequency of $\sim 0.6$--$0.7$ kHz, consistent with differential rotation in the remnant and inner disk layers, indicating a dynamical coupling between rotational structure and neutrino emission variability. Finally, we quantify how the same quadrupole-dominated radiation geometry induces a latitude-dependent equilibrium electron fraction. The polar material is driven close to the neutrino-equilibrium target, whereas equatorial material remains systematically more neutron-rich and below equilibrium.

Harmonics are common features of quasi-periodic oscillations (QPOs) in black hole X-ray binaries; however, their physical origins remain poorly understood. Using broadband Insight-HXMT data, we investigated the Type-C QPO harmonic in MAXI J1348-630. The harmonic is significantly detected exclusively during the hard intermediate state and displays prominent energy-dependent properties: while it is strong in the soft X-ray band (< 10 keV) with its fractional rms amplitude even exceeding that of the fundamental QPO, it is much weaker in the hard X-ray band (> 10 keV), where its rms amplitude is generally several times lower than that of the fundamental. Furthermore, the harmonic shows no significant phase coupling with the fundamental in the soft X-ray band, whereas strong coupling is present in the hard X-ray band. These features point to a complex, energy-dependent origin for the harmonic. We propose that the hard X-ray harmonic may arise from nonlinear distortion of the fundamental waveform within the corona, while the soft X-ray harmonic is likely produced via a distinct physical process, such as the reflection emission from the inner disk.

The source 4U 1812-12 is a persistent, weakly variable low-mass X-ray binary containing a neutron star. The source was observed by NICER between 2019 and 2021 and, more recently, by NuSTAR in 2025. During the NICER and NuSTAR observations, the source was detected in a hard spectral state with a bolometric luminosity of $\sim 1.90\times 10^{36}$ ergs s$^{-1}$. Its $3-70$ keV NuSTAR spectrum is characterized by a soft thermal emission from the disc, a hard Comptonized emission from the corona, and its reflection from the accretion disc. The NuSTAR energy spectrum exhibits the clear presence of disc reflection features, fitted using a self-consistent relativistic reflection model {\tt relxill}. Our reflection modeling indicates a moderately ionized accretion disc (log\:$ξ\sim2.72$) extending close to the neutron star surface ($R_{in}\lesssim 1.72\:R_{ISCO}$), and viewed through a small inclination angle ($i\sim 25$ degrees). Assuming that the magnetic field ($B$) truncates the disc, we found $B\lesssim 2.54\times 10^{8}$ G, comparable to the typical values observed for NS LMXBs. The $1.0-9.5$ keV NICER spectra are also characterized by a soft thermal component and a dominant hard Comptonized component. During NICER observations, the disc temperature exhibits a small variation within $\sim 0.69-0.84$ keV. In contrast, the power law photon index, $Γ$, exhibits a large variation of $\sim 0.8-1.5$, implying a substantial change in the Comptonized emission. Moreover, NICER timing analysis reveals broadband aperiodic variability with significant QPO-like features at $0.379\pm 0.008$ Hz and $0.724\pm 0.025$ Hz, having fractional rms amplitudes of $2.9\pm 0.6\%$ and $4.1\pm 0.5\%$, respectively.

The disruption of a star by the tidal field of a super-massive black hole may provide insights into dormant and otherwise hard-to-study galactic nuclei. State-of-the-art numerical tools have not converged on the importance of the strong relativistic effects in the disruption of stars by potentially spinning black holes. We present a specialised numerical tool to perform global hydrodynamic simulations of stellar tidal disruptions in curved spacetimes. We quantify the role of impact strength and black hole spin onto the stellar structures and mass fallback rates. We adapted the code SPHINCS_BSSN to perform General Relativistic Smoothed-Particle Hydrodynamics (GRSPH) simulations of stellar tidal disruptions in Kerr metric. We coupled the code with a Newtonian self-gravity module, and we added the option to use an entropy evolution formulation to handle numerically challenging situations. Besides describing the implementation and code validation, we present a set of 18 simulations of parabolic tidal disruptions of stellar polytropes to investigate the effect of impact strength and black hole spin. We demonstrate that SPHINCS is capable of performing GRSPH simulations, reproducing benchmark tests to machine precision. Our stellar tidal disruption simulations show that the fallback rates agree with state-of-the-art modelling. Deep events result into structures where self-gravity plays no role, the mass fallback rates peak at lower values, and rise-to-peak timescales decrease with impact strength. In these cases the black hole spin affects noticeable these quantities increasing (decrease) both fallback rate peak and rise-to-peak timescale for prograde (retrograde) spin. Last, fallback rates tend to decay with the characteristic $t^{-5/3}$ on long timescales. The results show that SPHINCS can simulate high-resolution stellar tidal disruptions in Kerr metric at a reasonable computational time.

Gamma-ray bursts (GRBs) are the most energetic explosive events in the Universe, yet the physical mechanism of their prompt emission remains a mystery. Especially, it is unclear whether the energy dissipation mechanism in the GRB jet is dominated by kinetic energy or magnetic energy. Here, we studied the pulses in the prompt emission of the second brightest GRB to date, GRB 230307A, which was accurately measured by the Gravitational wave high-energy electromagnetic counterpart all-sky monitor (GECAM), with focus on the cumulative distributions of peak counts and duration of pulses as well as the waiting time between pulses. We find that these cumulative distributions show scale-invariant behavior, well consistent with the prediction of the self-organized criticality (SOC) theory. This is the first robust evidence of an SOC feature in the prompt emission of a single GRB. Moreover, the statistical properties of pulses in the prompt emission of GRB 230307A are very similar to those of solar flares. Our findings suggest that the prompt emission of GRB is powered by the dissipation of magnetic energy in the ultra-relativistic jet, supporting the Poynting-flux-dominated prompt models.

During a tidal disruption event, the resulting debris stream cools down adiabatically due to the tidal stretching. As the temperature drops, the gas is expected to undergo chemical processes, which can release thermal energy into the stream, potentially affecting the subsequent gas evolution. For the first time, we investigate in detail this effect and its dynamical impact on the early-time evolution of the stream by making use of three dimensional hydrodynamic simulations coupled with a realistic equation of state. We find that a few days after disruption, the energy injected by hydrogen recombination and molecular hydrogen formation causes the stream thickness to grow much more rapidly. In the bound debris, this effect stops the stream's confinement by self-gravity before the gas reaches apocentre. As a result, the maximum stream thickness increases by a factor that ranges from a few, for the most bound gas, to a few tens for the near-parabolic gas, reaching $\approx 30 \, R_{\star} $ around the peak of the mass fallback rate. We discuss how this accelerated stream expansion may affect the subsequent evolution of the gas, estimate the luminosity powered by recombination in the unbound debris, and evaluate the potential influence of non-ideal magneto-hydrodynamic effects. By characterizing the thermodynamic and hydrodynamic properties of the stream before its return near pericentre, our results provide physically motivated initial conditions to self-consistently model the later stages of tidal disruption events, offering a promising pathway to unveiling the physical origins of their observed emission.

Fast radio bursts (FRBs) are luminous millisecond radio transients whose physical origin remains unsettled. A key diagnostic is whether their burst-energy distributions retain characteristic physical scales that are intrinsic and temporally stable within an individual engine. Here we report a 3.2-year monitoring campaign of the hyperactive repeater FRB~20220529 with FAST and Parkes, yielding more than 1,300 bursts spanning nearly five orders of magnitude in spectral energy density. The cumulative burst-rate distribution is described by an exponential-plus-power-law (EXP+PL) form, linking a low-energy exponential component with characteristic scale (E_0) to a scale-free bright-end tail. This scale remains invariant despite the burst rate declining by more than an order of magnitude, revealing a stable dissipation scale decoupled from the source's macroscopic trigger activity. Within a magnetar interpretation, this phenomenology is consistent with localized sub-critical reconnection episodes coexisting with plasmoid-mediated magnetic avalanches in a twisted magnetosphere. The invariant (E_0) constrains the dissipation region to the inner-to-middle magnetosphere and reveals a robust energy-release hierarchy beneath the variable activity of repeating FRBs, providing an observational benchmark for relativistic reconnection in an ultra-magnetized neutron-star environment.

Photospheric emission from relativistic gamma-ray burst (GRB) jets is a promising mechanism for producing the Band-like spectra observed in the prompt phase, yet the connections between jet structure, dissipation location, and polarization signatures remain unclear. We investigate time-dependent photospheric radiation transfer in structured relativistic jets by coupling two-dimensional axisymmetric special relativistic hydrodynamic simulations with Monte Carlo photon propagation. Photon escape and subphotospheric dissipation are characterized using the residual line-of-sight optical depth <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>τ</mml:mi></mml:mrow><mml:mrow><mml:mi>out</mml:mi></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mover><mml:mrow><mml:mi>Ω</mml:mi></mml:mrow><mml:mrow><mml:mo>̂</mml:mo></mml:mrow></mml:mover><mml:mo>)</mml:mo></mml:math> </inline-formula> evaluated along each photon trajectory, allowing a direction-dependent treatment of photon decoupling in structured jets. The radiative transfer includes Klein─Nishina Compton scattering and polarization evolution using the Mueller matrix formalism. We perform a systematic parameter study exploring the effects of viewing angle, electron─positron pair loading (Z<SUB>±</SUB>), and the optical-depth window of subphotospheric dissipation. The model produces time-resolved spectra, peak-energy evolution E<SUB>pk</SUB>(t), Band parameters, polarization degree Π(E, t), and last-scattering statistics. We find that jet angular structure and the geometry of the line-of-sight optical depth strongly regulate spectral evolution and polarization signatures. The dissipation depth and pair loading jointly control the stability of E<SUB>pk</SUB>, the formation of high-energy spectral tails, and the energy dependence of polarization. These results provide quantitative predictions for GRB prompt-emission spectra and polarization that can be tested with current and upcoming high-energy polarimeters.

The association of broad-lined Type Ic (Ic-BL) supernovae with long-duration gamma-ray bursts (GRBs) has been known for 28 yr. However, only about 70 GRB supernovae (GRB-SNe) have been identified, of which only one-half have spectroscopic classifications. At z = 0.153, GRB 260310A is the twelfth spectroscopically confirmed GRB-SN discovered within 1 Gpc, offering a critical opportunity to follow one of these rare supernovae in detail. We present optical to near-infrared imaging and spectroscopy of GRB 260310A and SN 2026fgk out to 65 days after discovery. The optical afterglow is among the brightest ever observed from a GRB. Spectra obtained more than 2 weeks after the explosion reveal broad absorption features that securely identify SN 2026fgk as a Type Ic-BL supernova (SN). Modeling of the multiwavelength (grizJK<SUB>s</SUB>) lightcurve shows that the SN is approximately one-half the luminosity (k<SUB>98bw</SUB> = 0.4─0.6) of the canonical GRB-SN 1998bw. We derive a nickel mass of M<SUB>Ni</SUB> = 0.4─0.5 M<SUB>⊙</SUB>with a total ejected mass of M<SUB>ej</SUB> ≍ 4─6 M<SUB>⊙</SUB>and kinetic energy E<SUB>K</SUB> = (3─8) × 10<SUP>51</SUP> erg. The GRB exploded at an extremely large offset of 15 kpc from its host galaxy. Long-slit spectra reveal a "bridge" of nebular emission extending along the galaxy's disk to the GRB location, which has a subsolar metallicity (∼0.4 Z<SUB>⊙</SUB>), compared to a near-solar metallicity for the host galaxy. This indicates that the large offset arises from the galaxy's extended light profile rather than an isolated environment.

The origin of late-time radio flares in tidal disruption events remains unclear. In particular, the peculiar radio flare observed in AT2018hyz has motivated two leading scenarios: a delayed outflow launched ∼1000 days after discovery, or an off-axis relativistic jet directed far from our line of sight. Very long baseline interferometry imaging provides the most direct way to distinguish between these scenarios. In this paper, we calculate synthetic radio images for both models and examine their observational signatures. The motion of the emission centroid is the most powerful diagnostic for breaking the degeneracy. In the delayed-outflow scenario, the centroid motion is confined within a non-relativistic distance, whereas in the off-axis jet scenario it exhibits apparent superluminal motion. Detecting such superluminal motion would therefore provide a smoking-gun signature of the off-axis jet interpretation. We also find that the jet image exhibits characteristic features, including a nonmonotonic evolution of the image aspect ratio. These results are expected to be generic and applicable to other jetted explosions, such as microquasars and gamma-ray bursts.

In 2026 March, the Einstein Probe (EP) discovered its nearest (z = 0.0343) Fast X-ray Transient (FXT), EP 260321a, the first EP FXT to provide a strong match to expectations for X-ray "shock breakout" (SBO) emission. Here, we present our multiwavelength follow-up campaign of EP 260321a and its broad-line Type Ic (Ic-BL) supernova (SN) counterpart, SN 2026gzf. We show that our radio follow-up extending over 5.8─54.5 days post-FXT rules out an on-axis jet counterpart of isotropic-equivalent kinetic energy E<SUB>K</SUB> ≳ 10<SUP>49</SUP> erg for circumburst densities n &gt; 10<SUP>−2</SUP> cm<SUP>−3</SUP> and assuming microphysical parameters ϵ<SUB>e</SUB> = ϵ<SUB>B</SUB> = 0.1. Our radio data also constrain a median mass-loss rate of <inline-formula> <mml:math><mml:mover><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>̇</mml:mo></mml:mrow></mml:mover><mml:mo>≲</mml:mo><mml:mn>1.2</mml:mn><mml:mo>×</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⊙</mml:mo></mml:mrow></mml:msub><mml:mspace></mml:mspace><mml:msup><mml:mrow><mml:mi>yr</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> </inline-formula> for a Wolf─Rayet progenitor. In addition, we derive SN 2026gzf's properties, including <SUP>56</SUP>Ni mass, diffusion timescale, and expansion velocities, from our ∼nightly cadence optical data and compare them with those of optically discovered Type Ic-BL SNe, finding that SN 2026gzf is well within the 90% confidence interval across all properties. We further fit SN 2026gzf's light curve and determine that combined emission from both interaction with CSM and <SUP>56</SUP>Ni radioactive decay provides the best fit with plausible model parameters. Finally, using the rate of Ic-BL SNe from the Zwicky Transient Facility Bright Transient Survey and assuming all Type Ic-BL SNe produce EP 260321a-like FXTs, we infer an expected rate of EP-detected SBOs of 4.4─16 yr<SUP>−1</SUP>. This is inconsistent at the 90% confidence level with current EP detection rates, potentially indicating that most Type Ic-BL SNe produce less luminous X-ray SBO signals compared to EP 260321a.

Real-time ranking of optical transient candidates during gravitational-wave and multimessenger follow-up is challenging when only sparse early-time, multi-band photometry is available. We present KilonovaSCORER, an open-source framework for scoring and ranking in this regime. It quantifies the consistency of each candidate with a physically motivated kilonova model grid in absolute magnitude space using two complementary per-observation metrics, P<SUB>tail,KNe</SUB> and P<SUB>near,KNe</SUB>. These are aggregated into a cumulative ranking score via inverse-variance weighting in logit space, naturally accounting for heterogeneous observational uncertainties across bands and epochs. A sequential Approximate Bayesian Computation diagnostic tracks photometric consistency across epochs, penalizing candidates whose temporal evolution is incompatible with kilonova expectations. We validate the framework on AT 2017gfo and SN 2025ulz, and test it against supernova simulations under a realistic Rubin/LSST Target-of-Opportunity strategy. The framework recovers kilonova candidates with high confidence while ruling out supernova contaminants within five days of the gravitational-wave trigger. In our LSST ToO simulations, median cumulative scores for thermonuclear and core-collapse supernova contaminants fall to zero by 3─4 days post-trigger, whereas kilonova medians remain ≳0.4. KilonovaSCORER supports real-time workflows for ToO teams and LSST alert brokers, integrates with follow-up coordination platforms such as the Tool for Rapid Object Vetting and Examination, and is publicly available at https://github.com/phelipedarc/KilonovaSCORER/tree/main.

We present an extensive radio monitoring campaign of the unique extragalactic transient GRB 250702B, with observations spanning 0.65-233 GHz from 6-356 d (observer frame) post-discovery. The radio emission shows a smoothly evolving peaked synchrotron spectrum consistent with an adiabatic shock expanding into a stratified ambient medium ($n_e\propto R^{-k}$; $k= 1.5-2$). We detect significant variability in the low frequency ($\leq3$ GHz) light curves which we interpret as interstellar scintillation, placing an approximate bound on the blast wave image size of $1.2\times10^{16}\lesssim R_{\perp} \lesssim 5\times10^{17}$ cm. The temporal evolution of the flux density and critical synchrotron frequencies suggest the shock that powers the radio emission is potentially a wide-angle $θ_j\gtrsim15$ deg, low Lorentz factor ($Γ\lesssim10$) jet, or a narrow $θ_j\lesssim2$ deg highly relativistic jet. A narrow jet is expected for a stellar-mass black hole engine, such as a helium star merger, and the beaming-corrected kinetic energy in this scenario is consistent with the known distribution for long GRBs ($E_K\sim10^{51}$ erg). The wide-angle jet scenario would instead require a progenitor involving prolonged accretion. We derive and show an intermediate or stellar-mass black hole tidal disruption event are viable possibilities. The beaming-corrected kinetic energy in this scenario is on the low end of the known distribution for relativistic SMBH TDEs ($E_K\sim10^{50}$ erg). We disfavour an SMBH TDE due to lack of compatibility with the observed timescales. The detection of a jet shut off within the next year would favour a WD-IMBH TDE due to the shorter theoretical duration of super-Eddington accretion than the main-sequence TDE channels.

Accreting millisecond pulsars (AMSPs) and transitional millisecond pulsars (tMSPs) are neutron star low-mass X-ray binaries that can evolve into recycled radio millisecond pulsars. In both types of systems, X-ray pulsations have been detected during phases of X-ray activity when matter accretion through a disc was turned on. On the other hand, when accretion stops, and these systems enter the quiescent low-luminosity X-ray state, and only tMSPs become visible as radio pulsars. Despite several attempts, radio pulsations have never been detected in quiescent AMSPs, except for IGR J18245−2452. We present the results of two observational campaigns performed on the AMSPs IGR J00291+5934 and MAXI J1957+032 with the Five-hundred-meter Aperture Spherical Telescope (FAST) in L band (1─1.5 GHz). The two sources have most likely been observed in quiescence, as suggested by the upper limits on their X-ray and optical flux obtained with Swift and the Las Cumbres Observatory, respectively. We performed a deep search for coherent periodicities in radio, but found no significant candidate signal at the known spin frequency of these sources or at other frequencies. Assuming a pulse duty cycle of 10%, we derive upper limits on the pulsed radio flux density of 3.3 μJy and 5.6 μJy for IGR J00291+5934 and MAXI J1957+032, respectively, which are the most stringent limits for any known persistent AMSP so far.

State transitions during outbursts of black hole X-ray binaries (BHXBs) exhibit complex, rapidly evolving disk─corona coupling. Understanding this dynamic phase is essential for deciphering accretion physics and the mechanisms that drive outbursts, yet it remains poorly understood because of the scarcity of high-quality, high-cadence observations. Here, we present an analysis of observations from the Hard X-ray Modulation Telescope during the 2023 outburst of the newly discovered low-mass BHXB Swift J1727.8─1613. Follow-up, high-cadence monitoring reveals pronounced variability in disk emission, attributable to fluctuations in the accretion rate. These disk fluctuations exhibit damped amplitudes and shortened flare periods. This evolving disk emission modulates the supply of soft seed photons to the corona, producing a dynamically changing positive correlation between the photon index Γ and the Comptonization luminosity L<SUB>Comp</SUB>. As the transition proceeds, the correlation shifts toward higher Γ and a narrower range of L<SUB>Comp</SUB>. We further suggest that the damped disk variability arises from fluctuations generated at large disk radii and propagating inward, possibly linked to the thermal─viscous disk instability.

Gradual crustal deformation can generate strongly twisted magnetic fields around magnetars, potentially triggering giant flares with total energies exceeding 10<SUP>44</SUP> erg. In this letter, we present the first relativistic magnetohydrodynamic simulation of a surface shear-driven magnetar eruption, capturing reconnection-driven plasma heating, the ejection of relativistically hot plasma, and the formation of a hot fireball confined within the inner magnetosphere. We find that magnetic reconnection in the equatorial current sheet launches a hot trailing outflow capable of powering the initial spike observed in giant flares, while simultaneously leaving behind a thermally stratified fireball with sufficient thermal energy to produce the pulsating, decaying tail. Together, these features provide a self-consistent physical framework for understanding the observed energetics of magnetar giant flares. The eruption also expels a magnetically dominated giant plasmoid carrying up to ∼9% of the magnetosphere's total magnetic energy. Furthermore, our simulation demonstrates how the plasmoid drives the formation of a blast wave—an important ingredient in models linking magnetar eruptions to fast radio bursts.

Using deep near-infrared and optical JWST and Hubble Space Telescope imaging, we identify a new host-galaxy candidate for the short GRB 061201 at an angular offset of ∼2″ from the optical afterglow, with a best-fit photometric redshift of z ∼ 1.2. We compare it with the prior putative host candidate at z = 0.111. The new candidate has a chance-coincidence probability of P<SUB>cc</SUB> = 0.18, above the classical 0.1 threshold but consistent with a physical association given the depth of the JWST imaging. In contrast, the lower P<SUB>cc</SUB> = 0.11 of the z = 0.111 galaxy is driven primarily by bright-tail statistics. A high-redshift origin is favored by three lines of evidence. First, although both scenarios satisfy the Amati (E<SUB>p,i</SUB>─E<SUB>iso</SUB>) relation, the beaming-corrected energy at z = 0.111 makes GRB 061201 an outlier in the Ghirlanda (E<SUB>p,i</SUB>─E<SUB>γ</SUB>) relation. Second, near-infrared data disfavor an AT2017gfo-like kilonova at z = 0.111. Third, afterglow modeling favors the high-redshift scenario, with the Akaike information criterion (AIC) for the z = 1.2 model lower than that for z = 0.111 by ∆AIC = 16.35. Adopting the z ∼ 1.2 candidate as the host gives a physical offset of 16.4─16.9 kpc (substantially reduced from ∼42 kpc implied under the low-redshift scenario) and a stellar age of ∼2 Gyr, consistent with the general population of short gamma-ray bursts (sGRBs). Furthermore, a low-redshift origin implies a binary neutron-star merger rate of ∼1400 Gpc<SUP>−3</SUP> yr<SUP>−1</SUP>, in tension with gravitational-wave constraints. We conclude that a moderately high-redshift host provides a more self-consistent physical framework for GRB 061201, demonstrating the capability of deep JWST imaging for historically hostless sGRBs.

We model the multiband lightcurves of 80 broad-lined Type Ic supernovae (SNe Ic-BL), including 11 associated with long-duration gamma-ray bursts (lGRBs), using a magnetar engine model with <SUP>56</SUP>Ni decay. The data are all consistent with a magnetar engine, yielding high-quality fits across the sample. The medians with 1σ regions for key parameters are initial spin period <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>P</mml:mi></mml:mrow><mml:mrow><mml:mi>i</mml:mi></mml:mrow></mml:msub><mml:mo>∼</mml:mo><mml:mn>2.0</mml:mn><mml:msubsup><mml:mrow><mml:mn>3</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.95</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>1.81</mml:mn></mml:mrow></mml:msubsup><mml:mspace></mml:mspace><mml:mi>ms</mml:mi></mml:math> </inline-formula>, magnetic field <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mi>p</mml:mi></mml:mrow></mml:msub><mml:mo>∼</mml:mo><mml:mn>3.9</mml:mn><mml:msubsup><mml:mrow><mml:mn>7</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.42</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>3.07</mml:mn></mml:mrow></mml:msubsup><mml:mo>×</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mn>15</mml:mn></mml:mrow></mml:msup><mml:mspace></mml:mspace><mml:mi>G</mml:mi></mml:math> </inline-formula>, ejecta mass <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mi>ej</mml:mi></mml:mrow></mml:msub><mml:mo>∼</mml:mo><mml:mn>2.3</mml:mn><mml:msubsup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.01</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>1.46</mml:mn></mml:mrow></mml:msubsup><mml:mspace></mml:mspace><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⊙</mml:mo></mml:mrow></mml:msub></mml:math> </inline-formula>, and <SUP>56</SUP>Ni mass <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mi>Ni</mml:mi></mml:mrow></mml:msub><mml:mo>∼</mml:mo><mml:mn>0.1</mml:mn><mml:msubsup><mml:mrow><mml:mn>8</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.09</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.14</mml:mn></mml:mrow></mml:msubsup><mml:mspace></mml:mspace><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⊙</mml:mo></mml:mrow></mml:msub></mml:math> </inline-formula>, with strong correlations observed for both M<SUB>ej</SUB>−P<SUB>i</SUB> (anticorrelation) and M<SUB>Ni</SUB>−M<SUB>ej</SUB> (correlation). Comparing the lGRB-associated and non-lGRB-associated samples using fitting parameters, we find no significant difference, although the lGRB-associated sample is slightly brighter, possibly due to observational bias (dimmer ones overshone by afterglow). Relative to ordinary Type Ic SNe, SNe Ic-BL have similar <SUP>56</SUP>Ni and ejecta masses, suggesting comparable pre-SN progenitor properties, with differences possibly arising from the presence of a magnetar engine that governs supernova (SN) dynamics. In comparison with other possible magnetar-powered stripped-envelope SNe (SESNe), including Type Ic superluminous SNe (SLSNe Ic) and fast blue optical transients (FBOTs), we confirm a strong universal M<SUB>ej</SUB>−P<SUB>i</SUB> correlation, indicating a common origin. SNe Ic-BL and SLSNe Ic have similar ejecta mass distributions, typically M<SUB>ej</SUB> ≳ 0.5 M<SUB>⊙</SUB>, while FBOTs mostly lie below this value. Differences between SNe Ic-BL and SLSNe Ic may arise from magnetar properties, with SN Ic-BL magnetars rotating faster and having stronger fields. Moreover, the P<SUB>i</SUB>−B<SUB>p</SUB> distribution of lGRB magnetars largely overlaps with that of SN Ic-BL magnetars. Motivated by binary simulation results, we propose a unified physical classification and progenitor framework for magnetar-powered SNe and ordinary SESNe.

Gamma-ray burst (GRB) X-ray light curves exhibit a variety of complex temporal structures, such as flares and plateaus. The origin of flares seen in many GRB early afterglows is still uncertain. Here we analyze a sample of 89 GRBs, 61 of them with flares, both with and without a "plateau" phase. We fit the Swift X-Ray Telescope light curves with synchrotron emission from a forward shock propagating into either a constant-density interstellar medium or a stellar wind and flares on top of that. We find that the flare light curves are not symmetric, with a decay time that is approximately three to four times longer than the rise time. We do not find any differences in flare properties between GRBs with and without a "plateau" phase. Moreover, additional afterglow properties such as the electron power-law index and the end time of the plateau are consistent between bursts with and without flares. These results strongly indicate that flares originate from a mechanism distinct from that producing the plateau and afterglow. When looking at the prompt-emission properties, we do find some tendencies: GRBs with flares tend to be brighter and longer lasting than GRBs without flares. We therefore conclude that, unlike plateaus, flares are unlikely to arise from an external origin and are more plausibly associated with prolonged central engine activity that lasts longer than the main episode that produces the prompt phase. While the plateau cannot share the same origin, our results indicate that it is unlikely to be dominated by late-time accretion episodes associated with flaring activity, although alternative forms of late-time energy injection remain possible.

Solar flares result from explosive magnetic energy release in the solar atmosphere, often linked to magnetic reconnection. While the underlying physical mechanisms are generally understood, the specific conditions that make an active region (AR) flare at a particular moment remain poorly understood. We aim to investigate the role of magnetic vortices in the flare activity of solar ARs, and to assess whether the presence and evolution of such structures can serve as precursors to major flares. We applied the Integrated Averaged Current Deviation (IACD) method to vector magnetic field data from Helioseismic and Magnetic Imager (HMI)/SHARP for three ARs—AR 11157 (nonflaring), AR 11158, and AR 12673 (both flaring). Time series of IACD values were analyzed using the power spectral density, zeta function scaling, kurtosis, and current helicity to characterize the temporal variability and energy distribution of the vortices. Long-lived magnetic vortices were observed to form and intensify before X-class flare events, followed by abrupt changes and collapse near flare onset. The IACD method isolated these structures more effectively than standard diagnostics, such as magnetograms, current density, or helicity maps. Flaring ARs exhibited flatter power spectra, lower kurtosis, and linear zeta scaling, suggesting more continuous energy accumulation. In contrast, the nonflaring AR displayed steeper spectral slopes, higher kurtosis, and nonlinear zeta scaling, indicative of intermittent and less coherent magnetic activity. Magnetic vortices detected via IACD are promising indicators of flare-productive conditions and may reflect topological changes in the magnetic field. Their temporal evolution provides a potential basis for improving flare forecasting tools.

Solar flare ribbons are extensive brightenings in the chromosphere during flares, often showing fine-scale structuring that reflects the underlying energy release. Using high-cadence imaging from the Swedish 1 m Solar Telescope/CRISP during an X1.5-class limb flare on 2014 June 10, we identify and track 232 coherent, thread-like substructures, which we term for the first time "riblets." From a statistical analysis, riblets have well-defined lifetimes and plane-of-sky speeds (typically 5─15 s and 50─150 km s<SUP>−1</SUP>, respectively), establishing them as distinct ribbon substructures. From analysis of their temporal distributions, their distance─time (X─T) evolution uniquely reveals approximately linear and nonlinear (accelerating/decelerating) classes, a discrepancy that may be influenced by projection geometry. From analysis of their spatial distributions, we find no clear correspondence between the properties of adjacent riblets, suggesting that local atmospheric conditions (fine-scale thermodynamic and/or magnetic structuring) govern their kinematics more than spatial variations in electron beam energy flux. From analysis of their spectral distributions, clusters of riblets do show temporal and spatial coincidence with hard X-ray emission signatures, consistent with episodic electron beam injection into the chromosphere. Using Fermi/Gamma-ray Burst Monitor spectroscopy, we derive thick-target parameters suitable for flare simulations, with representative values δ<SUB>low</SUB> ≍ 5.93, E<SUB>c</SUB> ≍ 24.7 keV, and an implied beam energy flux <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>F</mml:mi></mml:mrow><mml:mrow><mml:mi>beam</mml:mi></mml:mrow></mml:msub><mml:mo>≍</mml:mo><mml:mn>1.5</mml:mn><mml:mo>×</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mn>10</mml:mn></mml:mrow></mml:msup></mml:math> </inline-formula> erg cm<SUP>−2</SUP> s<SUP>−1</SUP> (based on RHESSI footpoint area). Together, these results identify riblets as the fundamental building block of flare ribbons and provide quantitative constraints for forward tests of riblet formation mechanisms.

The launch of the James Webb Space Telescope (JWST) has enabled the discovery of a small but increasing sample of high-redshift core-collapse supernovae (CC SNe), which provide new tests of massive star evolution in the early Universe. In this study, we report the analysis of SN 2023aeaf discovered in COSMOS-Web survey observations, which at z = 3.195 has one of the highest SN spectroscopic redshifts to date. Using two epochs of JWST photometry separated by ∼1 month in the rest frame, we photometrically classify SN 2023aeaf by comparing the JWST photometry to spectrophotometric CC SN and Type Ia (SN Ia) models and UV observations of SNe from the Swift telescope, finding that SN 2023aeaf is highly likely to be a Type II SN. A spectrum of the SN+host galaxy was also obtained ∼30 rest-frame days after discovery but shows no clearly identifiable SN features, with Hα emission from the host potentially masking emission from the SN. Although the limited photometric coverage prevents strong constraints on the explosion properties, we find that the data are most consistent with a ∼12M<SUB>⊙</SUB> progenitor with ∼0.5M<SUB>⊙</SUB> of circumstellar material. We next use the host-galaxy spectrum and photometry to model the host spectral energy distribution (SED) using the Prospector Bayesian inference framework. We find that the host is a star-forming galaxy with an sSFR of <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>log</mml:mi></mml:mrow><mml:mrow><mml:mn>10</mml:mn></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mi>sSFR</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:mrow><mml:mi>yr</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mo>−</mml:mo><mml:mn>10.1</mml:mn><mml:msubsup><mml:mrow><mml:mn>7</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.10</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.13</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula>, a stellar mass of <inline-formula> <mml:math><mml:mi>log</mml:mi><mml:mo>(</mml:mo><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⋆</mml:mo></mml:mrow></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⊙</mml:mo></mml:mrow></mml:msub><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mn>9.0</mml:mn><mml:msubsup><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.04</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.03</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula>, and a gas-phase metallicity of <inline-formula> <mml:math><mml:mn>12</mml:mn><mml:mo>+</mml:mo><mml:msub><mml:mrow><mml:mi>log</mml:mi></mml:mrow><mml:mrow><mml:mn>10</mml:mn></mml:mrow></mml:msub><mml:mo>(</mml:mo><mml:mi>O</mml:mi><mml:mo>/</mml:mo><mml:mi>H</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mn>7.82</mml:mn><mml:mo>±</mml:mo><mml:mn>0.02</mml:mn></mml:math> </inline-formula>. SN 2023aeaf joins a growing sample of early Universe CC SNe with high luminosities, dense circumstellar material (CSM), and low-metallicity environments.

The origin of the large-scale poloidal magnetic field required to power relativistic jets in collapsars remains uncertain. While such a field may be inherited during proto-neutron-star collapse, the efficiency of this process is unclear, motivating an in situ mechanism to generate poloidal fields out of the predominantly toroidal fields produced by stellar differential rotation. We present the first 3D general-relativistic magnetohydrodynamics collapsar simulations initialized with toroidal magnetic field profiles that closely follow those of precollapse stellar models. As the toroidal field in the disk becomes dynamically important, it seeds the dynamo, producing coherent poloidal magnetic loops that appear at <inline-formula> <mml:math><mml:mo>∼</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mn>100</mml:mn><mml:mo>)</mml:mo></mml:math> </inline-formula> gravitational radii and are then advected inward along paths that may deviate from the disk midplane. The resulting poloidal fields thread the black hole (BH) and launch highly variable, wobbling relativistic jets on timescales of order seconds, with the onset depending on the initial magnetic field and the plasma circularization radius. Although the jets are highly variable and misaligned with the BH spin axis, they sustain ≳10<SUP>50</SUP> erg s<SUP>−1</SUP>, comparable to that inferred for long gamma-ray bursts (LGRBs). We identify magnetic-flux inversions driven by the stochastic dynamo, leading to the formation of striped jets that could be imprinted in LGRB light curves. These results demonstrate that accretion-disk dynamos provide a robust pathway for jet production in collapsars across a broad range of progenitors.

We have recently shown evidence that the most relativistic jets (with Lorentz factor <inline-formula><tex-math>$\gt 2$</tex-math></inline-formula>) from stellar-mass black holes in X-ray binary systems may be locked to a fixed axis, likely the spin axis of the black hole. Slower, mildly relativistic jets (with velocities typically <inline-formula><tex-math>${\sim} 0.3c$</tex-math></inline-formula>) are often seen to precess and can be associated with both neutron stars and black holes. In this paper, we demonstrate an additional clear link between highly obscured systems and these lower velocity, precessing jets. We speculate that this link may be due to mass-loading of the jets close to their launch sites, since these obscured systems are likely to be examples of (sometimes persistent, other times transient) super-Eddington accretion. The fastest relativistic jets are now seen to be both locked to a fixed direction, likely the black hole spin axis, and to be launched in low-density environments, while jets launched in dense environments are generally slower and very likely to precess.

We present a comprehensive multiwavelength study of the black hole X-ray binary 4U 1543−475 during its 2021 outburst, focusing on radio flaring episodes that are commonly interpreted as signatures of episodic jet production and are embedded within states when the X-ray emission was dominated by an accretion disc component. The radio monitoring reveals at least two discrete flares that coincide with periods of enhanced Comptonized X-ray emission. Broad-band spectral modelling shows a significant decrease in the reflection-to-disc flux ratio (by a factor of <inline-formula><tex-math>${\sim} 3\!-\!4$</tex-math></inline-formula>) during these episodes, consistent with a temporary change in the geometry of the inner accretion flow, although the data do not allow the causal sequence to be firmly established. Optical photometry exhibits variability that broadly tracks the reflection fraction, consistent with changes in the illuminating component. The accompanying spectral hardening indicates that the radio flares were associated with short-lived excursions toward a 'harder' state, departing from the soft state. X-ray timing analysis suggests that the radio flares may be associated with changes in the fractional rms variability; however, no consistent or unified pattern can be firmly established across different events. These results provide a multiwavelength observational example of radio flaring activity in a black hole binary and highlight the complex interplay between accretion flow geometry, coronal emission, and jet-related phenomena.

We present a simple time-dependent model of viscously spreading accretion disks around black holes (BHs) with masses ranging from 10−108M⊙. We apply the results to observations of late-time emission in tidal disruption events (TDEs) and luminous fast blue optical transients (LFBOTs) such as AT2018cow. Our model generalizes previous work by incorporating outflows during phases of super-Eddington accretion, non-conservation of mass and angular momentum in TDE circularization, irradiation of the outer disk by the inner accretion flow, and a range of viscous stress models. We show that many of the late-time plateaus in TDEs can be explained by disks that form with a large spread in angular momentum, due to angular momentum redistribution during circularization. Viscous spreading on year timescales is not required, although it is also compatible with the data. The significant range of peak TDE X-ray luminosities is also consistent with a range of disk spreading timescales. The collapse of radiation pressure dominated thin disks to the stable gas-pressure dominated phase underpredicts TDE plateau luminosities by orders of magnitude, strongly favoring thermally stable magnetically dominated disk models. Irradiation of the outer disk in TDEs due to misalignment of the stellar orbit and BH spin increases plateau luminosities and durations by factors of a few. Continued study of late-time TDE emission provides a unique opportunity to constrain the physics of disk formation and circularization, accretion disk warps, angular momentum transport, and other poorly understood aspects of disk physics. The models developed here can also explain the late-time optical-UV emission in the LFBOT AT2018cow for BH masses of ∼10−100M⊙. The very faint X-ray emission at late-times in AT2018cow is likely due to ongoing X-ray absorption. Our models predict that late-time X-rays at should eventually be detectable (again) in LFBOTs and that HST-JWST observations of AT2018cow may detect a break in the SED at near-IR-optical wavelengths, providing a powerful probe of the outer accretion disk thermodynamics.

We present a broadband spectral─timing analysis of the black hole X-ray binary GS 1354 ─ 64 using five NuSTAR observations obtained during its 2026 outburst, together with long-term soft X-ray monitoring from MAXI. The rms-normalized NuSTAR cospectra reveal persistent type-C low-frequency quasi-periodic oscillations (QPOs) in all observations, with centroid frequencies evolving from ∼ 1.4 to ∼ 6.1 Hz. The QPO quality factor varies non-monotonically and reaches its maximum value at an intermediate frequency, indicating that the coherence of the oscillation is governed more by the changing inner accretion-flow geometry than by the total flux alone. The 3─79 keV spectra are well described by the model constant*TBabs*(diskbb + cutoffpl + relxill), with the absorption column fixed at the Galactic value and the photon index and cutoff energy of the reflection component tied to the direct continuum. The source shows clear spectral softening, with the photon index increasing from Γ ∼ 1.74 to ∼ 2.53, while the inner disc temperature rises from ∼ 0.38 keV in the first two observations to ∼ 0.84─1.16 keV in the later observations. The cutoff power-law component dominates the broadband 0.01─100 keV flux throughout the outburst, whereas the reflection flux remains comparatively stable. The cutoff energy increases from ∼ 33─37 keV to values above 100 keV, indicating significant evolution of the high-energy continuum. We find a positive correlation between QPO frequency and photon index, while the QPO frequency does not follow the total flux monotonically. These results are consistent with a truncated-disc interpretation in which the optically thick disc may evolve inward during the outburst, enhancing coronal cooling and contributing to the observed increase in QPO frequency. Within the framework of Lense─Thirring precession, the QPO evolution can be interpreted as arising from changes in the geometry of the inner hot flow.

Repeating fast radio bursts may reside in young supernova remnant (SNR) environments whose evolving plasma contributes to the observed dispersion measure (DM). We use two-dimensional axisymmetric hydrodynamic simulations to study the interaction between a continuous anisotropic wind from a young neutron star and homologously expanding supernova ejecta. We follow the evolution to approximately 160 yr and calculate the source-local DM along different viewing directions, using a passive tracer to separate wind and non-wind contributions. In the fiducial model, the strongly polar-focused wind inflates a low-density cavity, while the swept-up shell remains broadly rounded and the DM shows moderate angular variation. The DM is dominated by ejecta and swept-up non-wind material. The solid-angle-averaged ambient-subtracted excess DM declines throughout the evolution, approximately following $t_{\rm age}^{-2}$ during the first several tens of years and becoming modestly steeper later. Variations in wind and ejecta parameters modify the normalization, early evolution, and viewing-angle dependence, but the angle-averaged DM declines in all models, while different bipolar wind profiles produce similar long-term evolution. For FRB 20190520B, the fiducial model reaches a decline rate comparable to the source-frame value inferred from observations at approximately 20 yr, when the mean excess DM is approximately $1.8 \times 10^2$ pc cm$^{-3}$. Thus, such a young environment can retain a substantial electron column while producing a rapid secular decrease. Repeater diversity suggests that SNR-driven expansion may coexist with additional time-dependent plasma structures or ionization changes.

EP241113a, a soft X-ray transient detected by the Einstein Probe (EP), has an isotropic-equivalent energy ($E_{γ,\rm{iso}}\sim 10^{51}{\, \rm erg}$) comparable to classical gamma-ray bursts (GRBs) but an exceptionally low peak energy ($E_{\rm{p}} \lesssim 1 \, {\rm keV}$), placing it off the canonical $E_{\rm{p}}$--$E_{γ,\rm{iso}}$ (Amati) relation at $>3σ$ confidence. The afterglow of EP241113a is also unusual, with an extremely low plateau luminosity and no jet break up to the latest observation. Such properties have been interpreted as arising from an energetic ``dirty fireball,'' i.e., a relativistic jet with energy comparable to classical GRBs but much higher baryon loading, leading to a bulk Lorentz factor of $Γ\sim 20$. In this {\it Letter}, we propose that low $E_{\rm{p}}$ arises from dissipative photospheric emission in such a low-$Γ$ jet. As $Γ$ decreases, the photosphere shifts outward while the internal shock radius moves inward, causing dissipation to occur well below the photosphere and making the prompt emission photosphere-dominated. Meanwhile, the thermalization radius also moves outward, reducing the comoving radiation temperature where the spectral peak is established. Combined with weaker Lorentz boosting, these effects naturally shift the observed $E_{\rm{p}}$ into the soft X-ray band. We further suggest that such a low-$Γ$ jet may be powered by neutrino--antineutrino ($ν\barν$) annihilation in a hyperaccreting disk formed during the core collapse of a massive star. Compared with the Blandford--Znajek process, neutrino annihilation can produce a dirtier jet through baryon entrainment from a neutrino-driven wind. EP241113a-like events therefore offer new insight into the dissipation physics and launching mechanisms of jets from collapsing massive stars.

We present James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the field of the fast X-ray transient (FXT) detected by Einstein Probe, EP250207b, to resolve any ambiguity about the host galaxy and redshift of the FXT. EP250207b was originally associated with a nearby galaxy at z=0.082, based on its low chance alignment probability, and a binary neutron star merger origin was proposed. However, we report the detection of a background galaxy at z=3.2 at the location of EP250207b. Assuming this galaxy is the actual host galaxy, the rest-frame energetics and timescales of the event change. Furthermore, the available data are not able to rule out the presence of a supernova associated with EP250207b if at this redshift. We model the X-ray, optical, near-infrared and radio light curves using a tophat jet model implemented in Redback and find that they are consistent with an on-axis gamma ray burst afterglow. The energetics and host galaxy properties do not allow us to distinguish between a collapsar and a merger driven event.

We present XRISM/Resolve spectroscopy of Her X-1 across three X-ray eclipses observed in September 2024, resolving its iron K complex through the ingress, mid-eclipse, and egress phases. The 5 eV high energy resolution of Resolve enabled us to disentangle and detect all primary components of the iron K complex: neutral iron fluorescence (Fe K$α$ and K$β$), highly ionized emission lines (Fe XXV He$α$ and Fe XXVI Ly$α$). The neutral Fe K$α$ emission is not significantly detected during mid-eclipse, indicating a compact origin near the neutron star. At ingress and egress, the line centroid exhibits red- and blue-shifts of $\sim 200$ km s$^{-1}$ after correcting for the systemic velocity and the neutron star's orbital motion. This residual shift corresponds to Keplerian rotation at a characteristic radius of $r_{\rm disk} \sim 6.6\times10^{6}$ km, suggesting an association with the outer accretion disk. In contrast, the highly ionized Fe XXV He$α$ and Fe XXVI Ly$α$ lines remain visible during eclipses, indicating an extended origin. Photoionization modeling (SPEX pion model) yields $\log_{10}(ξ/{\rm erg\,cm\,s^{-1}}) \sim 3.4$ and $N_{\rm H} \sim 3.1\times10^{22}$ cm$^{-2}$ consistent with the ionized disk wind of Her X-1. Flux-ratio diagnostics constrain the geometric inner boundary of the clumpy disk wind to $R_{\rm in} = 3^{+5}_{-2} \times 10^{10}$ cm ($1σ$), consistent with the Compton-heated thermal winds. The inferred mass outflow rate is $\dot{M}_{\rm out} \approx 3.2 \times 10^{-9}\,M_{\odot}$ yr$^{-1}$ (half the supplied mass), consistent with absorption line measurements of the disk wind obtained out of eclipse.

The nature of the circum-burst medium serves as a key diagnostic for probing the progenitor systems and the physics of relativistic jet propagation in gamma-ray bursts (GRBs). In this work, we systematically infer the density profile index $k$ (where $n \propto r^{-k}$) from the change in the temporal decay index at the jet break ($Δα$). Within the framework of the uniform jet model, the two quantities are linked by the relation $Δα= (3 - k)/(4 - k)$. We apply this diagnostic to a substantial and uniformly selected sample of 170 GRBs with clear jet breaks, identified from over 1,400 Swift/XRT X-ray afterglows observed from 2004 to 2024. By fitting the light curves with a broken power-law model, we obtain $Δα$ for each burst and subsequently derive the corresponding $k$ value. We then use the derived $k$ values to classify the circum-burst environment of each GRB. Our results reveal a near-even split: 82 bursts ($\sim48\%$) are consistent with a constant-density interstellar medium (ISM, $k \approx 0$), while 88 bursts ($\sim52\%$) favor a wind environment ($k \approx 2$). For the 35 bursts with optical data, our X-ray-based classifications are generally consistent with independent multi-band analyses. Additionally, we derive jet opening angles and true beaming-corrected energies for bursts with known redshifts.

We conducted a comprehensive multi-wavelength analysis of a high energetic long-duration ZTF20abbiixp / GRB~200524A detected by \textit{Fermi} Gamma Ray Burst Monitor (GBM). Our study combines extended high-energy observations from multiple space-based observatories including \textit{Fermi} with broadband afterglow data spanning X-ray to radio wavelengths, complemented by extensive photometric and spectroscopic follow-up from several ground-based optical facilities worldwide like 3.6-m Devasthal Optical Telescope (DOT). ZTF20abbiixp / GRB~200524A exhibits almost negligible spectral lag, likely arising from the presence of multiple overlapping emission episodes, a property uncommon among long-duration bursts. The burst additionally shows a clear intensity-tracking evolution of the prompt-emission spectral parameters. The broadband afterglow light curve best fits with a broken powerlaw with a break at $10^{5}$ s since the GBM trigger. The electron powerlaw index (p) calculated from the temporal and spectral slopes fail to distinguish between a interstellar medium and a wind environment. Our custom-developed afterglow model fits the panchromatic data well, combining forward shock (FS) and reverse shock (RS) emission. The RS contribution required to fit the early time optical data. The inferred afterglow model parameters suggest that ZTF20abbiixp / GRB~200524A is a high energetic burst expanding into a dense ISM environment, with a relatively large value of the fraction of energy going to accelerating electron and magnetic field ($ε_B$).

We present long-term, three-dimensional simulations of ejecta from four binary neutron star mergers to second-long time-scales. Numerical-relativity data serve as boundary conditions for a general-relativistic hydrodynamics evolution incorporating an equation of state valid outside nuclear statistical equilibrium and an effective nuclear-heating prescription based on reaction-network calculations. We investigate the ejecta's dynamical and geometrical properties, the impact of nuclear heating, the formation and spatial distribution of elements, and compute multi-angle kilonova light curves. % Nuclear heating significantly affects ejecta dynamics, delaying homologous expansion beyond second time-scales and reshapes the spatial distribution of heavy nuclei. This effect is largest for asymmetric binaries with long-lived remnants; extending the evolution from ${\sim}150$~ms to ${\sim}1$~s widens the angular polar region containing 90\% of the heavy-element mass (\eg, $Z=56$, $Z=79$) from $|θ|\lesssim15^{o}$ to $|θ|\lesssim30^{o}$. Our nucleosynthesis results confirm that the $^{56}$Ni$\rightarrow^{56}$Co$\rightarrow^{56}$Fe decay chain dominates the heating at $\sim$100 days, with cobalt decay producing gamma-ray lines at 846.77 and 1238.288~keV. % Comparing kilonova ray-by-ray light curves obtained from multi-angle 3D profiles and averaged 2D profiles, we find the latter approach broadly robust, although 2D light curves should be treated as upper limits. Increasing dimensionality generally lowers the bolometric luminosity, with binary asymmetry strengthening the viewing-angle dependence. For observers aligned with a lanthanide curtain's densest region, 3D emission can match its 2D counterpart in brightness. We conclude that increased dimensionality alone is unlikely to reconcile current theoretical models with AT2017gfo observations.

A tidal disruption event (TDE) occurs when a star approaches a black hole (BH) and is disrupted by its tidal forces. Although several hundred TDEs have been identified to date, only a small fraction are accompanied by relativistic jets. These jets are thought to be Poynting-flux-dominated outflows powered by the Blandford--Znajek (BZ) mechanism. However, the origin of the magnetic flux and plasma needed to power BZ jets remain unclear. In this Letter, we propose a scenario in which magnetic flux is initially stored in a pre-existing low-Eddington accretion disk around BH, and is subsequently advected toward the BH by the super-Eddington accretion flow formed after the stellar disruption. We show that stars with low densities, such as red giants, can supply sufficient magnetic flux to power a BZ jet. Once sufficient magnetic flux accumulates near the BH, an equatorial current sheet forms where magnetic reconnection produces high-energy gamma rays. We find that photon--photon pair production by these gamma rays supplies the BH magnetosphere with sufficient plasma to launch and sustain a BZ jet. We further show that this mechanism simultaneously provides enough radiating particles to account for the observed prompt emission.

Approximately (20-50)$\%$ of the gamma-ray burst (GRB) X-ray afterglows exhibit the shallow decay features. Two popular energy-injection models had been proposed to interpret such observational phenomenons, the relativistic wind bubble (RWB) model with a Poynting-flux injection and the structured ejecta (SE) model with a dynamical energy injection. Polarization predictions of the two models had been investigated and can be used as a test of the two models. However, the impacts of the parameters on the model predictions were not studied and the comparisons with the detection ability of the forthcoming mission, enhanced X-ray Timing and Polarimetry (eXTP), had not been discussed. We considered the above issues and found that influences of the model parameters on the predicted polarizations of the two models are very limited. To perform a feasible polarization detection during the plateau phase, the priority ToO response is required. The detection probability of the GRB plateau phase is about $1/3$ for one pointing under the priority ToO. The polarization detection probability would depend on the ratio between the Poynting-flux injection to the dynamical energy injection, which is unclear currently. The predicted flux density and polarization degree (PD) of the RWB model could be well above the threshold flux and minimal detectable polarization degree of the polarimetry focusing array (PFA) on board eXTP, while the predicted PDs of the SE model would be difficult to be detected by eXTP/PFA. Therefore, a detection of a significant polarization signal during the GRB plateau phase would prefer the RWB model and the injected energy would be in the form of the Poynting flux, while a non detection of the polarized signal would indicate a dynamical energy injection of the SE model.

The $\sim10\%$ optical polarization observed at the early stage of GRB 091208B comes from the forward shock emission, which is higher than the conventionally predicted value. Polarizations of the forward shock radiation would depend on the observational geometry and the post-shock magnetic field structure. This magnetic field could arise either from the compression of a pre-existing magnetic field (i.e., the magnetic field in the outer medium) or from the shock-generated instabilities. In this paper, we use a synchrotron radiation model to fit the light curve and polarization observations of GRB 091208B. Two scenarios are considered: one is the case of a slightly off-axis observer, and the other is with a large-scale ordered magnetic field component in the burst environment. We found both scenarios could interpret the observations of GRB 091208B. For the slightly off-axis observation scenario, the observational angle is restricted to be within the range of (1.02, 1.05) times the jet half-opening angle. For the large-scale ordered magnetic field component scenario, the ratio between the ordered component to the random component is constrained to be around 1.

Relativistic-reflection measurements of Cygnus X-1 disagree on the extent of disk truncation and commonly infer supersolar iron abundances. We analyze a selected sample of 26 archival NuSTAR observations obtained between 2012 and 2024 using two configurations from one reflection-model family, with posterior modes sampled by preconditioned sequential Monte Carlo. In the baseline recovered modes, disk-surface ionization increases with photon index (Pearson r = +0.59), with state medians rising from log xi approximately 3.3 in the hard state to approximately 3.9 in the soft state. The corresponding free-emissivity fits give median inner radii of 6.6, 4.4, and 4.0 R_ISCO in the hard, intermediate, and soft states. Fixing q = 3 moves three of eight soft-state observations to the ISCO and one to 2.4 R_ISCO; four fixed-q fits have lower chi^2 than the sampled free-q solutions, showing that those runs missed higher-likelihood regions. The inferred radii are therefore model- and mode-dependent, and the spectra neither require nor exclude an ISCO disk or R_in greater than or approximately 20 R_ISCO. Baseline fitted abundances span A_Fe = 1.6-8.6, with a median of 4.9. Two observations separated by 7.2 hr yield A_Fe = 1.9 +/- 0.2 and 4.5 +/- 1.0, indicating that fitted abundance is not a direct composition measurement. Fixing A_Fe = 1.6 drives some densities toward the grid boundary and worsens the fits relative to A_Fe = 4.5. The wind parameters remain sensitive to the continuum, abundance, and orbital-phase sampling.

Hawking's area theorem states that the total event-horizon area of classical black holes (BHs) can never decrease. Existing gravitational-wave tests analyze single loud events (GW150914, GW230814, GW250114) and therefore certify the law only for the specific mergers observed. Here we perform the {\it first} population-level test, exploiting the recent decomposition of the GWTC-5 BBHs into a low-spin subpopulation of stellar-collapse origin and a high-spin subpopulation formed through hierarchical mergers of the former. If the high-spin BHs are merger remnants (i.e., second-generation BHs), the area law requires their horizon areas to statistically exceed the total pre-merger areas of the low-spin binaries. Using inspiral-only parameter estimation for 241 events, independent of merger-ringdown modeling, we find that both peaks in the horizon-area distribution of second-generation BHs lie above their first-generation counterparts, with probabilities of $0.9995$ ($3.3σ$) and $0.983$ ($2.1σ$), respectively. The area law thus holds statistically for the quasicircular, moderately spinning mergers that dominate current catalogs, which in turn underpins the robustness of our classification of stellar-collapse and hierarchical-merger BHs.

Relativistic capture and tidal disruption around a spinning black hole depend on both the magnitude and direction of the star's angular momentum, yet loss-cone models often assume fixed orbital inclinations by ignoring the associated diffusion. We show that this is not justified: for isotropic two-body relaxation near a small loss threshold, angular-momentum magnitude $L$ and inclination $x = L_{z}/L$ diffuse on comparable timescales, $t_{E} \gg t_{L} \sim t_{x}$. For Kerr capture, retaining inclination diffusion significantly amplifies the prograde--retrograde contrast while leaving the total inclination-integrated flux nearly unchanged. An almost correct integrated flux can hide a badly wrong angular distribution. The three-dimensional diffusion problem nevertheless retains enough angular structure to permit analytic treatment. By representing pericenter removal as a continuous sink, we obtain a closed-form loss flux solution for a nearly linear Kerr tidal-disruption boundary, finding close agreement with phase-resolved calculations. Inclination-dependent loss therefore requires inclination-resolved diffusion even when integrated rates appear robust.

The treatment of the stellar surface in binary neutron star simulations is crucial for the accuracy of the numerical evolution and the physical reliability of the predicted observables. Numerical artifacts associated with the treatment of steep gradients near the stellar surface can produce spurious heating during the inspiral, leading to an artificial increase of the internal energy and an unphysical expansion of the neutron star. In this work, we investigate the effectiveness of the entropy-based flux-limiting (EFL) scheme in mitigating these numerical effects within the finite-difference code BAM. We perform simulations of both isolated neutron stars and binary neutron star inspirals employing a representative set of hybrid equations of state. We show that the EFL scheme significantly reduces artificial surface heating. This reduction is observed consistently across all stellar models and binary configurations considered, demonstrating that the reduction of numerical heating is a robust feature of the EFL method.

We present distributions of gamma-ray burst observed and inferred properties for those GRBs with redshifts. We show that the isotropic energy distribution, which spans over four orders of magnitude, can be reproduced reasonably well under a simplistic assumption that every observed GRB originates from a quasi-universal jet with roughly the same decreasing power-law profile of energy as a function of angle, with a power-law index of $ 3 \lesssim ζ\lesssim 4$. The spread in the observed distribution can be explained by the variation in observer viewing angle alone. Furthermore, this power-law jet structure provides an even better fit to both the isotropic energy and luminosity distributions if the isotropic energy normalization evolves as a function of redshift in a manner that has been suggested by a range of previously published studies. The relatively steep power-law index of this jet is consistent with the structure predicted by simulations of weakly magnetized jets in collapsars, whereas simulations of hydrodynamic jets predict a structure shallower than what we find here. The predicted afterglow light curves within this model framework show steepening behavior at times commensurate with observed jet break times.

A warm neutron star is expected to have a fluid "ocean" of heavy elements at its outermost part of the outer crust. As is on the terrestrial ocean, the neutron star ocean also has surface gravity waves. Around a magnetic pole of a star, the ocean may have a dip due to the strong magnetic pressure coming into play in the hydrostatic balance of the ocean. The dip enables the surface gravity wave to be trapped around the magnetic pole to form eigenmodes. The frequency of the mode is much lower than the dynamical frequency at the stellar surface, owing to the weak Coriolis force and the gradient in the ocean's depth that makes the eigenmodes present. We solve the equation of surface gravity waves in the local $β$-plane approximation and obtain the spectrum of discrete eigenmodes. We see that there are no axisymmetric modes and that the mode frequency decreases and asymptotes to zero as the number of nodes of the corresponding eigenfunction increases. This is reminiscent of the g-modes in the context of asteroseismology. Observations of X-ray binaries containing neutron stars reveal that some of the systems exhibit low-frequency quasi-periodic oscillations (QPOs) whose frequency is $1-10^3$mHz. We investigate whether the eigenmodes considered here may explain the low-frequency QPO spectrum. It is suggested that some of the QPOs in the system whose neutron star spins at the period less than $10$s may be consistent with the model. As far as the spin period is larger than $10$s, the eigenmode frequencies are too low to explain the observed QPOs.

Continuous gravitational waves from accreting neutron stars in Low Mass X-ray Binaries are one of the main targets for current and next generation ground based detectors. In order to select the most promising astrophysical sources, however, reliable predictions for the signals are required, and it is therefore necessary to develop models that consistently account for the combined effects of magnetic stresses, accretion-induced heating, and the elastic response of the crust.}{We present a model for computing the quadrupolar deformation, incorporating for the first time the coupled effects of a poloidal magnetic field, deep crustal heating, and crustal elasticity. Perturbations to the star's structure driven by the Lorentz force density and by thermally-induced density variations are computed by solving a system of linearised deformation equations in the crust, for which we consider the full elastic response, while the ocean and core treated as barotropic fluids. We identify a threshold accretion rate whose value depends on crustal microphysics and the superfluid gaps in the core, above which magnetic stresses and asymmetric accretion drive deformations of opposite sign, while below this threshold their roles are reversed. The predicted eccentricities reach magnitudes up to $\varepsilon\sim 10^{-11}$, corresponding to characteristic gravitational-wave strains accessible to next-generation detectors such as the Einstein Telescope or Cosmic Explorer, but generally below the sensitivity of current LIGO, Virgo and KAGRA interferometers. These results are consistent with the non-detection of continuous gravitational waves from accreting neutron stars in Low Mass X-ray Binaries in recent observational campaigns, but highlight the need of reliable models to understand the impact of gravitational wave emission in these systems and select relevant targets for future searches.

The blazar TXS 0506+056 is the first source that was reported to be associated with high-energy extragalactic neutrino events and is one of the major targets for multi-messenger studies. We carried out multi-wavelength optical monitoring of this object on 24 nights in the period from 2018 to 2023. The overall light curves exhibit a dimming trend superposed by some small-amplitude fluctuations, and intraday variability was detected on four nights. Bluer-when-brighter behaviors were observed on both intraday and long timescales and were more pronounced on long timescales, while a weak redder-when-brighter trend was detected on one night. No significant time lags were found between variations at different optical wavelengths. We also retrieved the multi-broadband data from some monitoring programs. The data reveal complex, asynchronous flaring in different wavebands. A cross-correlation analysis shows that the high-energy emission (optical to gamma-ray) is co-spatial and leads the radio emission by a substantial time of about 800 to 900 days, suggesting that the radio emission originates from a downstream region of the jet. We performed time-dependent lepto-hadronic modeling of the spectral energy distributions for three representative epochs, the 2017 neutrino-associated flare, a post-flare phase, and a deep quiescent state, revealing an evolution in the radiative properties of the emission regions. The modeling results provide a phenomenological framework for interpreting the long-term multiwavelength behavior of TXS 0506+056 in a multi-messenger context.

The surge in publicly available photometric alerts from wide-field surveys requires automated tools for real-time transient classification. We present NMMA--Astro-COLIBRI, an on-demand Bayesian classification service that couples the Nuclear-physics and Multi-Messenger Astrophysics (NMMA) inference framework to the Astro-COLIBRI real-time multi-messenger platform. After the detection of an optical transient, if photometry is available, it is quality-filtered. The filtered photometry is fitted by nested sampling against a user-selected model from a library of eleven supernova templates; results are delivered to every user within minutes. Applying two or more models on the same optical transient, the service reports the corresponding log Bayes factors as a quantitative ranking of competing subtypes. We demonstrate the workflow on SN 2021ugl (ZTF21abotose), a Type IIb supernova initially mistaken for a kilonova candidate by automated real-time pipelines, comparing competing supernova and kilonova models. In an early-time configuration using only the first ~ 6 days of photometry in two bands (ZTF g and r), so ten days before spectroscopic confirmation, the empirical Type IIb template recovers the correct classification, favored over both the kilonova template and the kilonova-mimicking shock-cooling model. In the full 47-day, three-band baseline, it again achieves the highest evidence over every competing supernova and kilonova template. These results highlight the importance of a comprehensive supernova template library for kilonova discrimination in the multi-survey era.

We investigate the efficacy of semi-analytical global accretion solutions in capturing the flow properties observed in two-dimensional numerical simulations of shocked accretion onto black holes. A comparative analysis reveals that no single disc geometry adequately matches the simulation profiles across the entire radial domain. The pre-shock region exhibits closer agreement with the conical disc geometry, while the post-shock region is better described by the vertical equilibrium disc, where enhanced thermal pressure leads to substantial vertical expansion. Motivated by these complementary behaviours, we introduce a hybrid disc geometry in which the pre-shock flow follows the conical solution and the post-shock flow attains vertical equilibrium. This hybrid model satisfactorily reproduces both dynamical and thermodynamical properties of shocked accretion flows with the predicted Mach number and temperature profiles closely matching the simulations and the inferred shock location differing by $\sim10\%$. Within this framework, we delineate the shock parameter space spanned by the energy ($\mathcal{E}$) and angular momentum ($λ$) of the flow for weakly and rapidly rotating black holes and investigate the possible origin of Quasi-periodic Oscillations (QPOs) in black hole X-ray binaries (BH-XRBs). We constrain flow parameters that reproduce observed QPO centroid frequencies ($ν_{\rm QPO}$) demonstrating that oscillations of the shock front provide a self-consistent mechanism for both low and high frequency QPOs. Extending the analysis to ten Galactic BH-XRBs, we demonstrate that the observed $ν_{\rm QPO}$ are reproduced within physically plausible parameter ranges, which establishes shocked global accretion solutions as a potentially compelling framework for interpreting accretion driven temporal variability.

In recent years, afterglow emission in the very-high-energy (VHE) band above 100 GeV has been clearly detected for at least five gamma-ray bursts (GRBs 180720B, 190114C, 190829A, 201216C and 221009A). For some of these VHE GRBs, we previously proposed a two-component jet model, consisting of two uniform jets with narrow and wide opening angles to explain their multiwavelength afterglows including VHE gamma rays. In this paper, we show that the VHE spectra and light curves of GRBs 201216C and 221009A can also be reasonably explained by our two-component jet model, based on two top-hat jets propagating into a constant-density circumburst medium. We find that for the five VHE GRBs, the collimation-corrected kinetic energies of the narrow and wide jets have typical values of <mml:math><mml:mn>5</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>49</mml:mn></mml:mrow></mml:msup></mml:math> erg and <mml:math><mml:mn>5</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>50</mml:mn></mml:mrow></mml:msup></mml:math> erg, respectively. We discuss the similarities and differences among the VHE GRBs, and the implications for the structure of their jets. In agreement with previous studies, the narrow jet of GRB 221009A has an atypically small opening angle, so that its intrinsic, collimation-corrected energy remains within a plausible range despite the unusually large isotropic-equivalent energy.

We investigate the X-ray variability of the supergiant X-ray binary 4U 1907+09 using the new NuSTAR observation of 2024. Unlike the previous NuSTAR observations, the source shows significant flux variation during the current one. The light curve exhibits dips (off-state) and flares (on-state). The phase-coherent timing analysis during the on-state yields a pulse period of <mml:math><mml:mn>443.99</mml:mn><mml:mo>(</mml:mo><mml:mn>4</mml:mn><mml:mo>)</mml:mo><mml:mspace></mml:mspace><mml:mi>s</mml:mi></mml:math>, showing the pulsar's continued spin-down. The pulse profiles show an asymmetric double-peaked structure with a phase separation of 0.47 between the two peaks. A cyclotron resonance scattering feature (CRSF) is also detected at <mml:math><mml:mo>∼</mml:mo><mml:mn>17.6</mml:mn><mml:mspace></mml:mspace><mml:mrow><mml:mi>keV</mml:mi></mml:mrow></mml:math>, along with its harmonic at <mml:math><mml:mo>∼</mml:mo><mml:mn>38</mml:mn><mml:mspace></mml:mspace><mml:mrow><mml:mi>keV</mml:mi></mml:mrow></mml:math>, persisting across all flux states. Flux-resolved spectroscopy reveals that the CRSF energy remains constant despite a 25-fold change in flux. The spectral parameters, like photon index and e-fold energy, are out of phase with the pulse shape, whereas the cutoff energy is in phase with the pulse shape. The source's luminosity during the on-state is <mml:math><mml:mn>2.85</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>35</mml:mn></mml:mrow></mml:msup><mml:mspace></mml:mspace><mml:mrow><mml:mi>erg</mml:mi><mml:mspace></mml:mspace><mml:msup><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>, consistent with a "pencil" beam radiation pattern expected at this flux level from a collisionless gas-mediated shock. These results offer further insights into the accretion dynamics and magnetic field geometry of this system.

The pulse profile of Gamma-ray bursts (GRBs) is often described as fast rise and exponential decay (FRED). However, the exponential decay feature can be decisively identified from power-law decay only if the flux decays several orders of magnitude. GRB 221009A, the brightest GRB ever observed, should be able to test the FRED feature. Using data from Fermi-GBM, we examined the decay pattern of the burst across different energy ranges. Power-law and exponential models were fitted to different segments of the decay phase, and the posterior distributions of the model parameters were estimated using Markov Chain Monte Carlo (MCMC) method. The power-law decay fits the light curve better than the exponential decay in both the precursor and the main burst. This implies that they may originate from the same physical mechanism, which may improve our understanding of the mechanisms of GRB emission and the dynamics of the central engine.

Aims. We aim to identify the physical mechanism responsible for the observed X-ray emission and polarization of Cyg X-1 in the soft state. Methods. We performed a detailed spectral analysis of X-ray data obtained with NICER, NuSTAR, and INTEGRAL during observations simultaneous with IXPE on 2023 June 20, supplemented at higher energies with archival CGRO data. We developed a new model, retBB, which describes thermal disk emission and its returning reflection, and applied it together with accurate models of Comptonization and relativistically broadened reflection. Using the resulting spectral solution, we computed the expected polarization signal and compared it with the IXPE measurements. Results. Our spectropolarimetric modeling shows that the observed polarization is in agreement with being produced by Comptonization in a corona undergoing a semi-relativistic outflow at a velocity of ≃0.3c. Our spectral solutions admit either low or high black-hole spin values, depending on the adopted model setup. However, the observed polarization strongly favors a low spin. At high spin, the polarization angle would inevitably rotate significantly across the energy band, which is inconsistent with the observations. Apart from this rotation of the polarization angle, general relativistic effects do not play a significant role in producing the observed polarization. In particular, we find that, at most, returning disk radiation contributes only a minor amount.

Context. The event PS16dtm, which occurred in the centre of the narrow-line Seyfert 1 (NLS1) galaxy SDSS J015804.75−005221.8 (z = 0.080440), is one of the few candidates for a tidal disruption event in an already-accreting active galactic nucleus. Aims. We wish to shed light on the character of the tidal disruption event in this source since it exhibits unusual peculiarities, such as the double-peak optical/UV light curve and a low blackbody temperature with a lack of X-ray emission. Methods. We performed a spectral analysis of the source before and during the event. We modelled the time evolution of the luminosity profile using a numerical code that describes the viscous evolution of the flow. Results. Based on the combined spectral and timing studies, we interpret the event as the disruption of a ∼0.3 M<SUB>⊙</SUB> main-sequence star or as the gradual partial disruption of a low-mass giant star. The star is likely on a circular orbit, embedded in the accretion disc. We discuss that the evolution of the star rather suggests that the orbit is counter-rotating. We observe the system at a sufficiently large viewing angle that the actual disruption process is not directly observed. The disrupted star and inner disc are shielded from the observer by a gaseous envelope. Further observations of the system as it returns to the previous NLS1 state, particularly in the X-ray band, are needed to confirm the proposed scenario and to place constraints on the return to a regular NLS1 state.

Recent studies have revealed a notable timing feature in several black hole X-ray binaries (BHXBs) during the soft-to-hard transition at the outburst decay. Within a narrow frequency range, the phase lags between high- and low-energy X-ray light curves exhibit a sudden increase, accompanied by a drop in the coherence function. These narrow features have been associated with a quasi-periodic oscillation (QPO) appearing only in the imaginary part of the cross spectrum (CS). This QPO remains undetected in the power density spectrum and is known as an imaginary QPO. Motivated by these results, we analysed five years of NICER observations of the BHXB AT2019wey during its low-hard state (LHS) and hard-intermediate state (HIMS). We find an imaginary QPO in the CS of AT2019wey, with similar characteristics as those found in other BHXBs, making AT2019wey the fifth BHXB in which such QPOs have been found. As the source hardens, the frequency of the imaginary QPO drops from ∼5 Hz to ∼1 Hz, while its phase lag rises from ∼0.3 rad to ∼0.7 rad during the HIMS and from ∼0.5 rad to ∼0.6 rad during the LHS. During the HIMS, the phase-lag energy spectrum of the imaginary QPO shows a typical U-shaped profile, while the shape changes in the LHS. The rms spectrum of the imaginary QPO rises below ∼2 keV, peaks at around ∼2 keV and decreases at higher energies, which may be associated with the presence of a relatively cool corona. We compared the properties of the imaginary QPO with those of the type-B and C QPOs in BHXBs and find a tentative connection to type-C QPOs. Combining the imaginary QPOs detected in AT2019wey with those reported in other sources, we find a systematic increase in QPO phase lags with QPO frequency. However, we cannot conclude whether the phase lags of imaginary QPOs exhibit the inclination dependence previously observed in type-C QPOs.

Context. Quasi-periodic pulsations (QPPs) have been observed in a broad electromagnetic spectrum, encompassing radio, ultraviolet, white light, X-rays, and γ-rays. Yet, flare-associated QPPs remain rarely detected in mid-infrared emission. Aims. We explored dual-period QPPs in the mid-infrared waveband, hard X-rays (HXRs), and microwave emission during an X1.5 flare on 2024 December 30 (SOL2024-12-30T04:01). Methods. The X1.5 flare was captured simultaneously by the Accurate Infrared Magnetic field Measurements of the Sun (AIMS), Fermi, the Hard X-ray Imager (HXI), the Nobeyama Radio Polarimeters (NoRP), the White-light Solar Telescope (WST), and the Atmospheric Imaging Assembly (AIA). Dual quasi-periods were identified using fast Fourier transform and wavelet analysis methods. Hard X-ray images were reconstructed with the HXI_CLEAN algorithm. Differential emission measure (DEM) describes the temperature structure of optically thin plasmas in the solar corona, and its solution was applied in six AIA Extreme ultraviolet wavelengths. Results. Flare QPPs with dual quasi-periods at ∼8.5 s and ∼4.6 s were simultaneously detected in the AIMS 8─10 μm, Fermi 26─ 50 keV, and HXI 20─50 keV wavebands during the impulsive phase. Imaging observations show that the flare emission sources in the mid-infrared, HXR, and white-light wavebands are spatially coincident. Mid-infrared emissions are primarily localized at the loop top and double footpoints, which are connected by a hot plasma loop. These observational features support intermittent and rapid energy release via oscillatory magnetic reconnection during the solar flare. Differential emission measure analysis confirms fast sausage waves in flaring loops, while the WST observation indicates a white-light flare. Conclusions. We localized the flare QPPs with double periods in mid-infrared, HXR, and microwave emissions during a white-light flare. The flare-associated QPPs may be attributed to a quasi-periodic regime of magnetic reconnection, with the double periods likely modulated by quasi-harmonics of sausage waves.

We analyzed NICER observations of the 2025 outburst of the black hole X-ray binary 4U 1630−47 to investigate the spectral─timing properties of its transient low-frequency quasi-periodic oscillations (QPOs) and millihertz-scale quasi-regular modulation (QRM). During the rising phase of the outburst, the QPO centroid frequency increased from ∼0.24 Hz to ∼3.43 Hz. Wavelet-based state separation shows that the with-QPO intervals are associated with a higher inner disk temperature and a lower diskbb normalization than the without-QPO intervals, while the photon index (Γ) shows weaker changes within the uncertainties. Near the outburst peak, the source displayed a weak QRM at ∼0.07 Hz with a fractional rms amplitude of ∼4.7%, lower than that of the heartbeat state observed in 2023. Phase-resolved Hilbert─Huang analysis shows that the inner disk temperature is positively correlated with the X-ray flux, the diskbb normalization is anticorrelated, and Γ varies only weakly. Overall, the short-timescale spectral─timing variability is expressed most clearly through the disk-related parameters. The transient QPOs are therefore consistent with short-timescale disk-related variability during the rising phase, whereas the millihertz-scale QRM may represent a weaker heartbeat-like variability mode appearing near the outburst peak.

Aims. We investigate the evolution of X-ray reverberation lags in the black hole X-ray binary Swift J1727.8─1613 during its 2023 outburst, with the aim of probing the inner accretion flow geometry across spectral states. Methods. We analyzed NICER observations covering the low-hard state (LHS) and hard-intermediate state (HIMS). The time lags were computed using Fourier-based techniques, and we constructed lag─frequency and lag─energy spectra. To obtain a robust estimate of the soft lag amplitude, we focused on a frequency range in which the reverberation signal dominates and remains stable, thereby minimizing contamination from hard lags and phase-wrapping effects. Results. The soft lag amplitude increases rapidly from the LHS to the early HIMS and then stays near ∼10 ms throughout the HIMS. In the frequency range in which reverberation lags prevail, the lag shows little dependence on Fourier frequency. On the other hand, the observed low-frequency lags change from hard-lag dominated to soft-lag dominated, with amplitudes comparable to those measured at higher frequencies. Conclusions. These results suggest that the reverberation lag varies little during the HIMS, consistent with a relatively stable inner accretion geometry during this state. The apparent evolution of the lag amplitude from the LHS to the HIMS can be largely explained by the diminishing effect of hard lags and does not necessarily require significant changes in the intrinsic light-travel timescale. Swift J1727.8─1613 therefore provides a case in which the reverberation signal can be studied with reduced contamination over a broad frequency range, offering new insight into the evolution of the accretion geometry in black hole X-ray binaries.

We report the discovery of a new X-ray transient, EP J175257.3─351923 (EP250916a), by the Einstein Probe (EP) near the galactic plane. The outburst lasted for at least 250 days and reached a peak 2─10 keV flux of ∼4 × 10<SUP>−10</SUP> erg cm<SUP>−2</SUP> s<SUP>−1</SUP>, exhibiting a fast-rise exponential-decay (FRED) profile typical of X-ray binary outbursts. The source remained in the hard state throughout the outburst, with only modest variations in the photon index (∼1.6 − 2.2) and no evidence of a spectral state transition. Broadband spectral modeling suggests a truncated disk, a weak reflection component, and a high-energy cutoff at ∼217 keV, consistent with hard-state accretion in black-hole systems. No reliable optical or radio counterpart has been detected within the Swift/XRT error circle. The inferred X-ray-to-optical and X-ray-to-radio flux ratios are consistent with a low-mass X-ray binary (LMXB) classification. Neither pulsations nor significant aperiodic variability were detected. Although the compact object cannot yet be firmly identified, the timing, spectral, and multiwavelength evidence favors the classification of EP J175257.3─351923 as a black-hole (BH) LMXB candidate, highlighting EP's potential to uncover a faint, previously hidden population of X-ray binaries.

We present FAST-MEPSA, an optimised version of the MEPSA algorithm developed to detect peaks in uniformly sampled time series affected by uncorrelated Gaussian noise. Although originally conceived for the analysis of gamma-ray burst (GRB) light curves (LCs), MEPSA can be readily applied to other transient phenomena. The algorithm scans the input data by applying a set of 39 predefined patterns across multiple timescales. While robust and effective, its computational cost becomes significant at large re-binning factors. To address this, FAST-MEPSA introduces a sparser offset-scanning strategy. In parallel, building on MEPSA's flexibility, we introduce a 40th pattern specifically designed to recover a class of elusive peaks that are typically sub-threshold and lie on the rising edge of broader structures—often missed by the original pattern set. Both versions of FAST-MEPSA — with 39 and 40 patterns — were validated on simulated GRB LCs. Compared to MEPSA, the new implementation achieves a speed-up of nearly a factor 400 at high re-binning factors, with only a minor (<mml:math><mml:mrow><mml:mo>∼</mml:mo><mml:mn>4</mml:mn><mml:mtext>%</mml:mtext></mml:mrow></mml:math>) reduction in the number of detected peaks. It retains the same detection efficiency while significantly lowering the false positive rate of low significance. The inclusion of the new pattern increases the recovery of previously undetected and sub-threshold peaks. These improvements make FAST-MEPSA an effective tool for large-scale analyses where a robust trade-off between speed, efficiency, and reliability is essential. The adoption of 40 patterns instead of the classical 39 is advisable when an enhanced efficiency in detecting faint events is desired. The code is made publicly available.

In the era of time-domain astronomy, automated and efficient data processing is a crucial factor in ensuring astronomical scientific output. With the international collaboration trend in large-scale sky survey projects, data processing algorithms developed through multi-institutional collaboration face challenges in system integration, scheduling capability, extensibility, and traceability. In this work, we employ containerized execution modules to resolve dependency issues in multi-institutional software integration, implement complex workflow orchestration through message queue control modules, including cyclic dependencies, dynamic parallel relationships, and resource/priority-based scheduling. The system also non-intrusively implements the International Virtual Observatory Alliance (IVOA) provenance data model through aspect injection mechanisms, ensuring traceability and reproducibility of data products. In Einstein Probe (EP) data processing, the system has processed over 20,000 general observations, 170,000 FXT observations, and 21,000 VHF and Beidou alerts, supported over 300 agile algorithm updates, and detected more than 17,000 sources and nearly 200 transient events. The system demonstrates excellent generality and portability, smoothly migrating from the single-module LEIA to the 12-module EP, and supporting data fusion for new payload FXT and multiple data transmission links. This workflow management system provides a flexible, extensible, and standardized data processing framework for future large-scale astronomical observation projects, effectively addressing key issues such as multi-institutional collaboration, complex workflow orchestration, and scientific data reproducibility.

The management of observing proposals is a critical operational component for modern astronomical facilities. As missions grow in complexity, the demand for efficient, fair, and adaptable proposal handling systems is increasingly pressing. Existing systems are often monolithic and tightly coupled to a specific observatory, lacking the flexibility to be easily adapted. This paper introduces AstroPropose, a novel, general-purpose framework for creating and managing astronomical observing proposal systems, derived from the architecture of the operational Einstein Probe Observing Proposal System (EOPS). AstroPropose is centered on a powerful visual workflow engine, enabling administrators to define and deploy entire proposal workflows through a graphical interface. Key features include a dynamic form builder, a configurable workflow engine, and a flexible role-based access control (RBAC) system. We present the complete architecture and data model. As validation, we detail how the framework's design principles are embodied in EOPS, which has successfully managed two annual proposal cycles and handles daily time-critical Target of Opportunity (ToO) submissions for the Einstein Probe mission. To further demonstrate the framework's generalizability, we present a second case study: its adaptation as the proposal management prototype for the Chinese Space Station Telescope (CSST), a multi-instrument survey mission with a dual-phase review process. AstroPropose, validated through the successful operation of EOPS and the rapid prototyping of the CSST system, is being prepared for an open-source release to the astronomical community.

Fermi, Swift, and other observation satellites provided a dataset of 323 Long Gamma-Ray Bursts (LGRBs) with known redshift and well-defined spectral parameters spanning from February 1997 to September 2023. Among these, 123 GRBs (Gamma-Ray Bursts) observed by Fermi-GBM were selected to calibrate the relationship between the isotropic energy $E_{\rm{iso}}$ and the peak energy in the cosmological rest frame $E_{\rm{p,i}}$, known as the Amati relation. The analysis revealed that the $E_{\rm{iso}}$ of five LGRBs (GRB 980425, GRB 061210, GRB 070714B, GRB 071227, and GRB 080123A) significantly deviated from the Amati relation, suggesting that the jet of them may be off-axis to the line of sight. Utilizing the well-established $E_{\rm{p,i}}-\varGamma$ and $E_{\rm{p,i}}-E_{\rm{iso}}$ correlations, we estimated the viewing angles $\theta _{\rm{obs}}^{'}$ for these five LGRBs. The results show, $\theta^{'}_{\rm{obs}}=(0.42^{+0.29}_{-0.14})^\circ$ for GRB 980425, $\theta^{'}_{\rm{obs}}=(0.27^{+0.21}_{-0.10})^\circ$ for GRB 061210, $\theta^{'}_{\rm{obs}}=(0.10^{+0.09}_{-0.02})^\circ$ for GRB 070714B, $\theta^{'}_{\rm{obs}}=(0.13^{+0.09}_{-0.05})^\circ$ for GRB 071227, and $\theta^{'}_{\rm{obs}}=(0.12^{+0.14}_{-0.03})^\circ$ for GRB 080123A. Notably, all five $\theta _{\rm{obs}}^{'}$ values are small, with the maximum angle being $0.42^\circ$ for GRB 980425, the nearest long burst observed ($z=0.0085$). The results suggest that the majority of Long GRBs are likely detected on-axis, with only a small fraction of nearby Long GRBs being observed slightly outside the jet edge.

%Z CHIME/FRB Collaboration.; Andersen, B. C.; Bandura, K. M.; et al. 2020, Nature, 587, 54 FRB Collaboration.; Abbott, T.; Andersen, B. C.; et al. 2026, arXiv, 2601.09399 Li, C. K.; Lin, L.; Xiong, S. L.; et al. 2021, NatAs, 5, 378 Li, Y.; Zhang, S. B.; Yang, Y. P.; et al. 2026, Science, 391, 280 Liang, Y. F.; Li, Y.; Tang, Z. F.; et al. 2025, ApJL, 994, 32 Wang, F. Y.; Zhang, G. Q.; Dai, Z. G.; et al. 2022, NatCo, 13, 4382 Xu, J.; Xu, H.; Guo, Y.; et al. 2025, arXiv, 2505.06006 Zhang, J. S.; Wang, T. C.; Wang, P.; et al. 2025, arXiv, 2507.14707

Using magnetohydrodynamic fields and guiding-center test-particle calculations, this study investigates how time-dependent loop-top dynamics modulate the adiabatic energization of electrons in a solar flare. Our results indicate that a time-varying loop-top structure enhances acceleration efficiency compared to a quasi-steady one. In the quasi-steady velocity field, the net acceleration is suppressed due to the decelerating effect of betatron cooling. Conversely, in the unsteady velocity field, the betatron mechanism readily accelerates electrons within the compressed magnetic field at the edge of the loop top. These findings suggest that the acceleration of electrons at the loop top is driven not only by the static shape of the magnetic structure but also by dynamic events such as plasmoid collisions. While previous studies have primarily focused on acceleration processes within the reconnection outflow, such as at termination shocks or within plasmoids, our research highlights the importance of the acceleration and deceleration processes at the exit point where electrons escape from the loop top.

The Einstein Probe (EP) has detected several fast X-ray transients (FXTs) associated with broad-lined Type Ic supernovae (SNe), including EP240414a and EP250108a. The observations reveal common features among these FXTs, but the corresponding physical origin remains debated. By comparing the FXTs with low-luminosity gamma-ray bursts (GRBs; e.g., GRB 171205A), we propose a unified model that explains the common features in these events. In this model, a rapidly spinning magnetar generates a collimated Poynting-flux-dominated jet and an isotropic wind. As the jet propagates through the stellar envelope, it generates a hot cocoon. In addition, a pulsar wind nebula (PWN) is formed during the interaction of the wind and the ejecta. As the surrounding cocoon gradually becomes transparent, the emission from the PWN escapes and is observed. This model provides a unified explanation for the observations: (1) early thermal emission originates from the cocoon, (2) midterm nonthermal emission comes from the PWN, (3) late-term emission originates from SNe driven by <SUP>56</SUP>Ni radioactive decay and magnetar, and (4) the X-ray afterglow originates from the structured jet. Our research thus provides a natural explanation for the observed thermal-to-nonthermal evolution in such FXTs and reveals their shared physical origin with some GRB-SNe.

The study of multifeature cosmological correlations in gamma-ray bursts (GRBs) holds significant importance for establishing them as effective cosmological probes. As GRB observational datasets continually grow, the limitations of traditional linear models in handling multidimensional, nonlinear, and heavily selection-biased data have become increasingly apparent. And the calculation of intrinsic features heavily depends on specific physical models, which introduces prior biases. Thus, this study avoids strong reliance on physical models by starting directly from raw observational data and employing machine learning to autonomously uncover the nonlinear patterns linking redshift to observational features. Given the complexity of GRBs, traditional network models suffer from issues such as underfitting, weak feature interaction capabilities, and unstable training. We thus propose a hybrid model integrating XGBoost with a deep cross network (DCN), leveraging XGBoost's strength in efficient nonlinear transformation and feature interaction capture alongside DCN's capacity for automatic high-order interaction modeling through a fusion correction strategy and optimization mechanism to enhance training stability. Using public Swift satellite GRB data, we construct an input space of diverse observational features and systematically evaluate model performance via K-fold cross validation. Experimental results show that the proposed hybrid model can capture nonlinear relationships between multiobservational GRB features and redshift, outperforming linear regression and single-network baselines under the current sample. Regression diagnostics, normality tests, and correlation analyses further indicate that the residuals are approximately normal and that the predicted redshifts exhibit a statistically significant positive correlation with true values. Overall, this study provides a data-driven pathway to circumvent the limitations of traditional methods in addressing model dependence.

The IceCube neutrino observatory detects the diffuse astrophysical neutrino background with high significance, but the contribution of different classes of sources is not established. Because of their nonthermal spectrum, gamma-ray bursts (GRBs) are prime particle acceleration sites and one of the candidate classes for significant neutrino production. Exhaustive searches, based on stacking analysis of GRBs, however, could not establish the link between neutrinos and GRBs. GRB 221009A had the highest time integrated gamma-ray flux of any detected GRB so far. The total fluence exceeds the sum of all Fermi Gamma-ray Burst Monitor─detected GRBs by a factor of 2. Because it happened relatively nearby, it is one of the most favorable events for neutrino production from GRBs, yet no neutrinos were detected. We calculate neutrino fluxes for this GRB in the TeV─PeV range using the most accurate time-resolved spectral data covering the brightest intervals. We place limits on the physical parameters (Lorentz factor, baryon loading, or emission radius) of the burst that are better by a factor of 2 compared to previous limits. The neutrino nondetection indicates a bulk Lorentz factor greater than 500 and possibly even 800, consistent with other observations.

The gravitational lensing of multimessenger signals from binary neutron star (BNS) mergers, including gravitational waves (GWs), short γ-ray bursts (sGRBs), kilonovae, and afterglows, can serve as a unique probe to constrain the mass of the graviton and cosmological parameters. In this paper, we estimate the detection rates of lensed electromagnetic counterparts associated with lensed BNS GW events detected by Cosmic Explorer and Einstein Telescope. For kilonovae and afterglows, we further consider a complementary pointed follow-up strategy targeting preidentified galaxy-scale lens candidates within the GW localization region. By utilizing both numerical and observational constraints on BNS mergers, we find the following: (1) Future γ-ray telescopes, even with a sensitivity more than 10 times better than that of the Fermi γ-ray Burst Monitor, may only detect lensed sGRB prompt emission at a rate ∼ 0.1 yr<SUP>−1</SUP>, corresponding to ∼2 × 10<SUP>−3</SUP> detectable lensed BNS GW events. (2) For the known-lens pointed strategy, the identifiable lensed-host fraction is approximately 0.15─0.30 for the fiducial deep-lens-catalog case considered, suggesting a possible gain in per-lens sensitivity for faint kilonovae and afterglows. (3) A Nancy Grace Roman Space Telescope─like near-infrared facility could detect lensed kilonovae at rates of approximately <inline-formula> <mml:math><mml:mo>∼</mml:mo><mml:mn>0.4</mml:mn><mml:msubsup><mml:mrow><mml:mn>5</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.34</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.81</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula>, <inline-formula> <mml:math><mml:mn>0.5</mml:mn><mml:msubsup><mml:mrow><mml:mn>5</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.41</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.98</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula>, and <inline-formula> <mml:math><mml:mn>0.07</mml:mn><mml:msubsup><mml:mrow><mml:mn>8</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.059</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.139</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula> yr<SUP>−1</SUP> in the F106, F158, and F213 bands, respectively. (4) Lensed afterglows remain difficult to detect in the optical and radio bands, while ATHENA-like X-ray observations may detect 0.5─5 events over 10 yr.

The physical trigger of core-collapse supernovae (CC-SNe) is a central theme in modern surveys of the transient Universe. A multimessenger approach may provide a unique window to CC-SNe produced by a newly born neutron star (NS) or black hole (BH). Here, we apply joint electromagnetic-gravitational wave (EM-GW) analysis to the nearby event SN 2023ixf using a model-independent broadband search with a GW-energy detection threshold of a few times 10<SUP>−5</SUP>M<SUB>⊙</SUB>c<SUP>2</SUP> for chirps and 10<SUP>−2</SUP>M<SUB>⊙</SUB>c<SUP>2</SUP> for noise-like emission below 1 kHz. No joint LIGO Hanford-Livingston (H1-L1) signal associated with SN 2023ixf is observed over a 2.5 day window proceeding the first discovery. Together with EM photometric-spectroscopic data, we interpret this null result to indicate the absence of BH formation, in part by mass scaling of the BH powering GW170817B following GW170817. This multimessenger outcome thus supports scenarios consistent with NS formation and a progenitor mass likely below ∼20M<SUB>⊙</SUB> , independently also estimated by EM interpretations.

We present a study aimed at quantifying the detectability of radio counterparts of binary neutron star (BNS) mergers with total masses ≲3 M<SUB>⊙</SUB>, which may form neutron star remnants. We focus on mergers localized by gravitational-wave (GW) observations to sky areas ≲10 deg<SUP>2</SUP>, a precision that greatly facilitates optical counterpart identification and enables radio discovery even without detections at other wavelengths. Widely separated GW detectors are essential for building samples of well-localized BNS mergers accessible to radio telescopes, with minimum yearly detection rates (assuming the smallest values of the BNS local merger rate) ranging from a few with current GW detectors to hundreds with next-generation GW instruments. Current GW networks limit well-localized detections to z ≲ 0.2, while next-generation GW detectors extend the reach to z ≲ 0.8, encompassing the median redshift of short gamma-ray bursts (GRBs). With next-generation radio arrays operating at several tens of GHz and providing an order of magnitude improvement in sensitivity compared to the most sensitive ones available today, short GRB-like jet afterglows can be detected for a large fraction of the considered BNS mergers. At lower radio frequencies, detections with current radio interferometric arrays are feasible, though subject to synchrotron self-absorption effects. The enhanced sensitivity and survey speed of future radio interferometers operating at a few GHz, combined with the higher detection rate of well-localized BNSs enabled by next-generation GW observatories, are key to probing disk-wind and dynamical ejecta afterglows, as well as remnant diversity.

Gamma-ray bursts (GRBs) are among the most energetic extragalactic transients, providing a unique laboratory to investigate the physical origin of the diverse morphologies observed in high-energy light curves and spectra. These morphologies, interpreted through synchrotron and synchrotron self-Compton (SSC) closure relations (CRs), may depend on the circumburst density profile, energy injection, and the dynamical evolution of the reverse shock in either the adiabatic or radiative regime. We derive SSC CRs produced in the reverse-shock region for both thick- and thin-shell cases, considering constant-density and stratified circumburst media. The analysis assumes two power-law (PL) distributions for the electron spectral index and includes scenarios with and without energy injection from the progenitor. We show that, depending on the physical parameters, SSC emission from the reverse shock can reproduce the initial steep-decay and plateau phases commonly observed in GRB afterglows. We then compare the predicted spectral and temporal indices with those derived from PL and broken power-law (BPL) fits reported in the Second Fermi-LAT Gamma-ray Burst Catalog (2FLGC). Our results indicate that PL (BPL) light-curve fits preferentially favor scenarios with (without) energy injection. In scenarios with energy injection, the data preferentially support a constant-density environment over a stellar-wind profile.

We investigate the self-organized criticality (SOC) signatures of the precursor, main emission, extended emission (EE), and whole emission phases for three-phase gamma-ray bursts (GRBs). These parameters include the conventional duration-based index T<SUB>100</SUB>, as well as the newly introduced parameters T<SUB>MVT</SUB> and spectral lags (τ<SUB>32</SUB>, τ<SUB>42</SUB>). All cumulative distributions show power-law scaling in each phase for both Type I and Type II GRBs. For the first time, we demonstrate that the EE phase exhibits the same SOC scaling as the other phases. Phase-to-phase ratios show that spectral lag ratios are correlated, while duration-based ratios are not, indicating a common origin for spectral lags. Statistical comparisons among the four indexes reveal no significant differences across phases or within each phase, except in the EE phase where T<SUB>MVT</SUB> is marginally different from the spectral lags. When compared with the three-dimensional expectation α = 2.0 (duration-based), T<SUB>MVT</SUB> and T<SUB>100</SUB> show no significant deviation, whereas spectral lags seem to follow the fluence scaling α ≍ 1.5 as predicted by the fractal-diffusive SOC model. Thus, the same magnetic reconnection driven avalanche likely operates throughout the entire GRB, and these new parameters provide new observational diagnostics of the avalanche process.

Tidal disruption events (TDEs) have traditionally been discovered in optical sky surveys through targeted searches of nuclear transients. However, it is expected that some TDEs will occur outside the galaxy nucleus, arising from wandering black holes (BHs) originating in galaxy mergers. Here, we present observations of TDE 2025abcr, the first optical TDE discovered in the outskirts of a host galaxy. The TDE was identified by a custom "off-nuclear" implementation of the machine learning classifier tdescore, which classifies new ZTF transients based on their lightcurves. Follow-up observations confirm that TDE 2025abcr is a TDE-H+He, occurring 9<inline-formula> <mml:math><mml:mover><mml:mrow><mml:mo>.</mml:mo></mml:mrow><mml:mrow><mml:mtext>″</mml:mtext></mml:mrow></mml:mover></mml:math> </inline-formula>5 (9.3 kpc projected distance) from the nucleus of a massive galaxy (M<SUB>⋆</SUB> = 10<SUP>11.18±0.03</SUP>M<SUB>⊙</SUB>) with a central BH mass of 10<SUP>8.82±0.65</SUP>M<SUB>⊙</SUB>. TDE 2025abcr itself was likely disrupted by a much lighter BH (10<SUP>6.09±0.53</SUP>M<SUB>⊙</SUB>, as estimated with peak luminosity scaling relations). The BH was either dynamically ejected from the nucleus or lies at the center of a very faint tidally stripped dwarf galaxy undergoing a minor merger. Late-time observations of TDE 2025abcr could confirm the origin of this apparent "wandering" BH. The rate of highly offset (≳3 kpc) TDEs can be constrained to &lt;10% of the nuclear TDE rate, but our discovery implies that many dozens of similar sources will be detected by the Vera C. Rubin Observatory each year with resolvable offsets.

The potential association between gravitational waves (GWs) and fast radio bursts offers a unique multimessenger probe for cosmology. In this paper, we develop a redshift-independent framework to constrain cosmological parameters using the luminosity distance─dispersion measure (DM) relation, accounting for realistic astrophysical uncertainties. We perform a comprehensive comparative analysis across different GW detector sensitivities and modeling assumptions. Specifically, we investigate the performance of the current LIGO-Virgo (LV) network (at z &lt; 0.2) versus the future Cosmic Explorer (CE). Our study further evaluates the impact of different DM distributions—specifically the corrected Macquart's probability density function (Zhuge et al. 2026) and the log-normal distribution—and explores the influence of including or excluding host galaxy DM contributions. Using realistic simulated observations, we find that while the current LV network lacks the precision to provide meaningful constraints, CE will enable high-precision cosmology. Even without spectroscopic redshifts, CE observations can effectively break parameter degeneracies and robustly constrain both cosmology and host galaxy parameters. These results highlight the necessity of next-generation detectors.

We present a broadband X-ray study of the transient hyperluminous X-ray source (HLX), 2SXPS J111416.1+481833, in the galaxy NGC 3583, using archival XMM-Newton, NuSTAR, and Chandra data, and long-term Swift X-ray Telescope monitoring. The source episodically enters the hyperluminous regime with X-ray luminosities of L<SUB>X</SUB> &gt; 10<SUP>41</SUP> erg s<SUP>−1</SUP> and drops by a factor &gt;45 from its peak into a deep low state. We detect a clear spectral cutoff at ∼5─6 keV in the broadband spectra, which are well modeled by a soft thermal component combined with optically thick thermal Comptonization or an inner advection-dominated disk. In the XMM-Newton spectra, we detect a statistically significant (≳3.9σ) absorption line centered at E<SUB>line</SUB> ≍ 1.97 ± 0.04 keV with a width of σ<SUB>line</SUB> ≍ 74 ± 40 eV. We primarily interpret the line as a candidate proton cyclotron resonance scattering feature, implying a local magnetic field strength of B ∼ 4 × 10<SUP>14</SUP> G. Alternative interpretations, such as an origin in an ionized outflow, were explored and found to be less likely. We do not detect coherent X-ray pulsations, placing 90% confidence upper limits on the pulsed fraction of 19.3% in the 0.3─10 keV band and 36.3% in the 3─15 keV band. The combination of extreme luminosity, a hard spectral state, and the detection of a candidate cyclotron line provides strong evidence for a highly magnetized neutron star accretor.

Tidal disruption events (TDEs) serve as direct probes of the population of supermassive black holes in the center of galaxies and are nowadays regularly detected in optical wide-field time-domain sky surveys. Recent studies have demonstrated that a large fraction of TDEs can be uniquely identified in the infrared (IR) waveband, but these studies have, to date, been limited to relatively nearby events. In this work, we searched for highly luminous IR-bright TDEs that are rare and thus missed by searches in the local Universe. We performed a systematic search of the NEOWISE archive and developed a new selection criterion based on the evolution of the W1 − W2 color to select TDE candidates. We identified 10 IR bright TDEs with peak luminosities above L<SUB>peak W2</SUB> ≃ 3 × 10<SUP>43</SUP> erg s<SUP>−1</SUP> and estimated an event rate of <inline-formula> <mml:math><mml:mn>1</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.5</mml:mn></mml:mrow></mml:msubsup><mml:mo>×</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>10</mml:mn></mml:mrow></mml:msup></mml:math> </inline-formula> Mpc<SUP>−3</SUP> yr<SUP>−1</SUP> for the luminosity range of our sample. Compared to the existing local luminosity function of lower-luminosity events, we detect a suppressed rate for these highly luminous events. This turnover in the luminosity function can be naturally explained by the suppressed amount of TDEs taking place in systems with larger black hole masses, thereby confirming the TDE nature of our sources.

We present a comprehensive Bayesian spectral analysis of the black hole X-ray binary 4U 1630−47 during its 2022 outburst, using simultaneous NICER and NuSTAR observations. Using the traditional frequentist approach, we build our model combining reflection spectroscopy with continuum-fitting techniques and analyze the data. In the Bayesian framework, we jointly constrain the black hole's spin, mass, and inclination within a unified framework. Employing nested sampling, we capture parameter degeneracies and rigorously propagate both statistical and systematic uncertainties. Our results yield robust evidence for a high spin, with the exact value still subject to model-dependent systematic uncertainties from both approaches. Our Bayesian analysis infers spin <inline-formula> <mml:math><mml:msup><mml:mrow><mml:mi>a</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msup><mml:mo>=</mml:mo><mml:mn>0.9</mml:mn><mml:msubsup><mml:mrow><mml:mn>6</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.03</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.02</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula>, mass <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mi>BH</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>12.1</mml:mn><mml:msubsup><mml:mrow><mml:mn>9</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.46</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>1.22</mml:mn></mml:mrow></mml:msubsup><mml:mspace></mml:mspace><mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow><mml:mrow><mml:mo>⊙</mml:mo></mml:mrow></mml:msub></mml:math> </inline-formula>, and inclination angle <inline-formula> <mml:math><mml:mi>i</mml:mi><mml:mo>=</mml:mo><mml:mn>55.7</mml:mn><mml:msubsup><mml:mrow><mml:mn>5</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.37</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>1.60</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula> degrees, obtained via the relxillNS version of our model. It also demonstrates the power of Bayesian inference in providing valuable insights into the complex physics of black hole accretion and enabling high-confidence measurements of fundamental parameters.

GRS 1915+105 is a black hole X-ray binary whose ρ-class ("heartbeat") oscillations (∼50─100 s) are attributed to radiation-pressure instabilities in the inner accretion disk at near-Eddington luminosities. We present a phase-resolved spectral and timing analysis of 24 Swift XRT observations (1─10 keV) and broadband AstroSat SXT+LAXPC data (0.8─30 keV), dividing each cycle into five phases. The narrowband XRT fits show an apparent anticorrelation between the inner disk temperature (T<SUB>in</SUB> ∼ 1.7─1.5 keV) and apparent radius (R<SUB>in</SUB> ∼ 22─38 km) across the cycle. The broadband AstroSat fits, however, are statistically consistent with a constant disk temperature: a joint fit with T<SUB>in</SUB> tied across all five phases gives T<SUB>in</SUB> = 1.275 ± 0.020 keV (<inline-formula> <mml:math><mml:msubsup><mml:mrow><mml:mi>χ</mml:mi></mml:mrow><mml:mrow><mml:mi>ν</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msubsup><mml:mo>=</mml:mo><mml:mn>1.003</mml:mn></mml:math> </inline-formula>; ∆χ<SUP>2</SUP> = +4.3 for four added constraints), whereas tying the disk normalization as well is rejected (∆χ<SUP>2</SUP> = +175.9), leaving a ∼20% variation in apparent R<SUB>in</SUB> (18.1 ± 0.7 to 21.9 ± 0.7 km). The coronal electron temperature rises from ∼6 to ∼14.5 keV approaching the burst, with the photon index tracking it. We attribute the larger XRT disk swings to its limited bandpass, where coronal Comptonization is unconstrained, and the disk parameters absorb coronal variability. The dominant variability is therefore coronal, consistent with S. V. Vadawale et al., and the residual R<SUB>in</SUB> change is plausibly a color-correction effect. Hardness─intensity and color─color diagrams show clear spectral hysteresis. Our broadband coverage provides a phase-resolved test of disk constancy and favors coronal evolution as the driver of the spectral variability across the ρ cycle.

Fast radio bursts (FRBs) and gamma-ray bursts (GRBs) are both linked to compact-object activity, yet their possible connection remains unclear. Here we perform a systematic search for spatial and temporal associations between FRBs in the second CHIME/FRB catalog and Swift GRBs. Instead of using the positional ellipses reported in the catalog, the full CHIME localization probability maps are adopted for spatial cross-matching. This yields 130 candidate pairs and increases the number of spatially consistent matches by a factor of several. Applying a distance-consistency criterion based on dispersion-measure-inferred FRB redshifts and GRB distances inferred via the Amati relation reduces the sample to 37 pairs, including 26 GRB-preceding-FRB candidates (24 LGRB─FRB and 2 SGRB─FRB). Monte Carlo simulations show that the overall excess of associations is not statistically significant, and the distribution of matches across localization confidence levels is consistent with random expectations. These pairs are therefore not claimed as secure associations, but are used to constrain a possible subdominant FRB─GRB connection. These results place constraints on any FRB─GRB connection and highlight the need for improved localization and larger samples.

Recent observations have revealed significant soft X-ray polarization from several quiescent magnetars, including the intriguing 90° polarization angle (PA) swing as a function of photon energy for some sources. We thus present a general semianalytical framework for calculating energy-dependent soft X-ray polarization signatures from magnetars, consistently incorporating both quantum electrodynamics (QED) vacuum resonance in the atmosphere and resonant Compton scattering (RCS) in the magnetosphere. Starting from the polarized radiative transfer equation for RCS and treating vacuum-resonance-induced mode conversion as an input, we employ a first-order approximation in RCS optical depth to evaluate the effects of magnetospheric plasma density (which depends on magnetic twist), drift velocity, temperature, and viewing geometry on the observed radiation. Our analysis reveals that magnetic twist and plasma drift velocity are the critical parameters controlling the impact of RCS on both the absolute polarization degree and its variation across the soft X-ray spectrum. We find that sufficiently strong RCS can wash out the PA swing caused by vacuum resonance. Furthermore, in addition to the QED vacuum resonance effect, significant relativistic signatures arising from plasma drift velocity (β<SUB>0</SUB> ≳ 0.5) may introduce an extra 90° PA swing in the spectrum. Our calculation framework, based on the single-scattering approximation, bypasses the need for complex, multidimensional Monte Carlo simulations, providing an analytical pathway for modeling full-surface emission and rotational-phase-resolved radiation from magnetic neutron stars in support of current and future X-ray polarization missions.

We present detailed radio observations of the mid-infrared (MIR) tidal disruption event (TDE) WTP14adeqka. We detect rising radio emission starting ≍4 yr after the discovery of the MIR emission (and about 2 yr after its peak), peaking at ≍6.5 yr and declining thereafter, reminiscent of the delayed radio emission recently identified in optically discovered TDEs. The peak radio luminosity, νL<SUB>ν</SUB> ≍ 2 × 10<SUP>39</SUP> erg s<SUP>−1</SUP>, is comparable to the brightest radio emission in optical TDEs. Multifrequency radio observations at 8.9 and 9.7 yr reveal a nonrelativistic outflow with a mean expansion velocity of ≍0.022c (for an assumed launch at the time of disruption) and an energy of ≍10<SUP>49.8</SUP> erg, about an order of magnitude larger than in typical optical TDEs. More importantly, Very Long Baseline Array (VLBA) observations at the same epochs directly resolve the radio source and reveal an increase in the major axis of a slightly elliptical Gaussian from 0.11 to 0.13 pc (with no apparent astrometric shift), corresponding to an expansion velocity of ≍0.049c, and a potentially delayed launch by about 2 yr. The VLBA size measurements rule out an off-axis jet launched at the time of disruption, which would be ≳pc on these timescales; the possibility of a delayed jet can be evaluated with future VLBA observations. We conclude that MIR TDEs can launch energetic, nonrelativistic outflows. Ongoing radio observations of the full MIR TDE sample will reveal whether this behavior is ubiquitous.

TDE2025aarm is a nearby tidal disruption event whose early radio and X-ray emissions are exceptionally faint compared with previously observed tidal disruption events (TDEs). We examine whether these weak signals can be explained within standard outflow and disk-emission scenarios. The radio detection at 15 GHz with ∼10<SUP>36</SUP> erg s<SUP>−1</SUP> around 40 days after discovery is inconsistent with synchrotron emission from a quasi-spherical disk wind for reasonable circumnuclear densities and outflow velocities. Instead, the low luminosity and inferred self-absorbed spectrum imply a narrowly collimated outflow with a solid angle ≲0.1 sr, naturally identified with the unbound stellar debris. The X-ray emission is likewise unusually faint, with L<SUB>X</SUB> ∼ 10<SUP>39−40</SUP> erg s<SUP>−1</SUP> during the first few months. If interpreted as thermal emission from an obscured accretion disk, the inferred emitting area would correspond to an implausibly small X-ray-transparent region expected to vary on short dynamical timescales that are not observed. Alternatively, the same shock responsible for the radio emission can accelerate relativistic electrons that produce X-rays through synchrotron radiation and/or inverse-Compton scattering of optical/UV photons. Both mechanisms can explain the early faint X-ray emission, although their temporal evolution differs. Continued radio and X-ray monitoring of TDE2025aarm will provide a sensitive probe of the unbound debris, circumnuclear medium, and high-energy emission mechanisms in optical TDEs.

We analyze data from 142 of the 218 gravitational-wave (GW) sources in the fourth LIGO─Virgo─KAGRA Collaboration (LVK) Gravitational-Wave Transient Catalog (GWTC-4.0) to estimate the Hubble constant H<SUB>0</SUB> jointly with the population properties of merging compact binaries. We measure the luminosity distance and redshifted masses of GW sources directly; in contrast, we infer GW source redshifts statistically through (i) location of features in the compact object mass spectrum and merger rate evolution, and (ii) identifying potential host galaxies in the GW localization volume. Probing the relationship between source luminosity distances and redshifts obtained in this way yields constraints on cosmological parameters. We also constrain parameterized deviations from general relativity that affect GW propagation, specifically those modifying the dependence of a GW signal on the source luminosity distance. Assuming our fiducial model for the source-frame mass distribution and using GW candidates detected up to the end of the fourth observing run (O4a), together with the GLADE+ all-sky galaxy catalog, we estimate <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>H</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>75</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>9.1</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>12.8</mml:mn></mml:mrow></mml:msubsup><mml:mspace></mml:mspace><mml:mo>(</mml:mo><mml:mn>75</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>13.3</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>24.5</mml:mn></mml:mrow></mml:msubsup><mml:mo>)</mml:mo><mml:mspace></mml:mspace><mml:msup><mml:mrow><mml:mspace></mml:mspace><mml:mtext>km s</mml:mtext><mml:mspace></mml:mspace></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup><mml:mspace></mml:mspace><mml:msup><mml:mrow><mml:mspace></mml:mspace><mml:mtext>Mpc</mml:mtext><mml:mspace></mml:mspace></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> </inline-formula>. This value is reported as a median with 68.3% (90%) symmetric credible interval, and includes combination with the H<SUB>0</SUB> measurement from GW170817 and its electromagnetic counterpart. Using a parameterization of modified GW propagation in terms of the magnitude parameter Ξ<SUB>0</SUB>, we estimate <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>Ξ</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>1</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.4</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>1.0</mml:mn></mml:mrow></mml:msubsup><mml:mspace></mml:mspace><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.6</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>2.8</mml:mn></mml:mrow></mml:msubsup><mml:mo>)</mml:mo></mml:math> </inline-formula>, where Ξ<SUB>0</SUB> = 1 recovers the behavior of general relativity.

We revisit the mass of the neutron star in the ultracompact binary 4U 1820─30 in light of a recently reported transient absorption feature at about 3.8 keV, interpreted as a gravitationally redshifted, highly ionized iron line and implying <mml:math><mml:mrow><mml:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mi>z</mml:mi><mml:mo>≃</mml:mo><mml:mn>1.72</mml:mn></mml:mrow></mml:math>, a very high stellar compactness. We examine whether the mass-radius locus implied by this interpretation can be made compatible with external EoS-informed benchmarks and timing-based estimates. We map the compactness implied by the redshift onto the mass-radius plane, including rotational effects, and compare the resulting region with an EoS-informed 95% reference contour derived from NICER data of several neutron-star systems. While we do not perform a statistically self-consistent joint mass-radius inference for 4U 1820─30, we present a quantitative conditional consistency test in a common <mml:math><mml:mrow><mml:mi>M</mml:mi><mml:mo>−</mml:mo><mml:mi>R</mml:mi></mml:mrow></mml:math> framework, comparing published inputs under explicitly stated assumptions. We find that, if the maximum neutron-star mass is restricted to low values ( ≤ 2.3 M<SUB>⊙</SUB>), the redshift-implied locus shows at most marginal overlap with the EoS-informed contour, indicating substantial tension. Allowing a higher maximum mass enlarges the parameter space and can restore compatibility with that benchmark. However, such high masses remain in tension with previous touchdown-flux estimates, although they are not necessarily excluded by interpretations based on the highest detected quasi-periodic oscillation frequency. We conclude that the redshift interpretation of the 3.8 keV feature, the touchdown-flux estimates, and the QPO/ISCO interpretation do not naturally select the same mass-radius sector for this source under these assumptions. Reconciling them requires auxiliary assumptions with high leverage on the inferred compactness.

We investigate the dynamics of transonic advective accretion flows around spinning black holes in the presence of viscosity. The spacetime of a Kerr black hole is approximated using a pseudo-potential. We study viscously driven shock oscillations over a range of black hole spin parameters. Our results show that the frequency range of quasi-periodic oscillations (QPOs) obtained from the power density spectra depends strongly on the black hole spin. Low-spin systems predominantly exhibit low-frequency QPOs, whereas rapidly rotating black holes ( ≳ 0.9) produce QPOs spanning a broad range from low to high frequencies, comparable to those observed in black hole X-ray binaries. We further obtain a correlation between the QPO frequency and the power-law photon index (Γ-ν<SUB>QPO</SUB>) by computing the spectrum for a 10M<SUB>⊙</SUB> black hole.

We present a broadband spectral analysis of the neutron star LMXB GRS 1747-312 using ∼ 40 ks Astrosat data. The source was observed during the decay phase of the 2017 outburst, with an absorbed 1.0─5.5 keV flux of <mml:math><mml:mrow><mml:mn>1</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>67</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.07</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.04</mml:mn></mml:mrow></mml:msubsup><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>11</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> erg s<mml:math><mml:msup><mml:mrow></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> cm<mml:math><mml:msup><mml:mrow></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:math>, corresponding to a luminosity of <mml:math><mml:mrow><mml:mo>∼</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:mn>0.9</mml:mn><mml:mo>−</mml:mo><mml:mn>1.80</mml:mn><mml:mo>)</mml:mo></mml:mrow><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>35</mml:mn></mml:msup></mml:mrow></mml:math> erg s<mml:math><mml:msup><mml:mrow></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math>. The continuum is modeled with thermal Comptonization of blackbody emission and interstellar absorption. A mildly broad iron line at ∼ 6.4 keV is fitted with a disc reflection component. Narrow lines below 2 keV are described by a hot plasma using the xspec model apec. Additionally, there is a potential detection of an emission line at <mml:math><mml:mrow><mml:mn>4</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>19</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.10</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.12</mml:mn></mml:mrow></mml:msubsup></mml:mrow></mml:math> keV with width <mml:math><mml:mrow><mml:mi>σ</mml:mi><mml:mo>=</mml:mo><mml:mn>0.2</mml:mn><mml:mo>±</mml:mo><mml:mn>0.2</mml:mn><mml:mspace></mml:mspace><mml:mtext>keV</mml:mtext></mml:mrow></mml:math> and line flux = <mml:math><mml:mrow><mml:msubsup><mml:mn>13</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>9</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>10</mml:mn></mml:mrow></mml:msubsup><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> erg s<mml:math><mml:msup><mml:mrow></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> cm<mml:math><mml:msup><mml:mrow></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:math>. Examination of several short duration ( ∼ few kiloseconds) Swift observations at few times the AstroSat source flux, provided upper limits to the line flux <mml:math><mml:mrow><mml:mo>&lt;</mml:mo><mml:mn>30</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> erg s<mml:math><mml:msup><mml:mrow></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> cm<mml:math><mml:msup><mml:mrow></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:math>. The 4.2 keV line likely originates from reflection off the neutron star surface. Shifting the neutral Fe K<SUB>α</SUB> line from its rest energy of 6.4 to 4.2 keV requires a redshift of z ∼ 0.6, consistent with that expected from the surface of a non-spinning 1.4M<SUB>⊙</SUB>, 10 km radius neutron star. If confirmed, this feature provides a potential direct measurement of gravitational redshift, allowing us to place strong constraints on the neutron star's mass-to-radius ratio and gain valuable insights into the equation of state (EOS) of dense-matter.

We conduct timing and spectral analysis of the accretion-powered Be/X-ray binary, XTE J1829-098, by using three Nuclear Spectroscopic Telescope Array (NuSTAR) observations from 2018 and 2024. X-ray pulsations at ∼ 7.8 s were detected during all three observations, and the pulse profile exhibited an asymmetric sinusoidal shape that is consistent with earlier observations. The 3─40 keV continuum is well described with a cutoff power-law model, along with an Fe K<SUB>α</SUB> line at 6.4 keV and cyclotron resonant scattering feature (CRSF). The luminosity of the source during these observations lies in the range of <mml:math><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>5</mml:mn><mml:mo>−</mml:mo><mml:mn>10</mml:mn><mml:mo>)</mml:mo></mml:mrow><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mn>35</mml:mn></mml:msup><mml:mspace></mml:mspace><mml:mrow><mml:mi>erg</mml:mi><mml:mspace></mml:mspace><mml:msup><mml:mi>s</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:mrow></mml:math>. Our analysis reveals that the structure of the fundamental CRSF is complex and cannot be properly accounted for by adding a single absorption component to the continuum model. This complicated CRSF structure can be modeled either with a double CRSF at ∼ 14 keV and ∼ 17.4 keV or with a CRSF at ∼ 14 keV accompanied by a red wing. Both models provide comparable fits and are difficult to distinguish. We discuss the possible physical scenarios supporting the applicability of each model.

Swift J1727.8─1613 is a newly discovered transient low-mass X-ray binary harbouring a stellar-mass (<inline-formula><tex-math>$\sim 10\,\mathrm{ M}_\odot$</tex-math></inline-formula>) black hole. We present state-resolved VLT/X-Shooter optical spectroscopy of its 2023 outburst, sampling the luminous hard-to-soft and late soft-to-hard transitions. During the onset of the brightest radio flare, He II flux rises relative to adjacent epochs, with reduced peak-to-peak separation and full-width-half-maximum, consistent with enhanced irradiation shifting line emissivity to larger radii. We detect no contemporaneous change in the line base tracing the inner disc. The most dramatic change occurs at the onset of the dim-hard state, when strong, broad (higher-order) Balmer lines appear in absorption, and He II remains in emission, but becomes highly asymmetric. While the hardening of the X-ray spectrum likely promotes the reappearance of an underlying disc photosphere, the kinematic alignment between the Balmer absorption (<inline-formula><tex-math>$v_w\sim -750\, \mathrm{km\, s^{-1}}$</tex-math></inline-formula>) and the suppressed blue peak of He II suggests a unified origin in a massive, cool (<inline-formula><tex-math>$T\lesssim 10^{4}\, \mathrm{K}$</tex-math></inline-formula>) accretion disc wind. Radiative transfer simulations demonstrate that such asymmetric He II profiles are naturally produced in a rotating and accelerating outflow. Using the Sobolev approximation, we estimate the wind mass-loss rate to be <inline-formula><tex-math>$\dot{M}_w\gtrsim 10^{-9}\, \mathrm{ M}_\odot \, \mathrm{yr^{-1}}$</tex-math></inline-formula>, comparable to the instantaneous accretion rate and a significant fraction of the secular mass-transfer rate from the donor. If persistent at quiescent-level X-ray luminosities, this outflow could strongly impact the system's secular evolution.

We investigate late-time gamma-ray burst (GRB) afterglows produced by quasi-spherical outflows propagating into a stratified circumburst medium during the deep-Newtonian phase. Subrelativistic ejecta generated in compact binary mergers or core-collapse explosions naturally develop velocity structures, while additional energy injection from a long-lived central engine, through spin-down luminosity and/or fall-back accretion, can substantially modify the afterglow evolution. We develop an analytical framework for synchrotron emission from decelerated ejecta components undergoing energy injection in a stratified environment. The model provides multiwavelength light curves and corresponding closure relations for the deep-Newtonian regime. We apply this framework to the late-time multiwavelength observations of GRB 171205A. In addition, we constrain the physical properties of quasi-spherical outflows using observations of short GRBs associated with kilonova candidates, together with long-term radio upper limits obtained years after the burst in a broader GRB sample. Our results show that late-time observations can place meaningful constraints on the dynamics, energetics, and energy-injection history of subrelativistic quasi-spherical outflows from GRB progenitors.

We present the detection and detailed analysis of type-I X-ray bursts from 4U 1636─536 observed using X-ray SPECtroscopy and Timing (XSPECT) onboard X-ray Spectro-Polarimetry Satellite (XPoSat). Type-I X-ray bursts are the result of unstable thermonuclear burning of accreting matter on the neutron star (NS) surface. A total of nine type-I X-ray bursts are detected during the observations, out of which six bursts are a part of short wait time bursts or doublets with a consistent recurrence time of <inline-formula><tex-math>$\sim$</tex-math></inline-formula>7 min. The source was observed in the low flux state and remained in the same state throughout the XSPECT observations. We study the spectral properties of the system during the persistent emission as well as during the detected type-I bursts by means of time-averaged and time-resolved spectroscopy. The persistent emission spectra in the source could be well modelled using a Comptonized multicolour disc model with inner disc temperature, <inline-formula><tex-math>$kT_{\mathrm{ in}} \sim 0.37$</tex-math></inline-formula> keV, and an electron temperature, <inline-formula><tex-math>$kT_\mathrm{ e} \sim$</tex-math></inline-formula>2.17 keV. The nature of persistent emission between the doublets (<inline-formula><tex-math>$kT_{\mathrm{ in}} \sim 0.6$</tex-math></inline-formula> keV and <inline-formula><tex-math>$kT_\mathrm{ e} \sim 2$</tex-math></inline-formula> keV) is different compared to the overall persistent emission before the bursts. Time-averaged burst spectra are fitted using a blackbody model with temperature ranging between 1.3 and 1.8 keV. We report a clear distinction for the second burst in the doublets in terms of the almost constant recurrence time and decay time for the detected doublets. This observation is valid irrespective of the peak count rate during the burst. The consistently short decay time for these bursts could indicate a different fuel ignition compared to the singlet bursts observed in the source.

We present a detailed X-ray and radio study of the Be/X-ray binary pulsar 1A 1118-61 during its 2026 giant outburst using NuSTAR, Swift/XRT, and MeerKAT observations. Timing analysis reveals coherent pulsations with spin periods of ~408.77 s and ~408.30 s at luminosities of ~2.44 × 10<SUP>37</SUP> erg s<SUP>-1</SUP> and ~9.7 × 10<SUP>35</SUP> erg s<SUP>-1</SUP>, respectively. The pulse profiles show a clear dependence on both energy and luminosity. At higher luminosity, the profile is smooth and single-peaked, whereas at lower luminosity, an absorption-like feature appears, indicating changes in the accretion geometry. A quasi-periodic oscillation (QPO) at 0.106 Hz is detected during the NuSTAR observation at higher luminosity. Broadband spectral analysis using NuSTAR data reveals a cyclotron resonance scattering feature (CRSF) at ~53.9 keV during the high-luminosity observation, corresponding to a neutron star magnetic field strength of ~6.0 × 10<SUP>12</SUP> G. However, no CRSF is detected in the spectrum at the lower luminosity. An iron Kα line at 6.4 keV is evident in both NuSTAR observations, while an additional weak higher-energy iron emission feature at ~6.9 keV is detected in the high-luminosity NuSTAR observation. The Swift/XRT spectra in the 1-10 keV energy range are well described using an absorbed power law and a blackbody component. Radio observation with MeerKAT during the decay phase of the X-ray outburst results in a non-detection, providing a stringent upper limit on the radio luminosity. This indicates weak or absent radio emission from 1A 1118-61 at the time of observation.

The study of transient events, such as Gamma-Ray Bursts (GRBs) and extreme Supernovae (SNe), provides a unique probe of star-forming environments in the early Universe. Traditional global host galaxy studies often fail to capture the local conditions around the progenitor, leading to potential mischaracterizations due to metallicity gradients and spatial inhomogeneities. Using high spatial resolution integral field unit (IFU) data, such as those obtained with SHARP/VESPER, allows us to spatially resolve the host galaxies and accurately determine the physical properties (metallicity, stellar age, kinematics) at the precise explosion site. Here, we review the need for near-infrared spatially-resolved observations to link progenitor properties, such as the required low metallicity for long GRBs, to their birth environments, and highlight the crucial role of instruments like SHARP in advancing high-redshift transient science, including the characterization of SNe without associated GRBs and Kilonovae hosts.

The observational determination of the lower limit of neutron star masses is crucial for the physics of core-collapse supernovae. In this light, SMC X-1 is an important object because of its estimated pulsar mass lying near or potentially below the theoretical lower limit. SMC X-1 exhibits a double-peaked optical orbital light curve due to the tidal distortion of the donor star, and analysis of this allows us to constrain the binary parameters. In this study, we analyzed optical and X-ray light curves of SMC X-1 obtained by Transiting Exoplanet Survey Satellite and Monitor of All-sky X-ray Image. We found the systematic variations in the optical orbital light curves synchronized with the X-ray super-orbital modulation, regarding the following two aspects: the minimum at inferior conjunction and the double-peak asymmetry. To explain this behavior, we developed a modified ellipsoidal modulation model in which the precessing accretion disk changes the geometry of X-ray irradiation on the donor and that of optical irradiation on the disk. As a result, this model succeeded in reproducing the observed optical and X-ray light curves. Furthermore, we discovered that intense X-ray irradiation could cause the optical emission center to shift away from the gravitational center, potentially leading to an underestimation of the radial velocity of the donor by approximately 20%. Correcting for this effect yields an updated pulsar mass estimation of about <inline-formula><tex-math>$1.35\, M_{\odot}$</tex-math></inline-formula>.

GRB 230307A is one of the brightest long-duration gamma-ray bursts (GRBs) ever detected, yet its progenitor remains uncertain due to the variety of plausible astrophysical scenarios. In this work, we investigate four possible progenitors for GRB 230307A: a binary neutron star (BNS), a neutron-star/white-dwarf (NS-WD) system, a neutron-star/black-hole merger, and a tidal disruption event involving a white dwarf and a supermassive black hole. Additionally, we explore three distinct central engine models powering the kilonova associated with the BNS: radioactive decay of <inline-formula><mml:math><mml:mi>r</mml:mi></mml:math></inline-formula>-process nuclei in a two-component ejecta model, a magnetar-driven model including magnetic dipole spin down, and a combined model of magnetar spin down with <inline-formula><mml:math><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Ni</mml:mi></mml:mrow><mml:mrow><mml:mn>56</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math></inline-formula> radioactive decay. We perform Bayesian multiwavelength light curve analyses using physically motivated models and priors and evaluate model performance through Bayes factors and leave-one-out cross-validation scores. Our results show a statistical preference for a BNS or NS-WD progenitor producing a kilonova powered by a magnetar and <inline-formula><mml:math><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Ni</mml:mi></mml:mrow><mml:mrow><mml:mn>56</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math></inline-formula> decay, characterized by a <inline-formula><mml:math><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Ni</mml:mi></mml:mrow><mml:mrow><mml:mn>56</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math></inline-formula> mass of <inline-formula><mml:math><mml:mo>∼</mml:mo><mml:mn>4</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>4</mml:mn></mml:mrow></mml:msup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula> and an ejecta mass of <inline-formula><mml:math><mml:mn>0.06</mml:mn><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula>. Furthermore, under the assumption of a BNS origin within this model, we infer binary component masses of <inline-formula><mml:math><mml:msub><mml:mi>m</mml:mi><mml:mn>1</mml:mn></mml:msub><mml:mo>=</mml:mo><mml:mn>1.3</mml:mn><mml:msubsup><mml:mn>2</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>0.06</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.05</mml:mn></mml:mrow></mml:msubsup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula> and <inline-formula><mml:math><mml:msub><mml:mi>m</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo>=</mml:mo><mml:mn>1.2</mml:mn><mml:msubsup><mml:mn>4</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>0.04</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.02</mml:mn></mml:mrow></mml:msubsup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula>, with a dimensionless tidal deformability of <inline-formula><mml:math><mml:mover><mml:mi>Λ</mml:mi><mml:mo>∼</mml:mo></mml:mover><mml:mo>=</mml:mo><mml:mn>43</mml:mn><mml:msubsup><mml:mn>7</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>265</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>251</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula>. From the component mass posteriors, we infer that the observed offset can be explained by a natal kick as long as the systemic velocity is nearly aligned with the prekick orbital motion. In this case, the required kick velocity (comoving frame) and binary separation range within <inline-formula><mml:math><mml:msubsup><mml:mi>v</mml:mi><mml:mi>k</mml:mi><mml:mo>'</mml:mo></mml:msubsup><mml:mo>∼</mml:mo><mml:mn>100</mml:mn><mml:mi>─</mml:mi><mml:mrow><mml:mn>150</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>km</mml:mi><mml:mo>/</mml:mo><mml:mi>s</mml:mi></mml:mrow></mml:math></inline-formula> and <inline-formula><mml:math><mml:msub><mml:mi>a</mml:mi><mml:mn>0</mml:mn></mml:msub><mml:mo>∼</mml:mo><mml:mn>2</mml:mn><mml:mi>─</mml:mi><mml:mn>2.5</mml:mn><mml:msub><mml:mi>R</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:math></inline-formula>, respectively.

Bayesian inference, widely used in gravitational wave parameter estimation, depends on the choice of priors, i.e., on our previously existing knowledge. However, to investigate neutron star mergers, priors are often chosen in an agnostic way, leaving valuable information from nuclear physics and independent observations of neutron stars unused. In this work, we propose to encode information on neutron star physics into physics-informed prior distributions constructed with normalizing flows. These priors take input from constraints on the nuclear equation of state and neutron star mass distributions. Applied to GW170817, GW190425, and GW230529, we highlight two contributions of the framework. First, we demonstrate its ability to provide source classification and to enable model selection of equation-of-state constraints for loud signals such as GW170817, directly from the gravitational wave data. Second, we obtain narrower constraints on the source properties through these informed priors. As a result, these physics-informed priors consistently recover higher luminosity distances compared to agnostic priors. Our method provides a scalable way for classifying future ambiguous low-mass mergers observed through gravitational waves and for informing single-event gravitational wave data analysis with neutron star physics.

Relativistic magnetized shocks are natural sources of coherent radiation, representing a promising framework for fast radio bursts (FRBs). This Letter explores how the radiation reaction (RR) effect, triggered by high-energy photon emissions during shock radiation, significantly alters particle dynamics and coherent radiation properties. Using kinetic particle simulations, we demonstrate that RR severely suppresses electron energies from shock acceleration, resulting in multiple coherent gyration cycles at the shock front. It amplifies the intensity of coherent radiation and boosts energy efficiency by several fold, as compared to the single gyration cycle in the standard model of relativistic magnetized shocks. We further find that the coherent radiation spectrum from RR-mediated shocks is characterized by upshift peak frequency, broaden bandwidth, and narrow spectral peak. These RR-induced radiation changes may be related to several observed FRB phenomena, including the statistically positive correlation between luminosity and bandwidth in repeating and one-off FRBs, the narrow spectra seen in some FRB events, and the bimodal energy distribution reported in FRB 20121102A.

We revisit the physical origin of the achromatic peaks and breaks observed several thousand seconds after the burst in the multi-wavelength afterglows of XRF 080330 and GRB 080710. Using a numerical afterglow model that consistently incorporates finite ejecta thickness and a generalized external density profile, we perform Bayesian inference to estimate model parameters describing these events. Our analysis shows that the gradual rise and achromatic temporal features in both events are more naturally explained by jet dynamical evolution with finite shell thickness rather than by off-axis viewing effects. The inferred initial radial width of the ejecta is of order 10<SUP>13</SUP> cm for both bursts, implying a central engine activity timescale significantly longer than that suggested by the prompt gamma-ray duration alone. Taken together, these results demonstrate that early afterglow light curves are strongly influenced by transition dynamics when finite ejecta thickness is properly taken into account, thereby providing a physical link between the prompt and afterglow phases and highlighting limitations of simply applying the thin-shell approximation when interpreting early-time afterglows. Furthermore, Bayesian model comparison strongly favors a generalized circumburst density profile over the canonical uniform or steady-wind models, suggesting that fixing the external density structure to idealized profiles a priori may obscure crucial information about the progenitor's pre-burst activity.

The CUbesat Solar Polarimeter (CUSP) mission aims to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed at developing new CubeSat missions. We present the outcomes of the CUSP's Phase B study, which is ended on 2 July 2026. The design solutions adopted for the mission's most critical multi-physics design drivers will be discussed, these solutions have been formulated and applied to demonstrate compliance with system requirements at both the spacecraft and platform levels. Moreover, we will discuss the validation of the Payload model based on the environmental testing campaign (e.g., vibration) carried out on a demonstrator.

Submillimeter (sub-mm) follow-up of binary neutron star (BNS) mergers provides unique constraints on the early-time energetics and environments of relativistic outflows, capturing the spectral evolution at epochs where centimeter-band emission is often still optically thick or yet to peak. However, the practical scientific yield depends on instrument-specific thresholds, the observing cadence, and the distinct temporal contributions from isotropic ejecta versus beamed relativistic jets. With the upcoming Xue-shan-mu-chang 15-meter SubMillimeter Telescope (XSMT), facility-specific forecasts are needed to test for sustained engine energy injection, as expected for a long-lived magnetar remnant rather than a promptly formed black hole. We present a unified numerical framework that couples engine-driven ejecta dynamics to non-thermal synchrotron emission, accounting for synchrotron self-absorption and deep-Newtonian effects. Adopting fixed 5$σ$ (1 h) point-source thresholds of 1.5/2.9/10.2 mJy at 230/345/460 GHz, we construct parameter-space detectability maps and estimate event rates based on current BNS merger-rate priors. For a fiducial local event at 40 Mpc, we find that a magnetar-boosted ejecta afterglow peaks on timescales of weeks to months and remains detectable long enough to allow delayed follow-up, with an expected all-sky rate of $\dot N_{\rm ej} \approx 0.05$--1.7 yr$^{-1}$ at 230 GHz for $f_{\rm mag}=1$; this rate is an upper limit and scales linearly with the long-lived magnetar fraction. Conversely, while relativistic jets produce intense early-time signals, their detection is constrained by narrow beaming and fleeting visibility. Our framework provides a quantitative basis for prioritizing gravitational-wave triggers and maximizing the scientific yield of XSMT in the multi-messenger era.

Quasi-periodic eruptions (QPEs) are rapid, recurring soft X-ray bursts, whose nature is still in dispute. A compelling case of QPEs has emerged in the slowly evolving optical transient AT2019wzc, possibly associated with the tidal disruption of a post-main-sequence star by a supermassive black hole. Further evidence of a tidal disruption event (TDE) is crucial to understand the nature of AT2019wzc and establish the link between TDE and QPEs. Here we report the detection of a narrow, blueshifted N VI absorption line in its high-resolution X-ray spectra obtained by XMM-Newton, but weak or undetectable absorption lines from other elements of similar ionization states such as carbon and oxygen. The absorption line features can be reproduced by an ionized gas with ionization parameter $\log ξ\sim 0.3\ {\rm erg~cm~s^{-1}}$ and column density $N_{\rm H}\sim 10^{20}\ {\rm cm^{-2}}$, under the condition of a nitrogen abundance of $11.6_{-7.8}^{+19.6}$ times the solar value. This abnormal nitrogen abundance favors a TDE origin for AT2019wzc, and the absorbing gas may originate from the outflow induced by self-collision of the TDE's debris stream.

We investigate the influence of the global structure of accreting millisecond X-ray pulsars (AMXPs) on accretion-induced spin-up, using three equations of state (EoS) models representing neutron stars, quark stars, and strangeon stars. By applying the classical accretion torque formalism, and deriving the accretion rate and magnetic field from observations of three AMXPs --- XTE J1751-305, SAX J1808.4-3658, and IGR J00291+5934 --- we derive mass contours in the plane of luminosity versus spin-up rate. Our results show that the inferred masses are broadly consistent across the three EoS models, indicating that the method is insensitive to the specific EoS and can serve as an evolutionary channel for constraining the masses of pulsar-like compact stars. Notably, we find that SAX J1808.4-3658 and IGR J00291+5934 are constrained to very low masses, while XTE J1751-305 yields a mass consistent with the typical range for pulsars. Our analysis suggests that future improvements in distance and spin-up measurements would refine these mass constraints, which could offer crucial evidence to distinguish different EoS models. This paper also presents the idea that using EoS-specific parameters could yield new insights.

Cross-spectral methods have become essential for studying accretion physics in X-ray binaries and active galactic nuclei, where coherence and phase lag measurements constrain physical models and reveal variability components invisible in power spectra alone. Recent multi-Lorentzian fitting techniques have uncovered new quasi-periodic features through joint analysis of power spectra and cross-spectra, but testing these methods requires synthetic data with realistic statistical properties. We present an algorithm for generating pairs of time series with arbitrary power spectra, coherence functions, and phase lag profiles. The method extends existing methods by decomposing the dependent time series into coherent and incoherent components, where the coherent part is constructed through a complex transfer function applied to a reference series. We derive the transfer function and normalization required to preserve target spectral shapes while achieving specified cross-spectral properties. We obtain approximate analytic expressions for the variance of coherence and phase lag estimators and construct the approximate covariance matrix relating these quantities to the underlying power and cross-spectra. When fitting models jointly to power and cross-spectra, these correlations must be incorporated into the likelihood. We apply the method to a two-Lorentzian model with component-specific phase lags and demonstrate close agreement between input models and generated power spectra, coherence function, and phase lag profile across four decades in frequency.

The emission mechanism and host galaxy preference of optical/UV tidal disruption events (TDEs) are still not entirely understood. We present observations of the TDE AT 2022csn, which is one of the most distant (d_L~726 Mpc) and luminous (L_peak=2.487^(+0.073)_(-0.067)*10^(44) erg/s) optical/UV TDEs observed to date. Although it is a spectroscopically normal H+He TDE, it shows some photometric peculiarities, exhibiting a pronounced double-peaked light curve (with peaks separated by 18.30 pm 2.84 days in the g-band), and lying in the low-temperature and large-radius end of the optical/UV TDE population. The host galaxy of AT 2022csn shows evidence for a significant starburst within the last ~Gyr consistent with other optical/UV TDEs, but also narrow emission lines that place it within the Type II AGN region of the BPT diagram. Interaction between the TDE and a pre-existing AGN accretion disk might explain the peculiar photometric properties. However, it is puzzling that a TDE would be visible in a Type II AGN, where according to the AGN unification picture the central region around the supermassive black hole is obscured. We suggest a few scenarios to reconcile this. AT 2022csn together with AT 2019ahk, which shows similar properties, may belong to a new subset of low-temperature, high-radius TDEs in galaxies with Type II AGN emission features.

Recently, the Large High Altitude Air Shower Observatory (LHAASO) detected ultrahigh-energy (UHE; photon energy E>100TeV) $γ$-ray emission toward SS 433, the microquasar embedded in the W50 nebula, making it a promising Galactic PeVatron candidate. We reanalyze the archival XMM-Newton observations covering the bipolar jets and the thermal X-ray shell north of SS 433, and derive spatially resolved profiles of the nonthermal X-ray intensity and photon index along both jets. The jet emission softens with distance from the source, implying a correspondingly evolving electron population. In particular, a hard electron component appears close to the jet bases, which can account for the UHE emission from SS 433 via inverse Compton radiation if the magnetic field remains approximately uniform along the jets. The result, however, is highly sensitive to the magnetic field profile. For flux-conserving configurations in which the field decreases as the jet expands, the stronger field required in the inner regions may reduce the number of X-ray-emitting electrons and suppress their inverse Compton emission. Furthermore, electron transport calculations show that injection only at the jet bases cannot reproduce the observed intensity and spectral evolution, particularly the downstream re-brightening features, indicating additional particle injection and/or re-acceleration within the jets.

Fast radio bursts (FRBs) are intense millisecond-duration radio transients of extragalactic origin whose physical nature remains under active investigation, and which also serve as probes of the intergalactic medium. We report on the first FRB search campaign carried out at the Argentine Institute of Radio Astronomy (IAR) between December 2024 and March 2026, targeting nearby galaxy superclusters in the southern sky. We observed fields in the Ophiuchus, Shapley, and Sculptor/Phoenix supercluster regions with one of the two 30~m antennas of the IAR, using a ROACH-based backend with 400~MHz of bandwidth centred at 1400~MHz and a time resolution of 41--82~$μ$s, for a total net observing time of 212~h. The data were searched for dispersed single pulses with \texttt{PRESTO} in the dispersion measure range $100 \leq \mathrm{DM} \leq 500$, and candidates were classified with the FETCH machine learning classifier. The pipeline was validated on archival Parkes data containing known FRBs and on synthetic bursts injected into IAR observations. One FRB candidate, FRB~20251018, was identified on 18 October 2025 in an observation pointed towards the galaxy cluster A2870, in the Phoenix supercluster, with a dispersion measure of $243$, a signal-to-noise ratio of 8.2, and a FETCH probability of $p=0.99$. To the best of our knowledge, this would be the first FRB detected from South America. A set of more marginal candidates is also presented. These results demonstrate the capability of the IAR antennas to detect FRBs and support the continuation and extension of the monitoring campaign, including coincident dual-antenna observations and cross-matches with gravitational-wave events and electromagnetic transients.

We present the results of our intensive VLBI campaign on the black-hole low-mass X-ray binary (LMXB) Swift J1727.8-1613 during its 2023-2024 outburst. We observed the repeated quenching and re establishment of the highly-extended continuous jet during several transitions between hard-intermediate and soft-intermediate states, and the repeated ejection of transient jets. Using time-dependent visibility model fitting, we tracked the motion of nine discrete jet knots, obtaining some of the most precise measurements of transient jet proper motions and ejection dates in an LMXB. These ejecta were only detectable for a short time with VLBI, and some showed rapid intra-observation flux density variability that was not captured in image reconstructions. For the first time, we use time-dependent visibility modelling to fit a piecewise model for the jet knot flux densities, allowing us to create complex, non-parametric light curves of their intra-observation variability. We observed the launching of multiple ejecta across several state transitions, however, we could not identify a consistent signature of jet ejection in the available X-ray intensity or hardness data. We constrained the intrinsic speeds and bulk Lorentz factors of the jet knots, finding that Swift J1727.8-1613 launched both mildly relativistic and highly relativistic ejecta throughout its outburst. We used their proper motions to constrain a posterior distribution for the maximum inclination angle of the jet axis, which had 50th, 84th, and 99th percentiles of 40°, 50°, and 66°, respectively. These unique observations of the repeated ejection of transient jets by a single LMXB reveal that fixed parameters such as black-hole mass, black-hole spin, and spin-orbit misalignment do not uniquely determine the varying properties of transient jets, particularly their speeds and Lorentz factors.

Type I X-ray bursts are powered by unstable thermonuclear burning on the surface of accreting neutron stars in close binary systems. These brief X-ray flashes, with light curves featuring rise times of $1-10$ s, durations of $10-100$ s, and recurrence periods of hours to days, represent the most frequent stellar explosions in our Galaxy. With typical energies of $\sim 10^{39}-10^{40}$ erg, they rank among the most powerful astrophysical transients after supernovae and classical novae. To date, roughly 120 bursting X-ray binaries have been identified in the Milky Way. Several studies have been conducted to characterize the dynamics of these events, with emphasis on reproducing the observed recurrence periods and light curve shapes. In this paper we show, for the first time, that rotation is a key factor shaping the properties of Type I X-ray bursts in rapidly spinning systems. The inclusion of centrifugal forces, together with a suite of rotationally-induced mixing mechanisms, such as meridional circulation and shear-induced turbulent diffusion, reduce surface gravity, shortening the recurrence times and lowering burst energies. Rotation also modifies the extent of the nuclear activity during these events and affects the morphology of their light curves, which are distinctly broader for rapidly rotating neutron stars.

The nature of low-frequency quasi-periodic oscillations (QPOs) in black hole X-ray binaries remains unclear, and their relationship with the accompanying broadband noise (BBN) is still under debate. Here, we propose an improved variational mode decomposition (VMD) technique. Compared with the original algorithm that requires iterative, case-by-case parameter tuning, the new algorithm automatically and consistently determines the relevant VMD parameters based on the QPO central frequency and width measured from the power spectral density (PSD). This enables a more robust phase determination for QPOs and can also be applied to the study of BBN. We found that, for low-frequency type-C QPOs without significant harmonics in the black hole X-ray binary GX 339-4, the spectral properties of the QPOs and BBN are statistically consistent with each other: (1) the photon index is positively correlated with count rate as a function of phase, and (2) the PSD ratio spectra across different energy bands show no statistically significant QPO-like structures near the QPO frequencies, indicating that both components share the same energy dependence. These suggest that QPOs and BBN may be driven by the same physical processes. The QPO models based on geometric modulation struggle to account for the results, while those invoking corona oscillations are favored.

Recent X-ray and soft gamma-ray spectro-polarimetric observations of black hole X-ray binaries, such as Cygnus X-1, reveal discrepancies that challenge current models of accretion and high-energy emission. We present a general framework for broadband spectro-polarimetric modeling that simultaneously fits spectral and polarization data from independent instruments across different energy bands. The method robustly combines measurements with different energy binning and statistical significance while accounting for energy-dependent polarization mixing between model components. Applied to Cygnus X-1 over the 2 keV-2 MeV range, it provides an improved characterization of the soft gamma-ray hard-tail and constrains the optically thin synchrotron cutoff energy to $(3.9^{+0.6}_{-0.5}) \times 10^{2}$ keV. We discuss the implications for non-thermal electron acceleration in the context of Bohm diffusion and synchrotron cooling, and propose a new explanation for the observed polarization misalignment based on magnetic field helicity and Doppler boosting. The framework is broadly applicable to multi-instrument spectro-polarimetric analyses.

Nuclear transients are powerful probes of supermassive black hole properties, offering insight into black hole mass, accretion physics, and the structure of galactic nuclei. Among these, a growing class of events cannot be classified as either tidal disruption events (TDEs) or active galactic nuclei (AGN) flares, and their physical origins remain poorly understood. We present a multi-wavelength photometric and spectroscopic analysis of AT2021yky (ZTF21abzciqh), an ambiguous nuclear transient (ANT) at a redshift of $z = 0.076$. AT2021yky reached a peak bolometric luminosity of $L_{\rm peak} = (4.1 \pm 1.1) \times 10^{43}~\mathrm{erg~s^{-1}}$, with a rise-time of $18.2 \pm 0.7$ days. The early-time UV/optical emission is well described by a blackbody with a temperature of $T \simeq 1.4 \times 10^{4}$ K, cooler than most optically selected TDEs. No X-ray emission from the transient is detected, with a $3σ$ limit of $L_X \lesssim 3.4 \times 10^{41}$ erg s$^{-1}$ near peak. Spectroscopic observations reveal a largely featureless blue continuum with broad (FWHM$\sim 11,000$ km s$^{-1}$) H$α$ emission line that appears around 20$-$40 days post-peak. The host-galaxy emission-line ratios indicate the presence of an AGN, though the absence of optical or mid-IR variability and a non-AGN mid-IR color suggest it is weak. AT2021yky exhibits a rapid rise time comparable to that of luminous fast blue optical transients (LFBOTs), while its decay timescale and late-time broad H$α$ emission resemble those observed in TDEs. However, its cooler blackbody temperature and the absence of He II and Balmer emission lines other than H$α$ instead favour its classification as an ANT.

The discovery of delayed GeV emission from the extragalactic magnetar giant flare (MGF) GRB 200415A, located in the nearby Sculptor galaxy, revealed for the first time that these rare transients can launch relativistic outflows that power high-energy afterglows. Motivated by the recent identification of additional nearby MGF candidates in the archival data of the Fermi Gamma-ray Burst Monitor, we conduct a search for GeV counterparts with the Fermi Large Area Telescope (LAT). We analysed post-trigger time intervals taken in the range $10^{2}$--$10^{4}$\,s using a maximum-likelihood approach and performed a stacking analysis of all candidates with LAT coverage. In addition, we searched for photon triplets through \chng{a waiting time} analysis to identify events potentially associated with MGFs. We recover the known delayed signal from GRB\,200415A but find no GeV emission from the remaining six candidates. For three events, the earliest emission is unconstrained because the $10^{2}$\,s interval contains zero exposure after standard selections. For the events with LAT coverage, we obtain upper limits at 95\% confidence level of order $F_E \sim 10^{-9}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$ for the individual events, and a stacked population-averaged limit of $\approx2\times10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. We interpret these upper limits within the relativistic fireball framework, where the prompt spectral peaks favor a baryonic-poor regime ($η> η_*$). For the candidates with hard prompt spectrum and early LAT coverage, the baryonic-poor condition restricts the mass of relativistic ejecta to $M_b \lesssim 1\text{--}4 \times 10^{22}$\,g; the LAT upper limits confirm that the predicted GeV afterglow from such clean outflows falls below current instrumental sensitivity.

Gamma-ray bursts (GRBs) are promising cosmological probes, but their use as standard candles is limited by the circularity problem, necessitating model-independent calibration of GRB luminosity correlations. We revisit the two-dimensional (2D) and three-dimensional (3D) Dainotti correlations using Bayesian Neural Networks (BNNs) trained on the updated Observational Hubble Data (OHD) and Pantheon+ Type Ia Supernova sample. The reconstructed luminosity distances are used to calibrate the Platinum and Narendra et al. GRB samples. We constrain the parameters of the 2D Dainotti relation and the 3D fundamental plane, and examine the impact of calibration datasets and GRB sample selection. Calibration achieved using Pantheon+ yields tighter constraints than OHD, while the 3D correlation exhibits lower intrinsic scatter than the 2D relation. Our results demonstrate that BNNs provide a robust framework for model independent calibration of GRB luminosity correlations with reliable uncertainty propagation. Further, the underlying distance probe is a key factor in model-independent calibration, determining both the size of the GRB samples and the precision of the resulting constraints.

Thermonuclear X-ray bursts from the surfaces of neutron stars affect the surrounding accretion flow, revealing details of the underlying physical processes influencing accretion physics. In this context, we perform a spectral analysis of 5 long X-ray bursts that exhibit temporary rapid flux variations during the tails of their light curves. In all cases, spectra extracted from before, during and after the time of fluctuations show evidence for relativistic, ionized reflection with the blackbody from the neutron star often hidden from view. Both the properties of the reflecting region and the observed fraction of the blackbody vary as the fluctuations start and stop. These results are compared to a sample of 5 similar bursts without fluctuations in their light curves, and we find that spectra from 4 of the bursts in the control sample are best described by a simple absorbed blackbody. The bursts with fluctuations are longer and more energetic than those without fluctuations, in agreement with the prediction that radiatively-driven warps are impacting the accretion disks of the bursts with fluctuations. This result would also imply that these accretion disks must have large viscosity parameters. The lack of reflection in bursts from the control sample may result from lower accretion rates, as these disks would have smaller surface densities and would expand and become Compton-thin due to heating from the burst. High-throughput, time-resolved spectral analysis of X-ray bursts that undergo warping would give insight into the strength of accretion disk viscosity.

Solar flare hard X-ray imaging is a key diagnostic of flare energy release and electron acceleration. The ASO-S Hard X-ray Imager (HXI) employs 91 bi-grid sub-collimators, compressing the two-dimensional source distribution into a 91-dimensional counts vector---an inherently underdetermined inverse problem. The conventional CLEAN algorithm relies on a point-source prior and manual parameter tuning, while existing deep-learning methods (HXI-DLA) learn data-driven mappings without guaranteeing consistency with the forward physical equation. This paper introduces a physics-constrained deep learning framework whose core innovation is a counts mean--shape decoupling theory (DC--AC decomposition) derived from modulation imaging principles: the counts mean is proportional to total source energy and the normalized counts shape is determined by source position and scale, yielding two independently enforceable physical constraints. Based on this theory, HXI-PINN embeds the forward equation into both the network architecture---via ReLU non-negativity and counts-mean rescaling enforcing zero-error energy closure---and the optimization objective, where counts-domain constraints dominate the loss. Unlike data-driven approaches, HXI-PINN replaces heuristic regularization with executable hard constraints, ensuring every reconstruction satisfies the governing physics. Experiments on simulated Gaussian sources, soft X-ray morphologies, and a real HXI flare event confirm that the framework generalizes across source configurations, with advantages over CLEAN on ring-shaped sources and over HXI-DLA on complex morphologies. This work demonstrates that ``physical constraints + deep prior'' is an effective paradigm for underdetermined inversion---constraints anchor the solution in the feasible subspace satisfying the forward equation, while the deep prior selects the optimal solution within it.

We found two highly scattered Fast Radio Bursts (FRBs) during commissioning of the Commensal Realtime ASKAP Fast Transient COherent (CRACO) backend. FRB 240210D and FRB 240312D have scattering times of $34\pm6$ and $300\pm48$ ms, respectively, when scaled to 1 GHz. FRB 240312D originates near a spiral arm of a face-on galaxy at a redshift of only 0.05. Scintillation from a Milky Way screen constrains the distance of the scattering screen to $\sim 10$ pc from the source. FRB 240312D is therefore the first highly scattered FRB where scattering screens in the host galaxy centre, a background galaxy, or intervening structures can all be excluded, leaving only the circumsource medium. Integral field spectroscopy of the host reveals a Milky Way-like galaxy with a star-formation region at the FRB position. We find refractive scattering in a pulsar wind nebula as the most likely scattering origin. However, the explanation is not completely satisfactory as it requires a fine-tuned orientation. Hence, additional theoretical studies under different FRB progenitor models are needed. From the two FRBs, we calculate a total rate of $R_\mathrm{tot}=210^{+460}_{-180}\,\mathrm{events}\,\mathrm{sky}^{-1}\mathrm{day}^{-1}$ with durations between 55.2 ms and 1 s and above a fluence of 9 Jy ms consistent with the rate of shorter FRBs. This elevated rate suggests that the strong scattering seen in other FRBs likewise does not arise from chance-aligned sightlines, but is instead causally linked to the FRB sources.

An asymmetric relativistic outflow from a magnetar giant flare produces a step-like gravitational-wave (GW) memory. In the magnetized pair plasma surrounding the star, such GW memory drives transverse electromagnetic (EM) X-mode/fast-magnetosonic responses at the corresponding frequencies. We use this GW-memory spectrum to drive a global wave equation for a cold electron--positron pair plasma. With radiative boundary conditions excluding incoming waves, the driven response forms an outgoing EM mode that reaches the outer boundary of the source region. For central-engine energy-release timescales of $10$, $1$, and $0.15\,μ\mathrm{s}$, 90\% of the integrated response energy lies below 23.3 kHz, 233 kHz, and 1.51 MHz, respectively. The principal response occupies the 10 kHz--1 MHz frequency band; a substantial extension into the 1--3 MHz frequency band also appears when a submicrosecond asymmetric-energy component is present. The electromagnetic response can escape from the source region and potentially propagate in the interstellar medium.

The final stages of outbursts in transient X-ray pulsars (XRPs), which are characterised by a significant decline in the mass accretion rate, provide valuable insight into the physics of the accretion disc and its interaction with the strong magnetic field of the neutron star (NS). In particular, the `propeller effect', or centrifugal inhibition of accretion, has been proposed as a key mechanism governing both the onset luminosity and the timescale of the rapid transition to the quiescent state. In addition, it offers an independent method for estimating the magnetic field strength of the NS. On the other hand, the decrease in the mass accretion rate itself is driven by processes occurring in the accretion flow at larger distances from the NS. Recovering the information encoded in the light curve therefore requires sensitive high-cadence X-ray monitoring capable of capturing the rapid and often unpredictable transition from the accreting regime to the quiescent regime. In this study, we present the results of the first comprehensive monitoring campaign that tracks the entire transition to quiescence in the transient XRP 4U 0115+63 utilising observations by the NICER X-ray telescope. We show that the observed behaviour can be explained by the thermal-viscous disc instability model (DIM), with the emission observed immediately after an outburst possibly arising from the ongoing accretion from the recombined (`cold') disc and the subsequent quiescent emission being produced by the cooling NS. We further applied this model to a larger sample of XRPs encompassing a broad range of physical parameters. Ultimately, our findings indicate that the temporal behaviour of XRPs, including the quiescent state, can be consistently explained within the DIM framework without requiring the propeller effect as the primary mechanism governing the observed transition.

We analyze the prompt emission of the exceptionally bright GRB 220426A observed by Fermi/GBM, whose time-resolved spectra are among the narrowest measured in any GRB. Here we show that observations during the first $\sim 5$s are consistent with the signal being emitted while the jet was still in the initial radiation-dominated acceleration phase. The time-resolved spectra allow the effective launch radius, $r_0$, to be inferred with unusual precision. We find $r_0 \sim \mathrm{few} \times 10^{10}\,\mathrm{cm}$, increasing linearly with time. These findings match the theoretical predictions of the recollimation shock, suggesting that this GRB shows the first clear evidence for the existence and evolution of a recollimation shock. Using this interpretation, the linear increase observed is sustained over a period longer than expected from a pure jet breakout. Therefore, the jet collimation must have persisted even after breakout. We suggest that the collimation was maintained by a finite, dense, wind-like circumburst medium, which would reproduce the observed behavior of the prompt emission. We conclude that late-stage progenitor mass-loss can shape the earliest prompt emission and that the photospheric emission provides a way to probe both the jet collimation and the innermost region of the circumburst medium (CBM) surrounding the progenitor, independently of the constraints from interacting supernovae.

We present an analysis of ATLAS22kjn (AT 2022fpx), whose high-ionisation coronal lines (CLs) and pre-peak light curve bump provide distinctive opportunities to investigate the physical mechanisms powering tidal disruption events (TDEs). In addition to CLs, the optical spectra show common TDE features, including a strong, blue continuum and broad Balmer and He II lines. The CLs appear before UV/optical light curve peak, preceding the detection of X-rays by $\sim 300$ days and persisting after X-rays are no longer detected, suggesting the X-ray emission is obscured at both early and late times. Using the CL luminosities, we constrain the temperature evolution of the ionising source, finding a decrease of $\lesssim 10 \%$ over 500 days. In the UV/optical light curve, we observe a $9 \substack{+4 \\ -2}$ day bump that peaks $125 \substack{+5 \\ -3}$ rest-frame days before the peak of the main flare. Although we cannot definitively determine the origins of the bump, we find that its timescale and luminosity are most consistent with theoretical predictions for a precursor feature produced by a stream-stream collision or a wind-stream collision. ATLAS22kjn also shows a prominent dust echo in its mid-infrared (MIR) light curves, indicating a high dust covering fraction $f_c \simeq 0.40 \pm 0.03$, similar to the covering fractions of other CL-emitting TDEs. From the multi-wavelength observations of ATLAS22kjn, we estimate the size and relative radii of the emission regions in its nuclear environment and determine that the CL region lies between the broad line region and the MIR-emitting dust. ATLAS22kjn demonstrates the importance of multi-wavelength and early-time observations, and the utility of CLEs in characterising the otherwise unobservable EUV/ultrasoft X-ray emission of TDEs.

On 2025 August 18, the LIGO-Virgo-KAGRA collaboration reported a sub-threshold gravitational-wave (GW) candidate, S250818k, consistent with a binary neutron star (NS) merger potentially involving a sub-solar-mass compact object. Follow-up electromagnetic (EM) observations identified a Type IIb supernova, SN 2025ulz within the broad localization area of the GW signal. This potential link between a sub-solar GW event candidate and a Type IIb SN, while not confirmed given the low statistical significance of S250818k, has nonetheless sparked renewed interest in the "superkilonova" scenario, where sub-solar-mass NSs form through processes like the fragmentation of an accretion disk or core fission in a collapsing star. In this picture, the in-spiral and merger of a sub-solar NS-NS binary is followed by the merger of the NS-NS remnant with the central black hole (BH), producing chirp-like GW signals. For sub-solar NSs with masses in the (0.1-1)M_sun range and BH masses in the so-called lower mass gap range of ~(3-5) M_sun, these signals can persist in the 20 - 1024 Hz frequency band of ground-based GW detectors for (10^2-10^3)s, potentially offering an opportunity to probe the superkilonova scenario, as well as the lower mass gap between NSs and stellar-mass BHs. However, the complexity of the underlying astrophysics may yield waveforms that deviate from standard templates, limiting the use of matched filtering in real GW searches. We therefore explore the detectability of such signals using the Cross-Correlation Algorithm (CoCoA), a more robust though less sensitive cross-correlation method. We discuss general strategies for implementing CoCoA superkilonova searches via either targeted follow-up of candidate chirps identified in matched-filter searches, or EM-triggered searches of stripped-envelope core-collapse SNe.

We revisit Rossi X-ray Timing Explorer (RXTE) observations of GX~339$-$4 during the rising phase of its 2006/2007 outburst and apply a joint power-density-spectrum (PDS)--cross-spectrum (CS) decomposition to the type-C quasi-periodic oscillation (QPO) region. Within this framework, the QPO region is described by a narrow QPO fundamental and a neighboring high-frequency shoulder, whose amplitudes and phase lags can be measured separately. The shoulder is first detected at MJD~54142.04, mainly through the imaginary part of the CS and a narrow local structure in the phase-lag spectrum, before becoming a resolved high-frequency shoulder in the PDS. It follows the QPO frequency evolution on the high-frequency side, with $R_ν=ν_{\rm sh}/ν_{\rm QPO}\simeq1.04$--$1.18$. The QPO lag remains small, typically below $\sim0.17$~rad, whereas the shoulder carries a larger hard lag of $\sim0.5$--$0.8$~rad. Energy-resolved fits show the same separation: the QPO lag is close to zero or only weakly positive across most of the energy band, while the shoulder lag is systematically larger and generally increases with photon energy. The two components have broadly similar rms--energy shapes, although their relative strengths evolve during the observed sequence. Although the shoulder remains broad, with $Q\sim2$--$4$, its lag and rms--energy behavior resemble those of the type-B QPO detected shortly after our observations. This similarity raises the interesting possibility that the shoulder is related to an earlier, broader stage of the variability later seen as the type-B QPO.

We present complex energy-resolved properties of quasi-periodic oscillations (QPOs) in the black hole X-ray binary GRS 1915+105 using an observation from the LAXPC instrument onboard AstroSat. Power density spectra (PDSs) are constructed in multiple energy bands and modeled with multi-Lorentzian components to investigate the energy dependence of QPO properties. The QPO frequency shows a modest increase with energy. Dynamic PDS analysis does not reveal clear evidence for time-dependent evolution of the QPO frequency, suggesting that the observed frequency shift is not primarily driven by temporal variability. We perform simultaneous fitting of energy-resolved PDSs and find that a model in which the QPO feature is described by two Lorentzian components provides a better fit. The two components exhibit different evolution in fractional root mean square amplitude as a function of energy. We further examine the phase-lag properties by simultaneously modeling the PDS and the real and imaginary parts of the cross-spectrum and find distinct phase-lag behavior for the two components. Overall, these results indicate that the apparent energy-dependent evolution of the QPO feature may be a result of the presence of more than one variability component.

We present a first look at the Galactic population of heretofore known HMXBs as observed by SRG/eROSITA during its first four surveys. eROSITA's sensitivity of $\sim10^{-13}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$, translating to $10^{32}$-$10^{34},\mathrm{erg}\,\mathrm{s}^{-1}$ in luminosity for most known HMXBs in the Milky Way, has thus far never been reached by any wide-area survey instrument. We present the extended log N-log L distribution of known HMXBs reaching down to $10^{32}\,\mathrm{erg}\,\mathrm{s}^{-1}$ using eROSITA, and show the large scatter that can be induced by source intrinsic variability. We present sub-type resolved luminosity distributions, showing that the Supergiant X-ray binaries (SgXBs) and Be X-ray binaries (BeXRBs) occupy different parts of the overall distribution, and reanalyse RXTE/ASM data and MAXI for comparison to eROSITA. The luminosity regime uncovered by eROSITA allows a systematic study of the "transient" BeXRBs, which are typically below the detection threshold of monitors outside of outburst, and whose low luminosity behavior has been a longstanding question. Signatures of stable accretion at low luminosities have been observed with pointed instruments for a fraction of the overall sample, so far. With the eROSITA results, we posit that accretion outside of outburst is likely the norm, since a vast majority (> 80%) of BeXRBs are detected at luminosities at least an order of magnitude higher than expected for the most X-ray luminous Be stars. We discuss the observed luminosity in the context of cold disk accretion and the "propeller" mechanism. We highlight a small subpopulation of "isolated" Be-stars that reach luminosities comparable to the least luminous BeXRBs, hinting at the presence of compact object companions.

Supernova (SN) 2026sqf recently appeared in the nearby face-on spiral galaxy NGC 3310 and has shown signs of strong interaction with a circumstellar medium (CSM). Such intense interaction, rare among nearby SNe, offers a valuable opportunity to reveal details on the origin and nature of its progenitor system. We analyzed the early-phase spectra and light curves (LCs) of SN 2026sqf. The general shapes of the observed spectra, the strengths of the emission lines, and the LC evolution all suggest that SN 2026sqf belongs to the rare SN Ia-CSM subclass. If confirmed, this would be the closest known member of this class, at D~19 Mpc. We also report the first candidate progenitor system for a thermonuclear supernova identified in JWST pre-explosion imaging, and the first evidence for a (probable) carbon-rich AGB donor to the exploding white dwarf (WD). Our results are consistent with the expectation that SNe Ia-CSM emerge from a system consisting of a WD and an AGB star undergoing a common-envelope phase. The CSM mass and dust content are consistent with expectations for an AGB environment, but the narrow-line width exceeds superwind expansion velocities, favoring an episodic ejection. Binary interaction shortly before the explosion is a natural explanation, though the channel and timescale remain uncertain. Late-time follow-up, especially with JWST, will test the identification and the conclusions of our early-phase analysis.

Radio observations of gamma-ray burst afterglows provide insight in to the different emitting regions within the jet, its hydrodynamics and burst environment. In this paper, we present the second iteration of the Arcminute Microkelvin Imager - Large Array (AMI-LA) Gamma-ray burst (GRB) radio afterglow catalog. The catalog consists of 1035 observations of 210 bursts. Our observations range from 0.04 to 900days post-burst. We detect radio emission associated with 19 events with flux densities spanning 0.1-40mJy, and present a detailed analysis of six events whose light curves have not been published elsewhere. In our individual afterglow analyses, we find that our radio counterparts show evidence of reverse shock emission 60% of the time. Two of the detected afterglows show evidence of scintillation which we use to place source size limits at tens of days. Of the whole catalog, 50 events have redshift measurements which we use to find the radio afterglow luminosity distribution for our sample. Our luminosity distribution spans five orders of magnitude, a larger range than seen in optical and X-ray bands, indicative of the strong dependence of the radio counterparts on the GRB's physical parameters.

Galactic high-mass X-ray binaries (HMXBs) are important systems for studying accretion mechanisms onto compact objects and for investigating the complex stellar winds of massive stars. In particular, HMXBs hosting a neutron star allow us to reveal the structure of the accreted material in X-ray pulsars and consequently to investigate how matter behaves under extreme conditions of pressure and density. These are major scientific goals for XRISM and NewAthena. Here we report on the first out-of-eclipse XMM-Newton observation of the HMXB 4U 1538-52, complemented by NuSTAR coverage. Our campaign aimed to investigate stellar-wind variability and continuum changes with high-resolution spectroscopy at a critical orbital phase: when the neutron star is in inferior conjunction. Thanks to simultaneous observations covering both soft and hard X-rays, we obtain the most detailed X-ray view of the accreted material in 4U 1538-52 to date. In particular, we perform time-resolved spectroscopy down to the pulse period of the neutron star to highlight wind clumping properties and accretion structures. In this dataset, we observe a bright flare reaching $\sim$10$^{37}\,\rm{erg\,s^{-1}}$ probably induced by the accretion of a $10^{20}\,\rm{g}$ clump, followed by a luminosity dip forming a heartbeat-like episode. This event is followed by three local absorption peaks with local variability of the order of the pulse period, and a gradual hardening of the underlying spectrum throughout the observation. This could indicate the presence of both small-scale and large-scale overdense structures in the vicinity of the neutron star, which can be attributed to clumps and filamentary structures embedded in the accretion wake. These observational evidences are further supported and reproduced by 3D hydrodynamic simulations.

We report the discovery of AT 2020afjz (TSS2020a): the first hour-scale extragalactic transient discovered in optical wavelengths whose complete evolution -- from explosion onset to decay -- is temporally resolved, and the first such transient discovered by TESS. AT 2020afjz was identified as a $>10σ$ detection in the pilot HiLaTS program run within the TESSELLATE Sky Survey, which blindly searches for transient phenomena in TESS data with the TESSELLATE pipeline. Through cross-matching with legacy imaging, we associate it with DES J042144.37$-$383311.3, a member of an interacting galaxy pair at $z_{\rm phot}=0.67^{+0.07}_{-0.10}$. While AT 2020afjz is similar in duration and brightness to GRB afterglows, it exhibits a slow rise time of $\sim1$ hr and lasts for only 2.4 hr above the half-max brightness; modeling the TESS light curve with VegasAfterglow finds that it is best described as either an on-axis "dirty-fireball" or off-axis orphan afterglow. Each of these rare classifications hinge upon a non-detection at gamma-ray energies, but as Fermi-GBM was Earth-occulted at the time of explosion, AT 2020afjz's gamma-quiet nature cannot be definitively confirmed. Regardless, AT 2020afjz demonstrates TESS's power to discover fast extragalactic transients, and heralds a new population awaiting discovery with TESSELLATE.

We present observations of an X1.2 white-light flare on 2023 January 6, which exhibits a rare quadrupolar white-light source configuration. This event was observed by the White-light Solar Telescope (WST; 3600 Å) aboard the Advanced Space-based Solar Observatory and the Helioseismic and Magnetic Imager (HMI; 6173 Å) aboard the Solar Dynamics Observatory. Four flare-related footpoints (labeled as FP1--FP4) were nearly simultaneously identified in both WST 3600 Å and HMI 6173 Å continua, associated with a quadrupolar magnetic configuration and a failed filament eruption. The inner sources of FP1 and FP2 showed a similar enhancement of $\sim$65%/10% in the WST/HMI continuum, while the outer sources of FP3 and FP4 exhibited weaker responses. The inner footpoints had earlier responses in UV and EUV bands and were spatially coincident with the hard X-ray (HXR) footpoint sources. The two southern footpoints (FP2 and FP4) showed stronger HXR and white-light emissions than their northern counterparts (FP1 and FP3), with FP4 uniquely exhibiting a distinct HXR emission above 60 keV, in contrast to the absence of such an emission at FP3. Notably, faint WST 3600 Å enhancements at FP4 were observed during the gradual phase, temporally consistent with the fallback of filament material. This X1.2 flare presents a novel quadrupolar white-light structure, enriching our understanding of the generation and evolution of white-light flares.

We simulated various accretion disc structures with viscous hydrodynamic (HD) flow around a black hole (BH). We found that the structure of the accretion disc is significantly influenced by changes in the physical parameters of the initial inflowing gases. These physical parameters can be called outer boundary conditions (OBCs) at the outer-accretion boundary and represented on an OBC plane, which is primarily divided into hot-mode and cold-mode inflowing gases. We found smooth and shocked flows in the simulation, which follow the semi-analytical solutions with their OBCs. Interestingly, we observed that certain types of OBCs can produce shocks with jet-like features in the accretion flow. However, other OBCs can produce smooth or shock-free accretion flow, which may or may not have outflows. The smooth flows can display both the lowest and highest angular momentum distributions among the advective flows, depending on the OBCs. Additionally, the nature of the accretion flows can be either steady or quasi-steady, also influenced by the OBCs. We also observed that solutions corresponding to hot-mode gases have a greater tendency to generate outflows compared to those with the cold-mode. Therefore, this qualitative study of OBCs is crucial for understanding accretion physics, which can aid in modeling accretion discs, similar to the quantitative studies (which involve only changing mass accretion rates) of the inflowing gases. Thus, we assert that an accretion model should be based on both the qualitative and quantitative aspects of the initial inflowing gases.