高能暂现源 · 每日文献综述

SUNRISE III is a stratospheric balloon-borne solar observatory with a 1 m diameter telescope and three postfocus instruments, along with an image stabilisation system, all within a protective gondola. It samples the lower solar atmosphere, from the solar surface to the middle chromosphere, at a resolution approaching 50 km on the Sun. SUNRISE III flew successfully for 6.5 days suspended from a zero-pressure stratospheric balloon from northern Sweden to northwestern Canada in 2024 July, gathering around 200 TB of data. The present issue of the Astrophysical Journal Letters focuses on the first scientific results from the data collected during that flight. This Letter introduces this Focus Issue, providing a very brief overview of the capabilities of the instrumentation, the flight, and of the gathered data. Challenges for the measurements, data reduction, and interpretation are also briefly touched upon. The Letter ends with an overview of the first set of science results obtained from these data, as presented in the current Focus Issue.

Hellings-Nordtvedt theory is a vector-tensor theory in which a vector field <inline-formula><mml:math><mml:msub><mml:mi>A</mml:mi><mml:mi>μ</mml:mi></mml:msub></mml:math></inline-formula> is nonminimally coupled to curvature through two independent interactions <inline-formula><mml:math><mml:msup><mml:mi>A</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mi>R</mml:mi></mml:math></inline-formula> and <inline-formula><mml:math><mml:msup><mml:mi>A</mml:mi><mml:mi>μ</mml:mi></mml:msup><mml:msup><mml:mi>A</mml:mi><mml:mi>ν</mml:mi></mml:msup><mml:msub><mml:mi>R</mml:mi><mml:mrow><mml:mi>μ</mml:mi><mml:mi>ν</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula>. When supplemented by a potential whose zero-energy minimum occurs at nonzero <inline-formula><mml:math><mml:msup><mml:mi>A</mml:mi><mml:mn>2</mml:mn></mml:msup></mml:math></inline-formula>, the restricted <inline-formula><mml:math><mml:msup><mml:mi>A</mml:mi><mml:mi>μ</mml:mi></mml:msup><mml:msup><mml:mi>A</mml:mi><mml:mi>ν</mml:mi></mml:msup><mml:msub><mml:mi>R</mml:mi><mml:mrow><mml:mi>μ</mml:mi><mml:mi>ν</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula> sector is known to admit black-hole and neutron-star solutions with a global monopolelike asymptotic vacuum structure. We examine whether this structure is a generic consequence of the nonzero vector vacuum or instead relies on the special Ricci-tensor coupling. By analyzing the field equations near spatial infinity, we show that the asymptotic vacuum condition is incompatible with generic nonzero values of both couplings and instead selects two allowed single-coupling sectors. The <inline-formula><mml:math><mml:msup><mml:mi>A</mml:mi><mml:mi>μ</mml:mi></mml:msup><mml:msup><mml:mi>A</mml:mi><mml:mi>ν</mml:mi></mml:msup><mml:msub><mml:mi>R</mml:mi><mml:mrow><mml:mi>μ</mml:mi><mml:mi>ν</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula> sector reproduces the known global monopolelike asymptotics, whereas the <inline-formula><mml:math><mml:msup><mml:mi>A</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mi>R</mml:mi></mml:math></inline-formula> sector admits an asymptotically flat Schwarzschild metric with a nontrivial radial vector field. We further compute the Noether mass in the <inline-formula><mml:math><mml:msup><mml:mi>A</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mi>R</mml:mi></mml:math></inline-formula> sector, derive the corresponding Solar-System constraints, and construct neutron-star configurations. Although the weak-field deviation is constrained to be small, neutron stars can still show appreciable departures from both general relativity and the Ricci-tensor-coupling sector in their masses, radii, and moments of inertia. Our results identify that the <inline-formula><mml:math><mml:msup><mml:mi>A</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mi>R</mml:mi></mml:math></inline-formula> sector of massive Hellings-Nordtvedt theory as a viable and useful framework for studying strong-field compact objects with a nonzero vector vacuum while remaining compatible with weak-field tests.

Primordial black holes in the planetary-mass range have attracted renewed interest; however, the search for gravitational waves from such binaries remains challenging due to their long-lived nature. In this work, we present, define, and validate a fully operational search pipeline developed to detect planetary-mass binaries during their inspiral phase. We use the Viterbi algorithm, a dynamic programming technique that recovers the most likely track based on a Hidden Markov Model. To enhance its performance, we introduce a novel time-frequency representation of the data and a candidate isolation procedure that separates signals from background noise. The evaluation of candidates is carried out using the two detection statistics, $n_σ$ and NMSE, which quantify the power significance and the consistency with the expected binary evolution. We then validate the search method using O3 LIGO Hanford data with a population of injected signals. The pipeline is able to recover most of the signals with a fixed false-alarm ratio of $3\%$, covering Galactic scales across most of the parameter space and reaching luminosity distances $ \gtrsim 100$ kpc in the most sensitive region. For each candidate, we also obtain an estimate of the system's chirp mass, whose accuracy remains high throughout the detectable range, enabling a rapid characterization of the system upon detection.

Timelike geodesics, periodic orbits, and their associated gravitational-wave signatures are examined in the spacetime of a Bonanno--Reuter regular black hole, a geometry arising from Asymptotically Safe Gravity in which a running Newton coupling replaces the central singularity with a de Sitter core. The dimensionless parameter $α/M^2$ completely determines the strong-field dynamics. Increasing $α/M^2$ shifts the marginally bound and innermost stable circular orbits inward, systematically reducing their characteristic radii, angular momenta, and energies; the allowed phase space for bound motion contracts accordingly. Classifying trajectories via the rational frequency ratio $q = w + v/z$ reveals that periodic orbits experience a mild inward contraction, which reduces the energy necessary to sustain a specific topology. Within the numerical kludge framework, we calculate the gravitational-wave polarizations for extreme mass-ratio inspirals. The asymptotically safe correction induces a leftward phase shift that reflects shorter orbital periods, while mildly enhancing peak amplitudes owing to the smaller periastron distances reached in the deep strong-field regime. Waveform sensitivity displays a strong dependence on topology, with high-whirl orbits, which persist longer in the strong-field region near the horizon, showing markedly more pronounced deviations. Unlike environmental effects that inflate orbital scales, intrinsic quantum-gravity modifications generate distinct, observationally detectable signatures for future space-based detectors such as LISA, Taiji, and TianQin.

The population study of stellar-mass black hole (sBH) binaries with ground-based gravitational wave detection has achieved tremendous success in recent years. Future observation of extreme mass-ratio inspirals will similarly require proper population analyses that identify the formation channels, measuring the branch ratio(s) and characterizing major properties within each major channel. In this work, we propose that the measurement of eccentricity, inclination, and component mass provides critical information to distinguish different formation channels and probe detailed formation mechanisms. Focusing on the dry and wet extreme mass-ratio inspirals, we establish the theoretical expectation of these observables in each formation channel. We also discuss how their distributions can be used to probe lifetime and turbulence level of active galactic nuclei disks, accretion patterns of supermassive black holes, and population properties of sBHs within nuclear star clusters.

We present the first high-resolution near-infrared spectroscopy of the nucleus of the nearby, well-known Seyfert galaxy NGC 4151 (the 'Eye of Sauron'). Past studies of this source have revealed that it exhibits a variable absorption feature associated with the He Iλ10 830 Å emission line, that is potentially indicative of obscuration events affecting the central engine. Here, we took advantage of the IRTF/iSHELL and TNG/GIANO-B spectrographs to observe this feature with unprecedented spectral resolution (λ/∆λ &gt; 50 000); these allowed us to study the structure of the absorption trough and its variations in detail over a time span of ∼700 days. In order to infer a connection between the He I absorption variability and that of the X-ray ionising continuum, we also analysed the publicly available data collected by the Swift-XRT instrument over the same period of time, unveiling a potential driving mechanism in the changes of the outflow ionisation state due to the X-ray flux variations. We also derive outflow physical parameters ─ Ṁ<SUB>out</SUB> ≲ 10<SUP>−1</SUP> M<SUB>⊙</SUB> yr<SUP>−1</SUP>, r<SUB>out</SUB> ∼ 3 pc, v<SUB>max</SUB> ∼ 1000 km s<SUP>−1</SUP> ─ that are in line with those of comparable ionised winds found in similar targets, where the outflow is not powerful enough to trigger a significant feedback from the active galactic nucleus (Ė<SUB>kin</SUB>/L<SUB>bol</SUB> ∼ 0.001%). Such findings point at a scenario in which a dusty and clumpy outflow that obscures NGC 4151 up to galactic scales responds to changes in the ionising X-ray flux; this is similar to what happens in quasars with broad absorption lines and Seyferts with multi-phase outflows such as NGC 5548.

Rotating nonaxisymmetric neutron stars (NSs) are promising sources for continuous gravitational waves (CWs). CWs may, if detected, inform us about the internal structure and equation of state of NSs. Here, we present a narrowband search for CWs from known pulsars, for which a matched-filter search can be applied. Narrowband searches are robust to mismatches between electromagnetic (EM) and gravitational emissions, in contrast to fully targeted searches where they are assumed to be phase-locked. In this work, we search for the CW counterparts emitted by 34 pulsars using data from the first and second parts of the fourth LIGO─Virgo─KAGRA observing run. We use the 5n-vector narrowband pipeline, which applies frequency-domain matched filtering. In previous searches, it covered a narrow range in the frequency—frequency time derivative (f—<inline-formula> <mml:math><mml:mover><mml:mrow><mml:mi>f</mml:mi></mml:mrow><mml:mrow><mml:mo>̇</mml:mo></mml:mrow></mml:mover></mml:math> </inline-formula>) space. Here, we also explore a range in the second time derivative of the frequency <inline-formula> <mml:math><mml:mover><mml:mrow><mml:mi>f</mml:mi></mml:mrow><mml:mrow><mml:mo>̈</mml:mo></mml:mrow></mml:mover></mml:math> </inline-formula> around the EM observations. Additionally, for the first time, we target sources in a binary system with this kind of search. We find no evidence for CWs and therefore set upper limits on the strain amplitude emitted by each pulsar. For 20 analyses, we report an upper limit below the theoretical spin-down limit. The tightest constraint is for pulsar PSR J0534+2200 (the Crab pulsar), for which our strain upper limit on the CW amplitude corresponds to ≤0.04% of the spin-down power being radiated in the CW channel.

The high-mass (M ≳ 30M<SUB>⊙</SUB>) black holes (BHs) from the gravitational-wave (GW) observations of LIGO and Virgo came as a surprise to many astronomers. While the collapse of metal-poor massive stars could produce such BHs, gravitational lensing has been invoked to explain their high masses. Broadhurst, Diego, and Smoot (henceforth BDS) argued that the mass distribution of BHs in coalescing binaries is very similar to that of the galactic BHs, and the inferred high masses are the result of neglecting the lensing magnification. They also proposed a redshift distribution of binary BH (BBH) mergers to explain the observed LIGO-Virgo mass distribution. We ask whether such a model is consistent with different aspects of the GW observations: (1) the observed number of BBH mergers, (2) the distribution of their redshifted total mass and apparent luminosity distance, (3) the nondetection of strongly lensed events, and (4) the nonobservation of the stochastic GW background. By simulating lensed BBH mergers with the BDS model and comparing them with observations, we conclude that no choice of BDS model parameters is consistent with all aspects of the observations. Lensing magnification is not a viable explanation for the high-mass BHs discovered by LIGO and Virgo.

The observed gravitational-wave background (GWB) spectrum is higher in amplitude than model predictions by a factor of 2─3. Using a semi-analytic model, we evaluate the effect of a high-scatter supermassive black hole (SMBH) scaling relation (M<SUB>BH</SUB>─M<SUB>bulge</SUB>) on models of the nanohertz GWB. By implementing an intrinsic scatter of the M<SUB>BH</SUB>─M<SUB>bulge</SUB> relation, which is larger at higher redshift, but matches local observations, we find that the amplitude of GWB models increases to be consistent with the low-frequency end of the GWB spectrum. This amplitude increase is not uniform across frequencies, a strongly evolving scatter preferentially increases the number density of the most massive SMBHs which, in the GWB spectrum, minimizes the strength of the low-frequency turnover. Our models with positively evolving intrinsic scatter can reproduce the electromagnetically observed overmassive SMBHs at 4 &lt; z &lt; 6 without changing the M<SUB>BH</SUB>─M<SUB>bulge</SUB> normalization though we find that including moderate normalization evolution marginally improves fits to the GWB data. We conclude that the M<SUB>BH</SUB>─M<SUB>bulge</SUB> relation which best describes the available GWB and electromagnetic data sets has intrinsic scatter that evolves as <inline-formula> <mml:math><mml:mi>∊</mml:mi><mml:mo>(</mml:mo><mml:mi>z</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:msub><mml:mrow><mml:mi>∊</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msub><mml:mo>+</mml:mo><mml:mo>(</mml:mo><mml:mn>0.56</mml:mn><mml:mo>±</mml:mo><mml:mn>0.4</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:mn>1</mml:mn><mml:mo>+</mml:mo><mml:mi>z</mml:mi><mml:mo>)</mml:mo></mml:math> </inline-formula> and normalization that evolves as α(z) = α<SUB>0</SUB>(1 + z)<SUP>0.84±0.35</SUP>. The results of this work imply that the M<SUB>BH</SUB>─M<SUB>bulge</SUB> relation we see today is not universal throughout cosmic time and that a diversity of seeding models and growth mechanisms may be at play in the early stages of SMBH─galaxy evolution.

The variability characteristics of active galactic nuclei (AGNs) serve as a powerful diagnostic tool for probing the physical mechanisms underlying accretion processes and jet formation in supermassive black holes. Using long-term optical time-domain monitoring data from the Zwicky Transient Facility, this study applies the damped random walk model to fit the light curves of radio-quiet AGNs, enabling the extraction of their optical variability amplitudes and characteristic timescales. We investigate the origins of optical variability in radio-quiet AGNs. The main findings are as follows: (1) There is a significant correlation between variability amplitude and Eddington ratio; in contrast, variability amplitude is not significantly correlated with radio luminosity. These results indicate that the optical variability of radio-quiet AGNs is primarily driven by the Eddington ratio, rather than relativistic jets. (2) The relationship between variability amplitude and Eddington ratio in radio-quiet AGNs is highly dependent on the accretion mode. For the efficient accretion mode, the variability amplitudes of radio-quiet AGNs are inversely correlated with the Eddington ratio; in contrast, for the inefficient accretion mode, these variability amplitudes are positively correlated with the Eddington ratio. (3) A significant correlation between optical variability amplitude and X-ray luminosity is observed for radio-quiet AGNs. This finding may support a scenario in which variations in UV/optical emission and heating of the X-ray corona are jointly regulated by magnetohydrodynamic turbulence within the accretion disk.

We present the objectives, implementation, and preliminary results of the first Bulgarian polar astronomy project, "Impact of Solar Activity on Ionospheric Dynamics and High-Energy Particle Fluxes over Antarctica" (2024─2025), conducted at the Bulgarian Antarctic Base on Livingston Island. The study aims to investigate solar-terrestrial interactions, focusing on solar flares, coronal mass ejections, and their effects on ionospheric disturbances, geomagnetic field variations, and radiation levels. Four synchronized experiments were deployed: (1) HF radio spectrography for direct solar burst observations (50-1000 MHz), (2) VLF monitoring of ionospheric D-layer disturbances via long-range ground- based transmitters, (3) magnetometric measurements of local geomagnetic field variations, and (4) radiation dosimetry using passive detectors. Data analysis is ongoing, with plans for long-term monitoring to improve space weather forecasting and mitigate impacts on satellite communications. This initiative establishes Bulgaria's capacity for cutting-edge polar astrophysical research, supported by collaborations between the Institute of Astronomy and NAO, Technical University-Sofia, the Nikola Vaptsarov Naval Academy, and international partners.

Solar wind is a complex network of distinct magnetic flux tubes, each contained and separated by a current sheet. In this study, more than 50,000 current sheets in the solar wind are identified and characterized for the first time, using multi-point Cluster observations during solar minimum and maximum intervals. Flux tubes at Earth are found to have an average diameter of ∼1.5 Earth radii, nearly 45 times smaller than previously reported. Furthermore, 6 years of NASA's ACE solar wind observations at Sun-Earth Lagrange point L1 is used to show that only 30% of the observed flux tubes would directly impact the Earth's magnetosphere. It is shown that flying closer to the Sun-Earth line could help to improve the prediction accuracy of space weather monitors at L1 by up to 2.5 times.

In this work, we explore the possibility of Schwinger pair creation triggered by magneto-centrifugal effects in the magnetosphere of Sgr A*. We show that these effects become extremely efficient in the presence of a vortex-driven magnetic field, whose strength exceeds previous estimates by several orders of magnitude. The dynamics of magneto-centrifugally accelerated charged particles leads to charge separation, thereby parametrically exciting Langmuir waves. The associated electric field grows exponentially and, upon reaching the Schwinger critical threshold, initiates efficient electron-positron pair production, which is ultimately saturated by annihilation processes.

Upcoming stage 4 surveys, such as the Simons Observatory, LSST, and Euclid, are poised to measure weak gravitational lensing of the Cosmic Microwave Background (CMB) and galaxies with unprecedented precision. While the power spectrum is the standard statistic used to analyze weak lensing data, non-Gaussianity from non-linear structure growth encodes additional cosmological information in higher-order statistics. We forecast the ability of future surveys to constrain cosmological parameters using the weak lensing power spectrum and bispectrum from both CMB and galaxy surveys, including their cross-correlations. We consider an eight-parameter model (ΛCDM + ∑m<SUB>ν</SUB> + w <SUB>0</SUB>) and assess constraints for stage 4 survey specifications. In the absence of systematics, both the CMB and galaxy lensing bispectra are found to be detectable at high signal-to-noise. We test two priors: a "strong" one based on constraints from CMB temperature and E-mode polarization anisotropies, and a "weak" one with minimal assumptions. With the weak prior, the bispectrum significantly improves parameter constraints by breaking degeneracies. For strong priors, improvements are more limited, especially for the CMB bispectrum. On small scales, where non-linear effects dominate, the bispectrum's constraining power can rival that of the power spectrum. We also find strong synergy between CMB and galaxy lensing; combining both probes leads to tighter constraints, particularly on neutrino mass. It was recently found that the CMB lensing bispectrum is strongly affected by the Born approximation, so we also consider post-Born corrections but find that our main conclusions remain the same. These results highlight the potential of higher-order lensing statistics and motivate further work on neglected effects such as non-Gaussian covariance, instrumental systematics, and baryonic feedback.

We analyze GW production during self-resonant preheating with energy transfer via ϕχ <SUP>2</SUP>, in which ϕ and χ correspond to the inflaton and the daughter field, respectively. Linear Floquet analysis and nonlinear simulations show that ϕ fluctuations grow by parametric resonance, while χ undergoes tachyonic bursts. The dominant contribution to the GW spectrum arises from parametric resonance in the case of an effective quartic potential, with model parameters consistent with CMB observations. In a well-motivated α-attractor inflationary scenario, the present-day redshifted spectrum attains a peak amplitude of h <SUP>2</SUP>Ω<SUB>GW</SUB> <SUP>(0)</SUP> ∼ 10<SUP>-11</SUP> at a peak frequency f<SUB>p</SUB> <SUP>(0)</SUP> ∼ 10<SUP>7</SUP> Hz.

We present a comprehensive, two-decade, multiwavelength variability study of the blazar PKS 1510 - 089, one of the most prominent and extensively monitored flat-spectrum radio quasars. Using Fermi-LAT γ-ray data together with Swift-XRT and UVOT observations spanning 2005─2024, we trace the long-term evolution of its flux, interband correlations, and spectral behaviour across the optical, X-ray, and γ-ray bands. We find that all flux distributions prefer a double-log-normal fit. In case of the optical bands, this may be due to contributions from the accretion disk. The range of fluxes in a given band, as well as the fractional variability values are in-line with the expectations that high-energy parts of a given spectral component are more variable than low-energy parts. No obvious cross-correlations exist between the bands over the 20 years of observations. The X-ray and γ-ray spectra are variable, but do not show any trend with flux. These results are suggestive of different zones being active in the jet of PKS 1510 - 089 at any given time. In a previous paper, we used the same techniques to study the high-frequency-peaked BL Lac object PKS 2155 - 304. Both sources follow the aforementioned trend on the energy-dependent variability of the spectral components, as well as the lack of significant cross-correlations between the studied bands. While PKS 2155 - 304 exhibits a harder-when-brighter behaviour in its high-energy part of the synchrotron component, no such behaviour could be found in PKS 1510 - 089. Both sources show orphan flares, which can seemingly happen in any band. In summary, the long-term studies of these two sources reveal that the underlying physics is similar in these apparently different source classes, even though variability patterns keep changing and remain unpredictable.

We present a timing analysis of the high-cadence optical light curve of the high-redshift flat-spectrum radio quasar PKS 0805 - 07 obtained during TESS Sector 34 (MJD = 59230.90─59239.90). We search for short-timescale quasi-periodic oscillations (QPOs) using complementary time-series techniques, including the Lomb─Scargle periodogram (LSP) and the weighted wavelet Z-transform (WWZ), and evaluate their significance against red-noise variability using Monte Carlo simulations. The LSP reveals a dominant modulation at <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>≍</mml:mo><mml:mn>0.6</mml:mn><mml:mspace></mml:mspace><mml:msup><mml:mrow><mml:mi>d</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math> (P ≍ 1.7 d) exceeding the 99.99% confidence level, while the WWZ independently recovers a consistent timescale at the ∼ 99.9% level and shows that the signal is temporally localized rather than persistent across the full light curve. The modulation spans ∼ 5 coherent cycles, indicating a transient quasi-periodic feature. However, the detected significance only marginally exceeds the highest confidence threshold in the WWZ analysis, and the limited number of coherent cycles is insufficient for a firm confirmation. We discuss possible physical interpretations of the candidate modulation. In a disk-based scenario, orbital motion of a hotspot near the innermost stable circular orbit implies a black hole mass of M<SUB>BH</SUB> ∼ 7.2 × 10<SUP>8</SUP> M<SUB>⊙</SUB>, consistent with typical FSRQ values. Alternatively, magnetohydrodynamic kink instabilities in the relativistic jet can naturally produce day-scale variability for standard blazar parameters and account for the transient character of the signal. Compact SMBH binary and tidal disruption event scenarios are also considered but found to be disfavoured by the jet-dominated nature of the source, the optical rather than X-ray character of the modulation, and the black hole mass well above the regime associated with confirmed QPE host galaxies. We conclude that the candidate modulation is consistent with a compact, short-lived structure embedded within stochastic jet variability, and that high-cadence multiwavelength monitoring will be essential to confirm its recurrence and constrain its physical origin.

Gravitational-wave astronomy has developed enormously over the last decade, with the first detections and continuous development across broad frequency bands. However, the decihertz range has largely been left out of this development. Gravitational waves in this band are emitted by some of the most enigmatic sources, including intermediate-mass binary black hole mergers, early inspiraling compact binaries, whose mergers are seen by Earth-based detectors, and possibly primordial gravitational waves. To tap this exciting band, we propose the construction of a detector based on pulsar timing principles, the Artificial Precision Timing Array (APTA). We envision APTA as a Solar system array of artificial 'pulsars' ─ precision-time-reference-carrying satellites that emit periodic electromagnetic signals towards Earth or another satellite constellation receiver location. In this fundamental study, we estimate the clock precision needed for gravitational-wave detection with APTA. Our results suggest that six satellites and a clock relative uncertainty of <inline-formula><tex-math>$10^{-18}$</tex-math></inline-formula> at 1 s of averaging, which is currently attainable with ground-based atomic clocks, would be sufficient for APTA to reach pristine sensitivity in the decihertz band and observe <inline-formula><tex-math>$10^3$</tex-math></inline-formula>─<inline-formula><tex-math>$10^4 \, \mathrm{M}_\odot$</tex-math></inline-formula> black hole mergers and the early inspiral of heavy LIGO (Laser Interferometer Gravitational-wave Observatory)─Virgo─KAGRA (Kamioka Gravitational Wave Detector) sources. Future clock and oscillator technologies realistically expected in the next decade(s) would enable the detection of an increasingly diverse set of sources, allowing APTA to reach a better sensitivity than other detector concepts proposed for the decihertz band. This work opens up a new area of research into designing and constructing gravitational-wave detectors relying on principles used successfully in pulsar timing.

Recent evidence from Pulsar Timing Arrays (PTAs) for a nanohertz gravitational wave background is broadly consistent with theoretical expectations from a population of massive black hole binaries (MBHBs), although the inferred amplitude appears somewhat higher than predicted by standard models. Interpreting these observations requires a robust understanding of the merger timescales of MBHBs, and of their connection to host galaxy properties. In this work, we investigate the evolution of MBHBs selected from cosmological galaxy mergers in the ILLUSTRISTNG simulation. We re-simulate these systems at high resolution using the N-body code GRIFFIN to accurately resolve the dynamical friction and stellar hardening phases, and follow their evolution to coalescence with a semi-analytical model. We find that cosmological galaxy encounters and the resulting MBHBs are typically highly eccentric. We characterise the distribution of binary eccentricities at formation and at entry into the PTA band, and quantify the corresponding residence times. We identify the key parameters governing the duration of the different stages of MBHB evolution, and derive scaling relations linking galaxy and orbital properties to dynamical friction, hardening, and total coalescence times. These relations provide a framework for subgrid prescriptions in cosmological simulations. Applying these scaling relations to the full ILLUSTRISTNG merger population, we infer the probability distributions of galaxy merger and black hole coalescence times. We find that galaxy mergers typically complete within ~0.7 Gyr, while the total black hole coalescence time is ~1.0 Gyr. These short timescales imply efficient binary evolution, consistent with current PTA constraints.

Although the field of neutrino astronomy has blossomed in the past decade, physicists have struggled to fully map the high-energy neutrino sky. The Tau Air-shower Mountain-Based Observatory (TAMBO), a mountain-based neutrino observatory, aims to solve that issue—and find clues of new physics along the way.

The search for and the characterization of ultra-compact dual active galactic nuclei (AGN) are among the hottest topics of current extragalactic astrophysics. These systems involve two accreting massive black holes (MBHs) embedded within the same host galaxy, with relative projected separations from a few hundred pc down to a few pc. They are central to understanding hierarchical galaxy formation, black hole growth and demographics, and accretion─feedback coupling in the most extreme interaction phases. Even more compellingly, such tight pairs are the most direct precursors of gravitationally bound binary MBHs (sub-pc scale separation), which are among the loudest emitters of gravitational waves (GWs) in the low-frequency ranges. SHARP will deliver the first statistical census and physical characterization of ultra-compact dual AGN up to cosmic distances, finally bridging the observational gap between kpc-scale pairs and sub-pc GW-emitting binaries, and enabling a breakthrough understanding of MBH growth, feedback and co-evolution across cosmic time.

In this work, we investigate the General Relativistic Entropic Acceleration (GREA) framework, in which late-time acceleration emerges from entropy production associated with the cosmological horizon, and compare its performance with the standard ΛCDM description of the Universe. We first confront GREA with current background observations, including baryon acoustic oscillations, type Ia supernovae, compressed CMB information, and cosmic chronometers, with particular emphasis on the geometric horizon parameter −kη0. We then introduce a phenomenological extension of the theory by allowing for an additional dark energy component, Ωde, enabling the recovery of a ΛCDM-like expansion history as a limiting case. We perform a Bayesian parameter inference and model comparison analysis using both current data and mock datasets representative of future surveys, including SKAO, LSST, and ET. While current data statistically prefer ΛCDM when compressed CMB information is included, GREA remains competitive for low-redshift combinations. Forecasts indicate that gravitational wave standard sirens are expected to enhance the ability to discriminate between entropic-driven and dark-energy-driven expansion scenarios, and to identify the underlying cosmological model favored by the data.

Gravitational-wave signals are typically interpreted under the vacuum hypothesis, i.e., assuming negligible influence from the astrophysical environment. This assumption is expected to break down for low-frequency sources such as extreme mass ratio inspirals (EMRIs), which are prime targets for the Laser Interferometer Space Antenna (LISA) and are expected to form, at least in part, in dense environments such as active galactic nuclei or dark-matter spikes or cores. Modeling environmental effects parametrically is challenging due to the large uncertainties in their underlying physics. We propose a nonparametric test for environmental effects in EMRIs, based on assessing the self-consistency of vacuum parameter posteriors inferred from different portions of the signal. Our results demonstrate that this test can reveal statistically significant inconsistencies from vacuum signals—arising from, e.g., incomplete modeling, environmental effects, or deviations from general relativity—without introducing additional parameters or assumptions about the underlying physics.

Gravitational waves (GWs) in the <inline-formula><mml:math><mml:mrow><mml:mn>0.01</mml:mn><mml:mo>∼</mml:mo><mml:mn>1</mml:mn><mml:mtext> </mml:mtext><mml:mi>Hz</mml:mi></mml:mrow></mml:math></inline-formula> band encode unique signatures of the early Universe and merging compact objects, but they are beyond the reach of existing observatories. Theoretical models suggest that the Moon could act as a resonant detector, but the unknown influence of its rugged surface and heterogeneous interior poses a challenge to the accurate modeling of its response. Here, we address this long-standing uncertainty by constructing the first high-resolution, two-dimensional model of the lunar GW response, more realistic than previous ones. We achieve this by combining high-fidelity spectral-element simulations with the analytical power of normal-mode perturbation theory, thereby resolving topographical effects down to 2 km grid spacing while maintaining the capacity to discern global free-oscillation patterns. This dual-methodology approach not only recovers the expected predominant quadrupole (<inline-formula><mml:math><mml:mrow><mml:mi>l</mml:mi><mml:mo>=</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:math></inline-formula>) oscillation mode, but also exposes a systematic signal amplification in thick-crust regions. This enhancement is traced by our normal-mode analysis to a mode-coupling process, in which the original quadrupolar oscillation induced by the passing GW distributes energy into a series of higher-order modes, the hybridized eigenmodes of a laterally heterogeneous Moon. In certain narrow frequency ranges, we observe up to tenfold amplification spanning into the deci-hertz band, highlighting the power of numerical simulations in resolving these structurally fine-tuned features for designing future detectors. Our Letter establishes the Moon as a resonant GW detector albeit its complex topographical structures, and the resulting amplification maps provide a quantitative guide for the optimal landing site selection.

We report on two deep XRISM observations of the central and northern regions of Hydra-A's X-ray atmosphere covering the bubbles inflated by jets from the central galaxy's active galactic nucleus (AGN). We use spatial-spectral mixing that combines Chandra's high spatial resolution with XRISM's high spectral resolution to investigate atmospheric kinematics. The atmospheric velocity dispersion in the northern region, $σ_v = 140^{+30}_{-20}$ km s$^{-1}$, is comparable to that in the central region ($σ_v = 162 \pm 10$ km s$^{-1}$). We show that the motion of the large-scale cocoon shock front could be responsible for the large dispersion toward the north. The velocity dispersion in the northeast quarter of the central pointing, $σ_v = 260 \pm 50$ km s$^{-1}$, is among the highest dispersions measured. This region contains an X-ray-bright feature previously identified as metal-rich, possibly consisting of gas uplifted in the wake of previous-generation cavities. The dispersions in all other quarter regions are low ($σ_v \leq 120$ km s$^{-1}$) and consistent with previous XRISM results from other objects. The kinetic energy at the center is comparable to the enthalpies of the cavities, while in the north, it is roughly an order of magnitude smaller. The jet thus drives gas motion efficiently at smaller scales ($r < 95$ kpc) and inefficiently at larger scales ($95-317$ kpc toward the north along the jet). A bulk flow toward our line of sight of $-100 \pm 30$ km s$^{-1}$ in the southwest quarter of the central pointing is also observed, possibly due to sloshing.

Cosmic strings are not yet confirmed, topological defects formed in the early Universe. They can bend light or gravitational waves, which causes an effect similar to the gravitational lensing. Our goal is to check whether cosmic string could be detected as a lenses of gravitational waves by the Einstein Telescope (ET). To do that the apparatus of wave optics had been applied. Firstly we explored the amplification factor strength and behaviour. Next the wave effects in SNRs and waveforms was examined for different inclinations of the source. Lastly we estimated the string tensions based on eight mergers from the \textsc{StarTrack} simulation, using the Bayesian Inference methods. The wave effects were easily to see in waveforms, SNR and characteristic strains. Also most of the events could be detected, based on their Signal to Noise Ratio values. Almost whole characteristic strain lies in the range of the ET. When it comes to mock observations, we had got estimated vale of a logarithm of the CS tension equal to $\bar{\log{Gμ}}=-9.78^{+0.44}_{-0.49} $, which were consistent with injected value equal to $-10$. At the end we conclude that CSs could be detected by the ET.

Matched filtering remains the most sensitive method for detecting gravitational waves from compact binary coalescences. The efficiency of such searches depends on how well a discrete template bank covers the underlying parameter space. Conventional geometric, stochastic, and hybrid placement methods can lead to uneven coverage and redundant templates in higher dimensions. Hybrid methods are generally the most efficient among these, while stochastic methods are simpler to implement, particularly when the parameter-space metric is difficult to compute. In practice, both approaches rely on uniform random sampling, which often requires a large number of proposal points to achieve adequate coverage. We find that stochastic template banks constructed using low-discrepancy sequences achieve comparable recovery fractions while requiring 27.5\% fewer proposal points in two dimensions and 12\% fewer in three dimensions. The final template count changes only marginally ($\sim 1\%$), consistent with the metric-volume constraints of the covering problem. The primary benefit of low-discrepancy sampling is therefore a reduction in the size of the initial proposal set, leading to lower memory usage and reduced bookkeeping during bank generation. Since the final template count is governed mainly by the metric volume of the target parameter space, the wall-clock speed-up is more modest than the reduction in proposal count. Nevertheless, low-discrepancy sampling provides a simple and scalable improvement to stochastic template-bank generation for current and future gravitational-wave searches.

We consider the model of the regular black hole, which demonstrates that the gravastar is thermodynamically unstable towards the Schwarzschild black hole with singularity.

As a particular subclass of active galactic nuclei (AGNs), compact symmetric objects (CSOs) have attracted significant attention due to potential role as young AGNs. Several $γ$-ray emitting CSOs have been detected with Fermi Large Area Telescope (Fermi-LAT), which motivates further searches for more $γ$-ray emitting CSOs. We perform a systematic search for $γ$-ray emission from a bona fide CSO sample using 16 yr Fermi-LAT observation data. No significant $γ$-ray signal is found to be firmly associated with any CSO. Only one $γ$-ray signal is detected near CSO GB6 J0906+4124 with TS = 28.7 ($\sim4.9σ$) in 0.1--300 GeV band. Within the 68\% containment radius of Fermi-LAT average PSF at 3 GeV, GB6 J0906+4124 remains the plausible counterpart of the $γ$-ray signal, primarily due to its classification and broad spectral coverage. This work suggests that GeV emission from CSOs may either be uncommon or simply below the current Fermi-LAT detection sensitivity. Further multi-wavelength follow-up is needed to determine the origin of the $γ$-ray signal near GB6 J0906+4124.

Exotic compact objects (ECOs) may produce late-time gravitational-wave (GW) echoes, but sufficiently compact ECOs undergoing realistic accretion may eventually form an event horizon. We investigate the evolution of quasinormal modes and GW echoes during this transition. Modeling the exterior spacetime with the ingoing Vaidya solution, we numerically evolve scalar perturbations on the resulting dynamical background. We show that the echo signal is progressively compressed and suppressed during accretion and disappears once the growing event horizon engulfs the reflecting surface, after which the waveform smoothly approaches the standard black hole ringdown. Our results identify the disappearance of GW echoes as a characteristic signature of horizon formation and provide a framework for studying dynamical compact objects with future GW observations.

A study of cyclic variations in the number and parameters of coronal mass ejections (CMEs) in cycles 23-25 demonstrates differences in the behavior of strong and weak CMEs. Strong CMEs, unlike weak CMEs, follow the cyclic evolution of active regions. A method for separately predicting strong and weak CMEs is proposed. Based on global magnetic field (GMF) parameters, the timing of the minima of cycles 26 and 27 (CR~2375$\pm5$ and CR~2526$\pm5$), the duration of cycle 26 (151~CRs or 11.2838 years), and the periods of dominance of the GMF sectorial structure in cycles 25 and 26 ($\approx$CRs 2245-2355 and $\approx$CRs~2395-2546) were determined. Assuming similarities between cycles 26 and 20, a Wolf number forecast was made for cycle 26, based on which the number of strong CMEs in cycles 25 and 26 was predicted. Based on the ratio of weak to strong CMEs in cycle 23, a forecast of weak CMEs in cycles 25 and 26 was made. A comparison of the predicted and observed CME parameters in cycle 25 shows good agreement.

The James Webb Space Telescope (JWST) is opening a new window into the distant Universe by discovering galaxies and transients in the early Universe. We investigate a high-redshift transient candidate, beacon_1420-5253_4770. This object was initially identified as a high-redshift galaxy candidate at $z\sim14$. However, the source was not detected in epochs before and after the detection epoch, suggesting that the object is a transient source rather than a persistent galaxy. We classify the source by comparing the colors, magnitudes, light curves, and spectral energy distribution with various spectral templates of transients. Our analysis shows that the observed properties are consistent with a Type Ia supernova at $z\sim4.3$. Strong absorption by Fe-group elements seen in Type Ia supernova spectra can mimic the Lyman break used to detect high-redshift galaxies. At $z \sim 4.3$, corresponding to a cosmic age of only $\sim 1.5$ Gyr, our detection provides a direct probe of the delay time between star formation and supernova explosion. Our estimate of the event rate implies a minimum delay time shorter than 1 Gyr. We also discuss the implications of transient contamination for searches of galaxies in the early Universe.

We propose an extension of the quarkyonic matter framework that includes $u$, $d$, and $s$ quarks and the full baryon octet. Within this extended framework, we impose beta-equilibrium between baryons and leptons, while determining the quark fractions from the constituent quark contents of baryons. The hadronic sector of octet baryons is described by a recently developed density, momentum and isospin dependent effective interaction based on the N3LO Skyrme pseudopotential, whereas quarks and leptons are treated as free particles. We find that the quarkyonic mechanism can obviously reduce the critical density for hyperon appearance in neutron stars due to the fact that the nucleons are displaced to higher momentum states in quarkyonic matter and their chemical potentials rise accordingly. Furthermore, the quarkyonic mechanism can significantly stiffen the equation of state of hyperon star matter and thereby enhance the hyperon star maximum mass, thus helping to mitigate the hyperon puzzle.

After almost a century of speculation, the physical processes responsible for heating the Sun's corona remain uncertain. Observations of coronal loops at extreme ultraviolet and X-ray wavelengths have provided substantial insights, such as the constraint that heating rates increase monotonically with local magnetic field strength. However, there remain dozens of competing suggestions for detailed theoretical explanations of the heating. There remains a need to compare the predictions of these theories with one another, and with real data, so the most likely mechanisms can be determined. In this paper, we build three-dimensional simulations of the corona by starting with potential-field extrapolations for the magnetic field, filling in plasma densities and temperatures along individual loops using one-dimensional models, then computing optically thin intensities for comparison with data from the Solar Dynamics Observatory and Hinode. One new aspect of this forward modeling is the use of parameterizations for heating that include a wide variety of processes such as wave dissipation, reconnection, nanoflares, and turbulent cascade. We also include the effects of intermittent heating and cooling along unresolved strands in the form of time-averaged multithermal broadening of the local differential emission measure. After allowing the coronal heating prescription to vary freely, we found a narrow range of parameters that produce optimal agreement with a set of more than 500 independently measured intensities at solar minimum and maximum. The values of these parameters are consistent with those predicted by models of imbalanced magnetohydrodynamic turbulence, but this does not necessarily exclude contributions from other heating processes.

Ultracompact binary systems, consisting of two compact objects in an orbit <inline-formula><tex-math>$\lesssim 0.5 {\rm R}_\odot$</tex-math></inline-formula>, should exhibit measurable rates of orbital period change (<inline-formula><tex-math>$\dot{P} \ne 0$</tex-math></inline-formula>) due to the emission of gravitational waves (GWs). Measurements of <inline-formula><tex-math>$\dot{P}$</tex-math></inline-formula> have so far been limited to the shortest-period ultracompact binaries (<inline-formula><tex-math>$\lesssim 20$</tex-math></inline-formula> min). Among the AM CVn-type subclass, several works have proposed the presence of extra angular momentum loss beyond GW emission, with magnetic braking being a widely discussed mechanism. If present, this magnetic braking would dominate the angular momentum loss of AM CVn-type binaries with orbital periods <inline-formula><tex-math>$\gtrsim 30$</tex-math></inline-formula> min. In this work, we present a long-term eclipse timing study of two AM CVn-type binaries, YZ LMi and Gaia14aae, with respective orbital periods of 28.3 min and 49.7 min and continuous observations since 2006 and 2015. Both systems show <inline-formula><tex-math>$\dot{P}$</tex-math></inline-formula> consistent with zero within <inline-formula><tex-math>$2\sigma$</tex-math></inline-formula>. Their <inline-formula><tex-math>$3\sigma$</tex-math></inline-formula> upper limits are <inline-formula><tex-math>$1.1 \times 10^{-13}\, {\rm s \, s}^{-1}$</tex-math></inline-formula> and <inline-formula><tex-math>$9.7 \times 10^{-14}\, {\rm s \, s}^{-1}$</tex-math></inline-formula>, respectively. These non-detections are most simply explained by a scenario in which secular angular momentum loss is not substantially stronger than GW emission at all orbital periods, but is combined with deviations from the secular <inline-formula><tex-math>$\dot{P}$</tex-math></inline-formula> whose time-scales span decades but whose amplitude is <inline-formula><tex-math>$\lesssim 10^{-13}\, {\rm s \, s}^{-1}$</tex-math></inline-formula>. Our non-detections of <inline-formula><tex-math>$\dot{P}$</tex-math></inline-formula> represent a limit on the strength of any enhanced angular momentum loss beyond pure GW emission.

The extreme Oe star AzV 493 is known to show unusual photometric and spectroscopic variability that suggest the presence of an unseen companion in a highly eccentric and long-period (7.3 or 14.6 yr) orbit. We obtained a Chandra/ACIS observation near the putative periastron for the 7.3 yr orbit to test for transient X-ray emission that would confirm its binary nature. Our data only place an upper limit to the X-ray luminosity of L<SUB>X</SUB> &lt; 2.5 × 10<SUP>33</SUP> erg s<SUP>−1</SUP> based on the 0.5─8 keV flux limit. Additionally, we obtained 4 new spectroscopic observations with the M2FS spectrograph at Magellan and 20 archive FLAMES/GIRAFFE and X-Shooter spectra from ESO/VLT to further constrain the possibility of radial velocity (RV) variation. Statistical analysis of the RV measurements yields inconclusive results regarding the existence of variations. We discuss possible mass limits for a potential companion, which may be a black hole, in the event that the variations are real. The violet-to-red Balmer ratio has also recently inverted, which may be a further indication of a companion.

We present new regular black hole solutions in general relativity (GR) within a static, spherically symmetric framework governed by a variable equation of state, following the approach of [Class. Quant. Grav. 42, 025024 (2025)]. The matter supporting these geometries is identified as a purely magnetic monopole configuration of the Maxwell-Faraday tensor in the context of nonlinear electrodynamics (NLED). We explicitly reconstruct the corresponding NLED Lagrangian and analyze the asymptotic and central behaviors of the solutions. The geometric structure is examined through the metric functions, the regularity of the Kretschmann scalar, and the profiles of energy density and pressures, including a discussion of the resulting energy conditions. Using Event Horizon Telescope observations of Sgr A*, we constrain the model parameters by comparing the predicted size of the central dark region with the inferred observational images, taking into account the effective geometry experienced by photons in the presence of NLED. Finally, we investigate the dynamical stability of these configurations under scalar perturbations by computing the quasinormal mode spectrum and performing a time-domain analysis.

We present a framework to probe intrinsic stochastic fluctuation in the orbital phase evolution of long-lived double white dwarf binaries through gravitational-wave observations with LISA. To capture the essential structure of the fluctuation, we introduce a minimal quadratic statistic that isolates its temporal correlation. We derive a simple analytic scaling relation for the signal-to-noise ratio of this correlation statistic, explicitly showing its dependence on the total observation time and the intrinsic phase correlation time.

We study primordial gravitational waves (GWs) generated from first-order phase transitions (PTs) during cosmic reheating. Using a minimal particle physics model, and a general parametrization of the inflaton energy density and the evolution of the Standard Model temperature, we explore the conditions under which PTs occur and determine the corresponding PT parameters (the PT temperature, duration, and strength), which depend on the evolution of the background during reheating. We find that, in certain cosmological scenarios, PTs can be delayed and prolonged compared to the standard postinflationary evolution. Incorporating these PT parameters, we compute the resulting GW spectrum generated from the various processes occurring during a first-order PT. We find that, in comparison to the standard cosmological history, the GW amplitude and peak frequency can be modified by several orders of magnitude due to the cosmological evolution during reheating. In particular, the GW spectra could be within the reach of next-generation GW observatories.

Massive bosonic fields can form self-gravitating solitonic structures, which for vector fields are known as Proca stars. For ultralight fields, these structures can describe the cores of dark matter haloes surrounding the supermassive black holes at the center of galaxies. It has been argued that future gravitational-wave detectors might be able to probe the properties of dark matter structures. However, most of the analyses considering ultralight dark matter have focused on massive scalar fields. In this work, we study how Proca stars respond to a perturbing small object inspiralling in their interior, in the Newtonian limit. We consider both Newtonian spherically symmetric Proca stars and the non-spherically symmetric ground-state solutions. We compute the total energy lost by the orbiting object and compare it to the case where the bosonic star is composed of a scalar field. Our results show that, at the Newtonian level, the energy lost by the object in a Proca star ground-state is similar to that in its scalar field counterpart, with relative differences of at most $\sim 20\%$, while the maximum orbital energy loss rate in the ground-state configuration can be one to two orders of magnitude larger than in the spherically symmetric Proca star, when a central parasitic black hole is present. Our work motivates the need to study extreme mass-ratio inspirals into Proca stars using a fully relativistic setup, where differences between the Proca and scalar boson star ground states are expected to become more significant.

Primordial black holes (PBHs) evaporate through Hawking radiation, emitting high-energy photons and ionising their surrounding environment. As contributors to the density of dark matter, Omega_C, if 10^-18 solar mass PBHs are present in the Universe they delay recombination and move the surface of last scattering of the cosmic microwave background (CMB). We perform recombination simulations using the software Recfast, and calculate the PBH fraction of dark matter required to resolve the Hubble tension. We find that nominally a cosmic PBH energy density of Omega_PBH ~ 10^-3 Omega_C would cause an 8.9% increase in the value of H_0, enough to entirely reduce the tension between the early- and late-time observations. This PBH fraction is modified by Gray Body Factors affecting Hawking radiation. Furthermore, also fitting the CMB leaves the Hubble tension not fully relieved with our present PBH prescription. Until relevant non-gravitational properties of the dominant dark matter species are ruled out, we suggest that the hypothesis that the ionisation history of the universe matches the thermal history of the standard LCDM cosmology is too precarious to hang the expansion rate on, and that it is better to measure H_0 locally at z <~ 1.

The magnetosphere, protecting the Earth from intense solar activity, is also shaped by the solar wind, while its structure is still uncertain in observation. In this study, we map the X-ray emission in the magnetosphere, which is induced by the charge exchange between the highly-ionized solar wind and the neutral gas around the Earth, known as the magnetospheric solar wind charge exchange (SWCX). In particular, we extract the magnetospheric SWCX in the O VII line emission data adopted from the XMM Line Emission Analysis Program (X-LEAP). The observed magnetospheric SWCX shows an enhanced emission of approximately $I_{\rm OVII}^{\rm mag}\approx 2$ photons $\rm cm^{-2}~ s^{-1}~sr^{-1}$ toward the Sun, showing a consistent shape predicted by numerical simulations. Furthermore, this magnetospheric SWCX exhibits a dependence on the XMM pointing direction, which traces the path length of SWCX emission in the Earth's magnetosphere. Building on this directional dependence, we model the 3D magnetosheath structure using soft X-ray observations for the first time, constraining the averaged boundary geometry and SWCX emissivity distribution over 22 years. Finally, utilizing the XMM data, we derive an empirical O VII emission efficiency of $α_{\rm OVII}=(2.1\pm0.4) \times 10^{-16}\ {\rm eV\,cm^{2}}$.

We study the confinement phase transition in SU($N_{c}$) pure Yang-Mills theory at finite $θ\neq 0$ using the Improved Holographic QCD (IHQCD) model. We show that the critical temperature, as a function of $θ$ for large but fixed $N_{c}$, is reduced, thus decreasing the amount of supercooling in the confinement phase transition. Upon completion of the confinement phase transition, a network of domain walls can be produced, owing to the multi-branched vacuum structure of Yang-Mills theory at finite $θ$. We highlight the potential interplay between the produced domain walls and the confinement phase transition dynamics. We emphasize that DW production from bubble coalescence in strongly coupled non-conformal FOPTs is a dynamical process of vacuum assignment, hydrodynamics, and local reheating effects, all potentially affecting the approach towards the scaling regime. Lastly, we demonstrate the level of tuning necessary for potentially interesting imprints from gravitational waves through domain wall annihilation and its interplay with the confinement PT.

A robust understanding of selection effects in galaxy cluster studies is crucial for both astrophysical and cosmological applications. Examining clusters identified through different observational strategies, even in small numbers, helps to illuminate potential biases inherent to each method. We selected four rich galaxy clusters in the northern hemisphere whose early Swift X-ray Telescope (XRT) observations indicated unusually low central X-ray emission, making them unlikely to be detected in X-ray surveys. Spectroscopic follow-up confirms that all four systems are genuine galaxy clusters, rather than projections of multiple clusters or groups along the line of sight. Their optical richness, estimated using one of the baseline Euclid cluster richness estimators, implies masses of log M<SUB>200</SUB>/M<SUB>⊙</SUB> ∼ 14.6 and independently confirms the absence of additional massive structures along the line of sight. Deep XRT follow-up reveals highly disturbed X-ray morphologies: three clusters exhibit at least two distinct X-ray peaks, while the remaining cluster has an axis ratio exceeding 1.5. Spectroscopy shows that galaxies associated with different parts share the same redshift, demonstrating that these substructures are physically connected rather than chance projections. These clusters display low central X-ray surface brightness and total X-ray luminosities suppressed by roughly one dex for their richness, making them undetectable in X-ray surveys as eROSITA. We estimate ∼20% as a lower limit for the poorly sampled population, albeit based on a small sample. Our results demonstrate that even rich clusters in the northern z &lt; 0.3 Universe can be missed by X-ray selection and that the X-ray variety captured by X-ray surveys underestimates the true cluster diversity.

Gravitational waves from extreme mass-ratio inspirals (EMRIs) will enable subpercent measurements of massive black hole parameters and provide access to the demographics of compact objects in galactic nuclei. During the LISA mission, multiple EMRIs are expected to be detected, allowing statistical studies of massive black hole populations and their formation pathways. We perform hierarchical Bayesian inference on simulated EMRI catalogs to assess how well LISA could constrain the astrophysical population using parameterized population models. We test our inference framework on a variety of populations, including heterogeneous and homogeneous mixtures of parameterized subpopulations, and scenarios in which the assumed model is deliberately misspecified. Our results show that population parameters governing distributions with sharp features can be tightly constrained. Mixed populations can be disentangled with as few as ∼20 detections, and even with model misspecification, the inference retains sensitivity to key population features. These results demonstrate the capabilities and limitations of EMRI population inference, providing guidance for constructing realistic astrophysical population models for LISA analysis.

This study investigates the ionospheric response to the moderate geomagnetic storm of 25-26 August 2018 (Dst<SUB>min</SUB> ≈ -174 nT), using GPS-derived Total Electron Content (TEC) observations from six stations distributed across low and mid latitudes in the Indian and Asian sectors. A systematic detrending approach was applied to remove diurnal trends and long-period background variations, thereby isolating storm-driven disturbances. Continuous Wavelet Transform (CWT) analysis of the detrended TEC revealed statistically significant (95% confidence against first-order autoregressive (AR1) red-noise background) storm-associated oscillations in the 30-90-min range, corresponding to atmospheric gravity waves (AGWs) and medium-scale traveling ionospheric disturbances (MSTIDs). Low-latitude stations exhibited the strongest signatures, consistent with equatorial electrodynamic coupling and Equatorial Ionization Anomaly (EIA) modulation. A segmented CWT analysis effectively captured the pre-storm, main-phase, and recovery-phase evolution. These findings highlight coherent equatorward propagation of storm-induced AGWs/TIDs and demonstrate the utility of multi-station TEC-wavelet diagnostics for storm-ionosphere coupling studies.

Gravitational wave (GW) microlensing induced by compact dark matter (DM) offers an unparalleled opportunity to explore the fundamental nature of dark matter, as it enables the detection of optically invisible compact objects that are otherwise inaccessible to electromagnetic observations. Conventional approaches for identifying such lensed GW signals suffer from inherent drawbacks: matched-filtering algorithms struggle with the complex, parameter-sensitive diffraction patterns of wave-optics microlensing due to template bank limitations. Additionally, weak lensed signals can be easily obscured by background noise, leading to challenges in maintaining high detection reliability in low signal-to-noise ratio (SNR) regimes. In this work, we introduce the Wavelet Convolution Detector (WCD), a deep learning framework tailored to identify wave-optics diffraction imprints in lensed GW signals. The WCD integrates multi-scale wavelet analysis into residual convolutional blocks, enabling efficient extraction of subtle time-frequency interference structures characteristic of microlensing. To ensure generalization to realistic astrophysical scenarios, the model is trained on a physically motivated synthetic dataset that incorporates realistic distributions of compact DM masses, lens redshifts, and lensing probabilities. Evaluated on simulated binary black hole (BBH) events injected into Gaussian noise mimicking third-generation GW detector environments, the WCD achieves a test accuracy of 92.2% and an Area Under the Receiver Operating Characteristic Curve (AUC) of 0.966. At a stringent false positive rate of 3%, the model maintains a true positive rate of 86.5%, demonstrating robust performance in distinguishing subtle wave-optics imprints from stochastic noise. This makes the WCD a scalable, efficient tool for large-scale blind searches of compact DM candidates via GW microlensing in the upcoming era of third-generation GW detectors.

The Standard Model (SM) leaves several fundamental questions unanswered, including the origin of neutrino masses, the baryon asymmetry of the Universe, and the nature of dark matter. Motivated by these gaps, we investigate an extension of the SM with an additional local U(1)<SUB> X </SUB> gauge symmetry and a complex scalar singlet that spontaneously breaks this symmetry via its vacuum expectation value. The extended framework naturally accommodates three right-handed neutrinos (RHNs) to ensure anomaly cancellation and implements a type-I seesaw mechanism for active neutrino masses. We utilized Casas-Ibarra parameterization to systematically reconstruct the Yukawa coupling matrix which automatically satisfy the observed neutrino data. Furthermore, we estimate the key parameters of the first-order phase transition and compute the resulting stochastic gravitational wave spectrum, demonstrating that it can lie within the reach of forthcoming experiments such as LISA, DECIGO, BBO, and the Einstein Telescope. The right-handed neutrinos also open a viable path for thermal leptogenesis, providing a unified link between neutrino mass generation, baryogenesis, and gravitational wave signatures. Our results demonstrate that this minimal U(1)<SUB> X </SUB> scenario remains a promising probe for physics beyond the Standard Model, accessible through upcoming gravitational wave and neutrino experiments.

Astronomy and geology: the first deals with myriad, enormous, unreachable objects seen at great distances and at very low resolution; the other investigates one tiny (by comparison) object, Earth, at close range, from planetary scale down to almost atomic resolution in the laboratory. Planetary Science, studying solar system objects, to some extent bridges the gap between astronomy and geology. Astronomers mostly look up; geologists mostly look down. This paper lists cases where studies in one of those fields have contributed to understanding in the other. The central focus is on investigations of the Cretaceous and Paleogene pelagic Scaglia limestone in the Italian Apennine Mountains where geologic studies bear on five classes of solar system objects. The classes are separated by three orders of magnitude in size: the Moon-forming impactor, bolides that form craters on Earth, meteorites, meteor-forming grains, and extraterrestrial dust. The paper closes by calling attention to LIGO, the Laser Interferometer Gravitational-Wave Observatory, in which astronomical discoveries made by looking down have yielded a geological discovery done by looking up. LIGO has detected distant collisions involving black holes and neutron stars by measuring infinitesimal distortions of the Earth, which in turn have shown that at least some of Earth's supply of heavy elements has been created by collisions of pairs of neutron stars—a major contribution to geochemistry and cosmochemistry.

Current models predict that at distances beyond 80 kpc in the Milky Way halo, we can find the earliest escaped stars from the merging Sagittarius dwarf galaxy. However, observational data on the Sagittarius stream at these distances are limited. This study examines an overdensity of red giant branch (RGB) stars potentially linked to Sagittarius merger debris. Using the Magellan Inamori Kyocera Echelle spectrograph of Las Campanas Observatory's Clay Telescope, we measured the radial velocities and metallicities of these stars. We compared their properties with model predictions of Sagittarius's disruption and other stellar tracers from the Dark Energy Spectroscopic Instrument Data Release 1 and RR Lyrae catalogues. Our spectral analysis confirms the significant tight clustering of four of these RGB stars in full 6D phase space. This tight clump is embedded within a larger spur-like feature of the Sagittarius stream in the southern sky. A comparison with Sagittarius stream models further strengthens this hypothesis and shows that this far spur could be composed of stars originally in the halo of the Sagittarius dwarf galaxy, stripped in the earliest phases of the interaction. The metallicity dispersion of the four stars of <inline-formula><tex-math>$0.15 ^ {+0.17} _ {-0.08}$</tex-math></inline-formula> around the average of [Fe/H] = <inline-formula><tex-math>$-1.46 ^ {+0.11} _ {-0.09}$</tex-math></inline-formula> is very low. This study provides the first spectroscopic view of the distant southern spur of Sagittarius, composed of stars likely stripped from Sagittarius's halo.

Accreted stars in the Milky Way (MW) preserve information about their galaxies of origin in their chemo-dynamical properties. In this study, we use the chemo-dynamical signatures in the merger debris to approximate the posterior distribution of the properties of disrupted satellites at their accretion. Adopting a simulation-based inference framework, we train an ensemble of normalizing flows using samples of merger debris from the Auriga simulations. Applying this methodology to a local sample of accreted stars in the MW, we infer the lookback times, stellar and halo masses, and halo mass merger ratios of several known accretion events: Gaia-Enceladus-Sausage (GES), Helmi streams, Heracles, I'itoi, LMS-1/Wukong, Sagittarius, Sequoia, and Thamnos. Our predictions for accretion time and mass align with previous estimates and follow the expected stellar mass─metallicity relation across redshifts. The total accreted stellar mass from these events is predicted to be <inline-formula><tex-math>$2.2^{+1.1}_{-0.6}\times 10^{9}~\rm {M_{\odot }}$</tex-math></inline-formula>, with the GES and Sagittarius dwarfs being the largest contributors. The total accreted stellar mass from all disrupted progenitors is <inline-formula><tex-math>$1.4^{+1.1}_{-0.4}\times 10^{9}~\rm {M_{\odot }}$</tex-math></inline-formula>, which is consistent with previous mass measurements of this component. We provide a prediction for the evolution of the MW halo mass until the accretion of Sagittarius (<inline-formula><tex-math>$z\approx 1$</tex-math></inline-formula>): We find that the halo mass growth from the time of its first merger (<inline-formula><tex-math>$z\approx 5$</tex-math></inline-formula>) to <inline-formula><tex-math>$z\approx 2$</tex-math></inline-formula> significantly exceeds the total mass of the known progenitors accreted during that interval, suggesting the presence of unidentified substructures. Our estimate of the Galaxy halo mass after the Sagittarius merger, but prior to the accretion of the Magellanic Clouds, is <inline-formula><tex-math>$5.9^{+1.4}_{-1.1}\times 10^{11}~\rm {M_{\odot }}$</tex-math></inline-formula>.

Heliospheric density variations impart delays on pulse times of arrivals from millisecond pulsars. Improper modelling of these variations may affect gravitational wave detection and characterisation by pulsar timing arrays (PTAs). Currently, PTAs typically employ a time-varying, spherically symmetric heliosphere model, which does not capture the full spatial and temporal complexity of the heliosphere. Instead, we investigate whether a three-dimensional, time-dependent model of the inner heliosphere from interplanetary scintillation (IPS) measurements - the IPS-UCSD model - can be employed to mitigate the solar wind in PTA analyses. We applied the IPS-UCSD model to the MeerKAT PTA 4.5-year dataset to assess whether it could correct for heliospheric density variations, and the impact on GW sensitivity compared to a spherically-symmetric model. We find that the model does not accurately correct for heliosphere-induced timing distortions, leading to bias in recovered GW parameters. Using simulations, we show that the spherically symmetric heliosphere model also fails to fully capture heliospheric density variations like those in the IPS-UCSD model. However, if interstellar dispersion measure (DM) variations are also modelled, then the heliospheric model errors are partially absorbed by DM variations, reducing contamination of the GW signal. Therefore we find that a time-varying spherically symmetric model is sufficient to mitigate the effect of heliospheric time delays on recovered GW results at typical PTA radio frequencies, provided other signal components are also modelled. We propose that the most precisely timed pulsars may be used to improve data-driven heliospheric density models in the future.

We use diffuse Galactic high energy gamma ray data from LHAASO and Fermi-LAT to constrain gravitationally produced decaying dark matter (DM). Focusing on four benchmark candidates: a dark photon, a heavy right-handed neutrino (RHN), a pseudo-Nambu-Goldstone boson (pNGB), and a nonminimally coupled scalar we derive bounds on the DM mass and its couplings to the visible sector. For dark photons, RHNs, and pNGBs, the combined data constrain the effective interaction strength to <inline-formula><mml:math><mml:mo>≲</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>30</mml:mn></mml:mrow></mml:msup><mml:mo>)</mml:mo></mml:math></inline-formula> for DM masses <inline-formula><mml:math><mml:mo>≳</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:mi>TeV</mml:mi><mml:mo>)</mml:mo></mml:math></inline-formula>. For the nonminimally coupled scalar, this interaction strength is limited to <inline-formula><mml:math><mml:mo>≲</mml:mo><mml:mi>O</mml:mi><mml:mo>(</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>10</mml:mn></mml:mrow></mml:msup><mml:mo>)</mml:mo></mml:math></inline-formula>, for the same DM mass range. Moreover, photon-dark photon oscillations yield strong constraints for massive dark photon beyond 10 GeV, closing a region of parameter space previously left unconstrained.

We investigate primordial non-Gaussianity in the inflation without an inflaton framework, where scalar perturbations are generated at second order by primordial gravitational waves in Einstein gravity on an exact de Sitter background. Since scalar modes are produced nonlinearly from tensor modes, non-Gaussianity is an intrinsic prediction of the mechanism. We compute the corresponding scalar bispectrum, derive the relevant contribution to the three-point function of the scalar potential, and evaluate its shape numerically. Similar to the scalar power spectrum, we find that the bispectrum depends on the number of observed e-folds <inline-formula><mml:math><mml:msub><mml:mi>N</mml:mi><mml:mrow><mml:mi>obs</mml:mi></mml:mrow></mml:msub></mml:math></inline-formula> through the ultraviolet cutoff. We show that the bispectrum shape is enhanced toward squeezed configurations, but its amplitude becomes strongly suppressed once the scalar power spectrum is normalized to the observed value. The resulting non-Gaussianity at cosmic microwave background scales is therefore negligibly small, well below present observational sensitivity.

We investigate the observational constraints on <inline-formula><mml:math><mml:mi>α</mml:mi></mml:math></inline-formula>-attractor inflationary models and their postinflationary reheating dynamics in light of the latest cosmic microwave background (CMB) data from ACT DR6 combined with Planck18, <inline-formula><mml:math><mml:mi>BICEP</mml:mi><mml:mo>/</mml:mo><mml:mi>K</mml:mi><mml:mi>e</mml:mi><mml:mi>c</mml:mi><mml:mi>k</mml:mi></mml:math></inline-formula> 2018, and DESI (collectively denoted P-ACT-LB-BK18). Focusing on both E- and T-type attractor potentials, we analyze how inflationary observables, namely the scalar spectral index <inline-formula><mml:math><mml:msub><mml:mi>n</mml:mi><mml:mi>s</mml:mi></mml:msub></mml:math></inline-formula> and the tensor-to-scalar ratio <inline-formula><mml:math><mml:mi>r</mml:mi></mml:math></inline-formula>, are indirectly influenced by reheating parameters such as the reheating temperature <inline-formula><mml:math><mml:msub><mml:mi>T</mml:mi><mml:mi>RH</mml:mi></mml:msub></mml:math></inline-formula>, the inflaton equation of state <inline-formula><mml:math><mml:msub><mml:mi>w</mml:mi><mml:mi>ϕ</mml:mi></mml:msub></mml:math></inline-formula>, and consequently the inflaton's couplings to Standard Model (SM) particles. We incorporate constraints from primordial gravitational wave (PGW) overproduction via <inline-formula><mml:math><mml:mi>∆</mml:mi><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math></inline-formula> bounds, which are particularly relevant for stiff postinflationary dynamics. Additionally, we include theoretical constraints from Coleman-Weinberg (CW) one-loop radiative corrections, which impose an upper bound on the couplings, as well as inflaton self-resonance, a nonperturbative effect that can occur even in the absence of inflaton-SM interactions and imposes a strong lower limit, particularly significant for <inline-formula><mml:math><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mn>3</mml:mn><mml:mo>≲</mml:mo><mml:msub><mml:mi>w</mml:mi><mml:mi>ϕ</mml:mi></mml:msub><mml:mo>≲</mml:mo><mml:mn>0.7</mml:mn></mml:math></inline-formula>. Our analysis shows that E-models allow a broad range of reheating scenarios, including matter-like reheating (<inline-formula><mml:math><mml:msub><mml:mi>w</mml:mi><mml:mi>ϕ</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:math></inline-formula>), whereas the T-model yields more restrictive results and remains viable only for <inline-formula><mml:math><mml:msub><mml:mi>w</mml:mi><mml:mi>ϕ</mml:mi></mml:msub><mml:mo>&gt;</mml:mo><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn></mml:math></inline-formula> for all three nongravitational interaction channels, <inline-formula><mml:math><mml:mi>ϕ</mml:mi><mml:mo>→</mml:mo><mml:mover><mml:mi>f</mml:mi><mml:mo>̄</mml:mo></mml:mover><mml:mi>f</mml:mi></mml:math></inline-formula>, <inline-formula><mml:math><mml:mi>ϕ</mml:mi><mml:mo>→</mml:mo><mml:mi>b</mml:mi><mml:mi>b</mml:mi></mml:math></inline-formula>, and <inline-formula><mml:math><mml:mi>ϕ</mml:mi><mml:mi>ϕ</mml:mi><mml:mo>→</mml:mo><mml:mi>b</mml:mi><mml:mi>b</mml:mi></mml:math></inline-formula> are considered. For stiffer equations of state, an intermediate coupling window survives, bounded from above by CW corrections or <inline-formula><mml:math><mml:mn>2</mml:mn><mml:mi>σ</mml:mi></mml:math></inline-formula> observational limits, and from below by the combined PGW and self-resonance constraints. We derive updated bounds on inflaton couplings for both decay and scattering channels and identify parameter regions consistent with successful perturbative reheating. These results establish a robust connection between inflationary dynamics, reheating physics, and particle interactions, and provide concrete targets for upcoming precision CMB observations.

We examine extreme mass-ratio inspirals (EMRIs) as probes of beyond-vacuum general relativistic effects, accounting for both astrophysical environments and scalar Gauss-Bonnet (sGB) gravity. In beyond-vacuum scenarios, the evolution of an EMRI immersed in a cold dark matter environment modifies the gravitational wave flux and introduces additional dissipative effects such as dynamical friction. In parallel, in the beyond-general-relativistic settings such as in sGB gravity, the inspiraling object carries an effective scalar charge and emits scalar radiation. Both environmental and modified-gravity effects modify the flux-balance law, thereby inducing changes in the EMRI dynamics. Using a two-timescale analysis within the fixed-frequency formalism, we compute leading-order corrections to the energy fluxes for quasicircular, equatorial orbits in static, spherically symmetric spacetimes and construct the corresponding gravitational waveforms, which are used to quantify the accumulated gravitational wave dephasing and waveform mismatch relative to the vacuum general relativistic case. We further perform the Fisher information matrix analysis to estimate parameter correlations and the ability of future space-based detectors such as the Laser Interferometer Space Antenna to disentangle environmental and modified-gravity effects. Our results show that both dark matter and scalar-field effects can leave measurable imprints on EMRI waveforms and that a consistent beyond-vacuum treatment is essential for robust tests of gravity.

We derive new exact gravitational wave solutions with dynamical torsion and nonmetricity tensors in the framework of cubic metric-affine gravity (MAG). For this purpose, we consider the full algebraic classification of the gravitational field in general metric-affine geometries and impose a set of type N conditions on the field strength tensors that implement the kinetics of torsion and nonmetricity in a particular cubic MAG model, recently considered to eliminate ghostly instabilities from the vector and axial sectors of the theory. The new solutions represent pp waves characterized by a metric function that includes the dynamical contributions of the torsion and nonmetricity tensors provided by the field equations of the model. In particular, these quantities induce a scalar polarization mode in the gravitational-wave spectrum, thus offering a distinctive phenomenological signature beyond the ordinary tensor polarization modes of general relativity.

The recent success of gravitational-wave (GW) astronomy together with renewed plans for lunar geophysical instrumentation has revived interest in using the Moon as a resonant detector for midfrequency (mHz─Hz) GWs. In realistic observational scenarios, the GW strain amplitude is expected to be constrained independently by networks of GW detectors, which motivates an inverse, tomographic question: to what extent can measurements of the Moon's seismic response to known GWs be used to infer its internal structure? In this Letter, we develop a first-principles, perturbative framework that maps spherically symmetric perturbations of the elastic and density structure to measurable changes in observables, especially GW-driven modal amplitudes of the Moon. The formalism combines (i) a normal-mode representation of the elastic response, (ii) first-order perturbation theory for eigenvalues and eigenfunctions, and (iii) a linearized observation model that links frequency and amplitude observables to model parameters (bulk and shear moduli, density, and interface locations) and their perturbations. We show that the estimation errors of the Moon's elastic parameters can be reduced by about an order of magnitude with observations of calibrated GWs.

The cosmological collider program probes ultra-high-energy physics by searching for subtle oscillatory signatures induced by inflationary particle exchange. Due to the non-linear symmetries usually assumed in inflation, these signals do not appear in isolation; moreover, their amplitudes are bounded by perturbativity and unitarity. To comprehensively probe cosmological collider physics, we must jointly analyze the full multi-field inflationary Lagrangian: in this work, we conduct such a study, probing single, double, and triple scalar field exchange and self-interactions across a wide range of masses (in both the complementary and principal series), mixings, and sound-speeds. Using modern theoretical tools (including the cosmological bootstrap and the cosmological flow), we construct a vast library of tree-level primordial bispectra: combining these with recent measurements of the inflationary shape function from Planck and modern sampling techniques, we perform Bayesian exploration of the eight-parameter multi-field likelihood. Most previous studies of cosmological collider physics assume weak mixing, such that the coupling between the scalar field and the Goldstone mode can be treated perturbatively: in our companion study, we demonstrate that this assumption is incompatible with current datasets except at the smallest masses. By solving for the inflationary bispectra numerically using CosmoFlow, we perform the first analysis of the strongly-mixed collider, demonstrating that the Planck constraints are much tighter than the theoretical bounds, though the oscillatory contributions are heavily suppressed. Across the full multi-field landscape, we find no evidence for new physics with a maximal $χ^2$ improvement of $5.3$.

Recent DESI results are in tension with the constant dark energy density predicted by the $Λ$CDM model. If dark energy is associated with the vacuum energy of a scalar field in a metastable state, it will undergo a first-order phase transition through the nucleation of bubbles containing a reduced dark energy density. In this paper, we explore the consequences of this model, where dark energy density varies in both space and time. We model a single bubble spacetime using the Israel junction conditions and derive each of the usual distance measures in this inhomogeneous cosmology. We find that the model predictions of Alcock-Paczynski distortion have features that align strikingly well with the DESI measurements if the dark energy phase transition occurred at roughly a redshift of 1.4 and if the bubble of lower dark energy density has roughly 10$\%$ less dark energy density than the outer cosmology. Despite this feature, we find that the dark energy bubble is heavily constrained by the CMB which excludes the region of parameter space that reproduces the DESI BAO measurements. Still, the peculiar features in the distance measurements of the dark energy bubble cosmology serve as a useful toy model to motivate and inform future work in the currently poorly explored area of spatially varying dynamical dark energy.

We investigate how localized quantum corrections to the inflaton potential affect preheating dynamics and the resulting stochastic gravitational wave (GW) spectrum. When these corrections sufficiently suppress the quadratic term of the potential near its minimum, inflaton self-resonance can produce a peaked GW spectrum. On the other hand, we find that a smooth and enhanced spectrum can appear if the quadratic term acquires a negative coefficient. As a concrete realization, we analyze the $α$-attractor T-model with a one-loop Coleman--Weinberg correction induced by a heavy scalar and compute the resulting GW spectra using lattice simulations. The GW signals lie in the ultra-high-frequency regime at frequencies above the kHz range. These results suggest that GW signals from preheating may probe quantum corrections to the inflaton potential, thereby providing indirect information about the underlying UV physics.

We compute, for the first time, the Hawking emission spectrum of massive vector (Proca) fields by spinning Kerr black holes, determining the associated greybody factors and the resulting mass and spin loss functions. We show, in particular, that the scalar (longitudinal) polarization of the Proca field has a spectrum approaching that of a free scalar field in the massless limit (in which it becomes a pure gauge mode), although we find substantial differences for finite mass. The contribution of the two vector (transverse) polarization modes coincides, as expected, with the one obtained by Page for the Maxwell field in the massless limit. The black hole's evaporation rate is dominated by the scalar mode for slowly spinning black holes and by the two vector modes as the black hole approaches extremality. As for other fields, we find that Proca Hawking emission is Boltzmann-suppressed for Hawking temperatures $T_H\lesssim |μ-Ω_H|$, where $μ$ is the field mass and $Ω_H$ is the angular velocity of the black hole's horizon. This implies that highly spinning black holes can efficiently emit massive vector fields at temperatures parametrically below the field's mass. Finally, we also find that superradiant emission is more pronounced for massive vector fields, with a maximum amplification factor of $\simeq 7\%$ (compared to $\simeq 4\%$ for massless photons).

We investigate the nonradial quasinormal-mode spectrum of static charged Ellis--Bronnikov wormholes in Einstein--Maxwell theory minimally coupled to a phantom scalar field. The background solutions are known in closed form and comprise three classes: subcritical, critical and supercritical, which all approach the extremal Reissner--Nordström geometry at the boundary of their domain of existence. We derive the linear perturbation equations for axial and polar sectors, including the coupled gravitational, electromagnetic and phantom-scalar degrees of freedom, and compute the corresponding spectra by means of a Chebyshev spectral method. The uncharged limit reproduces the known Ellis--Bronnikov spectrum and exhibits the expected electromagnetic isospectrality. For charged configurations we track the axial and polar branches across the three families of solutions and identify the effect of the charge on the damping times and oscillation frequencies. In particular, we find that charge can substantially reduce damping rates as the extremal Reissner--Nordström limit is approached. We also uncover a nonradial polar instability, most clearly visible in the fundamental $l=2$ branch for sufficiently large wormhole masses. This instability is distinct from the familiar radial Ellis--Bronnikov instability and shows that the nonradial sector imposes additional constraints on the dynamical viability of charged wormholes.

The Nancy Grace Roman Space Telescope's Wide Field Instrument (WFI) will observe hundreds of thousands of bright stars across its Core Community Surveys, particularly in the dense stellar fields of the Galactic Bulge Time Domain Survey (GBTDS). Sources brighter than ~17th magnitude will saturate WFI detector pixels in typical survey exposures, with the brightest stars deeply saturating large pixel regions and potentially producing persistence signals that may impact subsequent observations. Prior detector characterization did not explore the regime of deep point source saturation. To address this gap, we conducted a bright star saturation test during WFI's second Thermal Vacuum test campaign (TVAC2) at BAE Space & Mission Systems in Boulder, CO. Using the Stimulus of Ray Cones (SORC) telescope simulator, we projected nine in-focus point sources through the F146 filter onto two Sensor Chip Assemblies (SCAs), with fluxes tuned to approximate stellar magnitudes ranging from ~4 to ~18 in ~170 s exposures. We present analyses of the saturation response and persistence properties of these detectors. We find that the saturated region of a ~4 mag source grows to ~150 pixels in diameter after ~170 s of illumination, compared to ~15 pixels for a ~12 mag source. Pixels adjacent to the expanding saturation front exhibit pronounced non-linear behavior consistent with charge leakage from saturated neighbors. For persistence, we find that the median signal in the first post-illumination dark exposure is broadly consistent across source magnitudes spanning ~4 to ~17, and that persistence decays to detector background levels ($\lesssim$0.05 e$^{-}$ s$^{-1}$) within approximately 20 minutes, consistent with flat field persistence measurements from the same TVAC2 campaign. These pre-flight characterization results inform community understanding of WFI detector response to prepare for Roman science.

We analyze the vacuum dynamics of Gravity from Entropy, including its algebraically constrained $G$-field formulation. Evaluating the curvature traces over zero-, one-, and two-form sectors, we show that the complete Minkowski Hessian is exactly that of the quadratic-gravity action $A R+B R_{μν}R^{μν}$, with $A=3β/\ell_{\rm P}^{4}$ and $B=5β^{2}/(2\ell_{\rm P}^{4})$. For diagonalizable curvature blocks, the same action reduces to a sum over eigenvalue logarithms and reproduces these coefficients exactly. A strict diagonal-curvature restriction on the perturbations is instead only a reduced subsector and excludes non-diagonalizable type-N wave curvatures. Linearizing the $G$-field equations and subsequently imposing the algebraic vacuum constraint reproduces the same reduced metric equation and covariant Minkowski Hessian. The spectrum contains the massless graviton, a scalar with $m_{0}^{2}=3/(5β)$, and an opposite-residue spin-2 branch with $m_{2}^{2}=-6/(5β)=-2m_{0}^{2}$. For the foundational choice $β>0$, conventional Einstein normalization therefore implies a tachyonic spin-2 instability. We also show that every four-dimensional Ricci-flat metric solves the local bulk equations through quadratic curvature order, while square-zero Ricci-flat pp-waves are exact local vacuum solutions of the analytic metric-only logarithmic branch. On the isolated massless transverse-traceless eigenspace, the quadratic translation current has the standard general-relativistic normalization.

We constrain the class of modified cosmologies derived in Paper I [1] from a generalized mass-to-horizon relation (MHR) that enforces thermodynamic consistency between the Cai-Kim horizon temperature and generalized horizon entropies. The modified Friedmann equations depend on an entropy exponent $m$, an MHR coupling parameter $γ$, and an entanglement-correction amplitude $f_B$, with standard $Λ$CDM recovered in the appropriate limit. Using Pantheon$+$/SH0ES Type~Ia supernovae, cosmic chronometers, DESI DR2 baryon acoustic oscillations, and Planck 2018 CMB distance priors, we constrain eight physically motivated sub-cases via Markov chain Monte Carlo and compare models through the Bayesian log-evidence. The entropy exponent is tightly bounded, $|m-1|\lesssim O(10^{-4})$ when the MHR coupling is fixed ($γ=1$), relaxing to $O(10^{-3})$ along the $m$-$γ$ degeneracy, excluding any macroscopically significant departure from standard horizon thermodynamics. Freeing the MHR coupling parameter or the entanglement amplitude raises the inferred Hubble constant to $h\simeq0.70$-$0.71$, reducing the CMB-SH0ES tension from ${\sim}4σ$ to ${\sim}1.2$-$2.6σ$, but no scenario fully resolves it within a flat universe. The Bayesian log-evidence nevertheless disfavors every extension relative to $Λ$CDM in all dataset combinations ($-16\lesssimΔ\ln Z \lesssim-1$): the improved fits obtained when the SH0ES calibration is included reflect an absorption of the Hubble tension by the additional parameters rather than genuine evidence for modified horizon entropy.

As they orbit within the Galaxy, stars swim through a vast population of interstellar objects (ISOs). In this paper, we use N-body simulations to show that a fraction of ISOs within $\sim$1 pc of the Sun (its tidal radius) may be captured during the flyby of another star -- a mechanism that requires no planets. Capture is most efficient when the impulse imparted by the flyby is comparable to the escape speed at the widest stable orbit, which is roughly 0.1 km/s for the Sun. ISO capture is dominated by the few highest-impulse stellar flybys, typically involving relatively slow encounters with massive stars. Most ISOs are captured in the outer parts of the Oort cloud, with semimajor axes greater than $\sim$50,000 au. Using Monte Carlo simulations, we show that the Sun underwent only a small number of ISO-capturing flybys in its history (median [mean] of 1 [1.7]). Using the {Ō}tautahi-Oxford population model, we estimate that a few times $\sim$$10^{4}$ `Oumuamua-sized ISOs were likely captured by the Sun. This only represents a $\sim$$10^{-8\pm1}$ contribution to the total Oort cloud population, yet it contains roughly as many present-day captured ISOs as Jupiter-assisted capture provides. Given that flybys are unavoidable in the Galactic field, most stars should, at any given time, host have sparse Oort clouds populated with ISOs captured during stellar flybys. Massive stars are both the main drivers of capture when they fly by a given star, and more efficient at capturing ISOs around themselves than low-mass stars.

Magnetic white dwarfs can convert photons into axions in their strong magnetic fields, with the conversion probability modulating the light curve as the star rotates. However, this observable is degenerate with intrinsic stellar variability if the background is modeled too simplistically. We develop a controlled background-degeneracy framework that computes the axion-induced modulation from reconstructed stellar magnetic fields while fitting it simultaneously with a flexible Fourier model of the intrinsic light curve. Applying this framework to TESS observations of PG~1015+014 using two independent magnetic field reconstructions, we find that a sinusoidal stellar background can produce an apparent preference for nonzero axion-photon conversion. This preference is absorbed once the background light curve includes the second harmonic, indicating that higher-harmonic stellar variability is a leading degeneracy for precise photometric axion searches in magnetic white dwarfs. Interpreting the two-harmonic fit as a conservative baseline, we obtain competitive constraints for sub-$μ\mathrm{eV}$ axions. We further derive an analytic target-ranking estimate for other TESS magnetic white dwarfs, identifying systems where phase-resolved magnetic modeling would be most valuable for competitive axion probes.

We investigate gravitational memory beyond null infinity by studying finite-distance null hypersurfaces endowed with Carrollian geometry. We show that the intrinsic degenerate geometry, optical data, and null Brown--York tensor of a finite null screen define a quasilocal Carrollian dissipative system, providing a natural framework to characterize the residual geometric response after the passage of radiation. To make this construction explicit and compare it with the standard asymptotic description, we use Robinson--Trautman spacetimes as an exactly solvable radiative setting. For asymptotically flat Robinson--Trautman geometries, we transform the solution to Bondi gauge and extract the asymptotic data directly in terms of the Robinson--Trautman field. In the linearized sector, the Bondi shear is purely electric and produces the standard displacement-memory effect associated with the relaxation toward the final Schwarzschild geometry. We show that the leading large-radius tracefree component of the finite-screen memory reduces to the Bondi displacement memory, while finite-distance corrections retain additional focusing, embedding dependence, angular drift, Coulombic data, and near-zone information. Thus, Bondi memory emerges as the universal asymptotic projection of a broader quasilocal Carrollian response. Late-time screens approaching the final Schwarzschild horizon exhibit exponentially decaying non-isotropic Carrollian data, leaving only the isotropic null Brown--York stress. As a by-product, we construct the Robinson--Trautman solution with nonzero cosmological constant to second order in the radiative amplitude and use the holographic dictionary to study the associated energy fluxes and find that the resulting charge does not obey a universal monotonicity property.

The Johannsen-Psaltis (JP) metric is constructed by introducing deviation parameters into the Kerr metric. The presence of these deviation parameters provides additional degrees of freedom for testing the extent to which astrophysical black holes are consistent with the Kerr paradigm and for examining the validity of the no-hair theorem. Since black hole imaging provides one of the most direct means of probing the properties of astrophysical black holes, a detailed investigation of the imaging characteristics of JP black holes constitutes a natural first step toward employing the JP metric to explore potential deviations from the Kerr geometry in astrophysical black hole systems. Accordingly, numerical backward ray-tracing simulations are employed to conduct a comprehensive investigation of the image structures, intensity distributions, and redshift-blueshift signatures of JP black holes with both closed and non-closed event horizons. The dependence of these imaging characteristics on the deviation parameter is also investigated systematically. Furthermore, the emergence of an extended inner shadow, distinct from the original inner shadow of the black hole, in images of JP black holes with non-closed event horizons motivated a detailed investigation of its physical formation mechanism using both numerical backward ray-tracing simulations and an approximate analytical framework.

Quantum field theory (QFT) in curved spacetime has led to profound predictions, including the Unruh effect and Hawking radiation, yet their direct observation remains extraordinarily challenging because of their extremely weak signatures. Here, we show that the transverse-field Ising model provides a quantum simulator for Majorana fermions in a Schwarzschild black hole background. Remarkably, different coordinate representations of the same spacetime-Schwarzschild, tortoise, Kruskal, and conformally flat-map onto distinct microscopic Ising spin models. Despite their microscopic differences, these models converge in the continuum limit to the same Majorana field theory, exhibiting an emergent form of general covariance. This provides a rare example of a fundamental symmetry of general relativity arising as an emergent property of a condensed matter system. We further demonstrate how black hole particle production can be simulated and detected through spin correlation measurements, and discuss experimental platforms capable of realizing these models. Our work establishes a practical route for investigating fermionic QFT in curved spacetime using controllable quantum many-body systems and tabletop experiments.

We study higher-rank generalized Killing tensors with mixed symmetries, providing a couple of examples and applications. It is shown that such objects naturally exist in higher-dimensional rotating black hole spacetimes, where they arise as partially contracted "squares" of Killing-Yano tensors and display interesting algebraic and differential properties. Motivated by conformal Killing tensors, a generalization of these objects that gives rise to parallel-transported tensors along null geodesics is proposed and shown to exist in higher-dimensional rotating black hole spacetimes.

We study the finite-time accretion of complex massive scalar wave packets by a Schwarzschild black hole in the test-field regime, with parameters motivated by ultralight fuzzy dark matter around supermassive black holes. Our goal is to determine how the scalar content of a localized configuration is redistributed after interacting with the black hole, and which spectral and multipolar components are more efficiently absorbed. We decompose the Klein--Gordon field into independent multipolar sectors and evolve nearly monochromatic Gaussian packets mode by mode, reducing the problem to a set of 1+1 dimensional evolutions. Accretion is quantified with the flux of the conserved Noether current through the horizon surface, providing a direct measure of the scalar charge absorbed by the black hole. For a carrier radial wavenumber $k_0$ and multipole index $\ell$, we construct accretion-efficiency maps in the $(k_0,\ell)$ plane that contain the fraction of accreted modal charge. These maps exhibit a transition between inefficient, partial, and efficient accretion regimes, which we relate to the structure of an effective potential. We show that the process is controlled by the ratio between the Schwarzschild radius $R_s$ and the reduced Compton wavelength $\lambdabar_C$. For $R_s \lesssim \lambdabar_C$, the transition is broad and dominated by the angular momentum barrier, while for $R_s > \lambdabar_C$ it sharpens across a narrower range of $k_0$ and a partial-accretion floor emerges at low $k_0$. These results provide a time-domain, Noether-charge-based classification of black hole accretion for massive scalar wave packets.

This work presents atmospheric modeling of multi-wavelength spectra for eight cool white dwarfs with carbon-enriched atmospheres, with four each of spectral type DQ and DQp. The Deslanders-d'Azambuja bands of C2 are detected in all six DQ stars that have data covering wavelengths shorter than 4000A. These bands are blue shifted in the DQp types, consistent with that observed for the Swan bands, and reproduced with the same pressure distortion model. The coolest stars in the sample show significant near-infrared flux suppression, identified here as collision-induced absorption from dense helium, with trace hydrogen [H/He]<-6, estimated from the absence of CH features. Notably, the near-ultraviolet through near-infrared spectral energy distribution of WD0038-226 is correctly reproduced using an atmospheric model with [H/He]=-6.8; however, beyond 2um the predicted flux is too high, and a carbon-free atmosphere is able to reproduce the mid-infrared observations well. The multi-wavelength coverage permits accurate modeling characterization of these stars, including constraining the ionization equilibrium in fluid helium and the resulting density profiles. The densities found in this work are 2-3 times higher than determined in previous studies, approaching 2g/cm3 for the coolest stars. Excluding the uncertain high masses obtained for the two coolest DQp stars, the sample stars have masses that are broadly consistent with those found for the wider white dwarf population, with no difference across the DQ-DQp temperature boundary.

We study the optical appearance of a rotating regular Bardeen black hole embedded in perfect fluid dark matter (PFDM) when photon propagation occurs through a plasma medium. Three plasma models are examined: a homogeneous distribution, a radially varying distribution, and a general distribution with both radial and angular dependence. The influence of plasma on photon motion and the resulting shadow morphology is analysed using shadow observables. To assess astrophysical viability, the plasma and PFDM parameters are constrained using the Event Horizon Telescope bounds on shadow circularity and fractional diameter deviation. The results demonstrate that environmental effects arising from both PFDM and plasma produce measurable modifications to the shadow, indicating that black-hole imaging can provide useful constraints on the surrounding medium as well as the intrinsic properties of regular rotating black holes.

We present an improved version and further validation of a method, originally introduced by Stodilka (2016), for reconstructing horizontal velocity fields in the solar atmosphere from physical parameters typically derived from spectroscopic observations through inversion techniques. This approach relies on the continuity equation and the assumption of negligible vertical vorticity. Several algorithmic modifications were implemented to allow application to large spatial grids, including the compact storage of a sparse matrix of the system of linear equations. The method was tested using snapshots from the realistic 3D MHD Bifrost simulation en024048_hion of the solar atmosphere, covering heights from 20 to 980 km. Horizontal velocities were reconstructed from model density and vertical velocity values. A sinc filter with a Lanczos window was applied to the reconstructed horizontal velocity maps to reduce artefacts related primarily to the use of horizontal periodic boundary conditions. In the photospheric layers, the reconstructed horizontal velocity fields show a high level of agreement with the model values, with the Pearson correlation coefficient in the range 0.8-0.9. The method performs best within granules, whereas larger discrepancies occur in intergranular lanes due to complex counter-streaming flows. In the chromospheric layers, the reconstruction quality decreases significantly with height, consistent with the increasing importance of vortex motions and the breakdown of the underlying assumption. The method provides a reliable and efficient tool for reconstructing horizontal flows in the solar photosphere. The proposed improvements make the method applicable to large observational datasets.

We present a semi-analytical framework for the halo mass function (HMF) in decaying dark matter (DDM) cosmologies, in which dark matter decays into a massive daughter particle inheriting a velocity kick $v_k$ and a massless dark radiation component. Building on the Press-Schechter formalism, we encode the DDM physics through a spherical collapse model that explicitly tracks the decay-induced mass loss, yielding a modified, mass-dependent critical collapse threshold $δ_c(M_0)$ and a mapping $M_{\rm coll}(M_0)$ between the initial Lagrangian mass and the collapsed halo mass. The critical threshold exhibits a characteristic transition between two analytically tractable plateaus: a large-mass limit, where all daughter particles are retained by the halo, and a small-mass limit, where all daughters escape and the collapse is equivalent to that of a dark matter species decaying entirely into dark radiation, making $δ_c$ independent of $M_0$ and $v_k$. We provide semi-analytical results and fits for both limits and a fitting formula for the transition, whose single free parameter $M_1 \propto v_k^3\,\tildeΓ^{-1/2} t_{\rm ta}$ has a transparent physical interpretation: it is the mass scale at which the kick velocity equals the halo orbital velocity. We validate our predictions against a suite of N-body simulations at $z=0$ and $z\approx 1$, finding good agreement across models spanning mild to strong HMF suppression relative to $Λ$CDM. Residual deviations for the largest kick velocities at $z=0$ are observed. Via a halo-by-halo comparison between simulations, we trace the discrepancy to the definition of the halo mass when daughter orbits extend beyond the halo boundary. The resulting fitting functions for $δ_c(M_0,Γ,v_k)$ and $M_{\rm coll}(M_0)$ provide an efficient and accurate route to DDM constraints from current and forthcoming probes of the halo mass function.

The cosmic infrared background, sourced by dust heated by star-forming activity, encodes the integrated history of the star-formation distribution over a large swathe of cosmic time. Decomposing the contribution from galaxies at different redshifts to the observed CIB maps provides a powerful tool for probing the large-scale star-formation history across cosmic times. To this end, we perform a forecast tomographic cross-correlation analysis using simulated photometric galaxy clustering and weak lensing data based on the Euclid mission survey specifications, combined with Planck CIB observations. The analysis adopts templates constructed from the halo occupation distribution model to measure the bias-weighted star-formation-rate density, $\langle b ρ_{\rm SFRD}\rangle$, as a function of redshift. We present forecasts for the $\langle b ρ_{\rm SFRD}\rangle$ constraints based on these simulations and quantify the expected constraining power using Fisher information matrix methods. Compared with results obtained using the same dataset as in previous works, we find that Euclid will tighten the constraints on $\langle b ρ_{\rm SFRD}\rangle$ across all redshift bins by a factor of about $2$ and provide more measurements over a deeper redshift range. This leads to improved constraints on all HOD parameters, typically reducing the marginalized uncertainties by a factor of about $2.5$, while also breaking parameter degeneracies.

Scattered light is a relevant noise source in current ground-based gravitational-wave detectors and a critical design concern for next-generation observatories. Beamtube scattered light estimates usually combine optical propagation simulations with analytical couplings that do not fully propagate the frequency-dependent optomechanical response of the interferometer to the strain readout. In this work, we use improved analytical transfer factors to convert scattered-light field perturbations in the Fabry--Pérot arm cavities into equivalent strain noise, consistently including radiation-pressure coupling, signal-extraction dynamics, microscopic detunings, and the homodyne readout angle. The formalism keeps the full amplitude and phase quadrature content of the scattered field, including the cross terms that arise in both diffraction and backscattering noise. For diffraction, we also identify the regime in which the linearized coupling to baffle motion is valid, avoiding unnecessary phase wrapping. Using representative LIGO, Cosmic Explorer (CE), and Einstein Telescope - Low Frequency (ET-LF) configurations, we show that legacy estimates are recovered in phase dominated regimes, but can differ when radiation-pressure coupling, quadrature correlations, or detuned signal extraction become important. In particular, the revised ET-LF estimate changes substantially with respect to previous beamtube noise budgets due to the detuned signal extraction cavity. These results provide a more complete framework for scattered light noise estimations for present and future gravitational-wave detectors.

Pulsar timing arrays (PTAs) are the only means to observe nanohertz gravitational waves (GWs). While current analyses primarily exploit the Earth term, the full detector response encodes additional directional information in the pulsar terms. We develop a GW sky-mapping framework based on the complete Earth--pulsar response and a tensor spherical harmonic decomposition of the GW field. This yields closed-form response functions for an elementary baseline and casts PTA sky reconstruction as a linear inverse problem. We show that a PTA behaves as a diffraction-limited GW observatory whose angular sensitivity is governed by the dimensionless parameter $ωL$, where $ω$ is the GW angular frequency and $L$ is the pulsar distance. The detector response exhibits four distinct regimes: an Earth-term dominated regime, a transition regime, a pulsar-term-dominated regime, and an exponential sensitivity cutoff at $l_{cut}\simeqωL$. This cutoff defines the fundamental angular resolution limit of PTA sky maps. Using Fisher-information and singular-value analyses, we show that the achievable angular resolution is constrained not only by the intrinsic detector response but also by the finite number of pulsars, their sky distribution, and timing noise. In particular, we find that coherent pulsar-term information can improve full-sky gravitational-wave mapping only for PTAs containing of order $N_{trans}\sim10^{11}$ precisely timed pulsars. This result demonstrates that, although the transition to a pulsar-term-sensitive regime exists mathematically, it is inaccessible for realistic PTAs and therefore provides a quantitative justification for the Earth-term approximation adopted in contemporary observations. Finally, we extend the formalism to stochastic GW backgrounds, establishing a unified mathematical framework for PTA sky mapping and anisotropy studies.

Near-IR spectra of a homogeneous sample of 15 red giants in the Galactic nuclear star cluster, within 1.5 pc of SgrA*, have been acquired with X-shooter at the Very Large Telescope. From these spectra, line-of-sight radial velocities and chemical abundances of iron, carbon, oxygen, and other alpha and light elements have been derived. By combining radial velocities from this study and proper motions from the literature, we computed the orbits of these stars, finding that they have been confined within 12 pc from SgrA* for their entire lifetime, thus strongly suggesting an in situ formation and evolution. Iron abundances between half and twice solar; approximately solar-scaled values within ±0.1 dex for α-elements, Ti, and V; some enhancement of [Na/Fe], [K/Fe], and [Al/Fe]; and some depletion of [C/Fe] with respect to the solar ratios have been measured. The inferred chemical abundance distributions are consistent with a formation from a gas enriched by type II and type I supernovae (SNe) over a prolonged timescale, closely matching those of the metal-rich populations of the inner bulge field and other complex stellar systems like Terzan 5 and Liller 1.

Probing inelastic dark matter at large mass splittings requires heavy target nuclei, an extended recoil-energy range, and the high-velocity tail of the dark-matter distribution. We exploit these features with the RES-NOVA prototype detector, featuring a PbWO4 cryogenic calorimeter, produced from archaeological Pb and operated at the deep-underground laboratory of Gran Sasso of INFN (Italy), analyzing a 32.4 g day exposure over 2.5 keV - 1 MeV under both the Standard Halo Model (SHM) and a Large Magellanic Cloud (LMC)-motivated velocity distribution. We extend direct-detection constraints beyond the 330 keV reach of established technologies (e.g. Xe-based TPCs), probing splittings up to 510 (780) keV in the SHM (LMC) benchmark, while future exposures will probe new regions of the parameter space.

GUEST is a space mission concept whose central objective is the detection of gravitational waves (GWs) in the microhertz band -- a physics-rich frequency window that no other present or planned detector can reach at a significant level. The concept is simple: two dense, passive spheres, covered with cube-corner retroreflectors, deployed in {highly eccentric} Earth orbits ($e \gtrsim 0.7$, period $P \gtrsim 33$ h), tracked continuously by the global network of satellite laser-ranging stations over a minimum observation time of 10 years, with an expected total duration of 30 years. The orbits themselves act as resonant detectors of the oscillating gravitational perturbations, with the microhertz sensitivity emerging from the selected orbital parameters. From the same data stream, GUEST delivers a programme of fundamental and applied science that cuts across particle physics, gravitational-wave astronomy, cosmology, astrophysics, and geodesy: the first coherent search for GWs from supermassive black-hole binaries in the $μ$Hz band, the exploration of primordial GW backgrounds in the unexplored energy-scale gap between pulsar-timing arrays and LISA, a dedicated probe of ultra-light dark matter in a parameter region untouched by any other experiment, a new way to search for ultra-light bosons, order-of-magnitude-improved tests of new gravitational interactions at astronomical ranges, and a step change in the absolute determination of $GM_\oplus$ that underpins the Global Geodetic Observing System and future navigation and Earth-observation missions. This white paper presents the motivation, scientific reach, and mission concept of GUEST.

We map the observational predictions of cosmological stasis in the inflationary gravitational wave background onto the sensitivity bands of current and planned gravitational wave detectors. Using the closed-form piecewise spectral template derived in the companion paper, we generate detectability maps for four stasis scenarios: canonical, dynamical scalar, vacuum-energy/matter, and vacuum-energy/radiation across the frequency bands probed by NANOGrav, SKA, LISA, DECIGO, BBO, the Einstein Telescope, and Cosmic Explorer. For scenarios in which the spectrum is suppressed, $w_s < 1/3$, the stasis feature is detectable by BBO in the region of $(w_s,ΔN)$ parameter space in which $w_s\gtrsim 0.2$ for tensor-to-scalar ratios close to the Planck upper limit, r = 0.036. For scenarios in which the spectrum is enhanced, $w_s > 1/3$, the stasis feature is detectable by BBO across the entire $(w_s,ΔN)$ parameter space for tensor-to-scalar ratios of $O(0.01)$. We characterize the Standard Model (SM) $g_*$ fine structure of the IGWB, showing that SM phase transitions introduce spectral steps of $\approx 20\%$ (electroweak, at $\sim 2.6\times10^{-6}$~Hz) and $\approx 53\%$ (QCD, at $\sim 3.6\times 10^{-9}$~Hz). For stasis scenarios with end-of-stasis temperatures below the QCD scale these steps fall inside the stasis band and constitute additional spectral features that complement the primary signature. Finally, we model the finite-width end-of-stasis transition phenomenologically, demonstrating that the spectral break at $f_{end}$ is smoothed over a log-frequency window $ΔN_\mathrm{trans}\times 3(1+w_s)/4$, and that the consistency relation $C^2=C^2(α)$ remains testable provided $ΔN_\mathrm{stasis}\gg ΔN_\mathrm{trans}$, a condition easily satisfied for all scenarios of phenomenological interest.

Gaia DR3 provides astrophysical parameters for hundreds of millions of stars, but the metallicities [M/H] from its GSP-Phot module suffer from systematic biases. We estimate stellar metallicities from Gaia DR3 data using the homogeneous spectroscopic iron abundances [Fe/H] of LAMOST DR10 as training labels. We cross-matched LAMOST DR10 with Gaia DR3 and trained a gradient-boosted decision-tree regressor (XGBoost) on 1.20 million AFGK stars using only Gaia-derived inputs and proxies. We validated the estimates on held-out LAMOST stars, GALAH DR4, APOGEE DR17, and 46 open clusters, and applied the model to measure the radial metallicity gradient of the Milky Way disk. On the held-out test set, the model achieves a mean absolute error of 0.052 dex and $R^2=0.94$ with negligible bias, compared with 0.242 dex for GSP-Phot on the same stars. The estimates transfer well to external surveys, with mean absolute errors of 0.066 dex for GALAH and 0.068 dex for APOGEE. For open clusters, the median difference between our estimated [Fe/H] and spectroscopic values is 0.041 dex, smaller than both GSP-Phot (0.248 dex) and a previous APOGEE-trained XGBoost model (0.067 dex). Applied to the Galactic disk, our model recovers a broken thin-disk radial gradient, with inner and outer slopes of $+0.119$ and $-0.058\,\mathrm{dex\,kpc^{-1}}$, respectively, and a break near 5.9 kpc, as well as an open-cluster gradient of $-0.066\,\mathrm{dex\,kpc^{-1}}$; both agree with previous high-resolution spectroscopic studies. Our [Fe/H] estimates are accurate to 0.05-0.07 dex for AFGK stars with $[\mathrm{Fe/H}]\gtrsim-2.5$; below this limit, the predictions should be treated as lower bounds. The catalogue and trained model are publicly available on Zenodo and are suitable for chemical studies of the Milky Way.

The frozen star is a model for the interior of an astrophysical black hole that is externally indistinguishable from the singular geometry of a Schwarzschild black hole, even though it contains no singularities nor trapped surfaces. The frozen star corresponds to a macroscopic ``BIon'', a solution of a certain type of Born-Infeld Lagrangian coupled to Einstein gravity, which consists of rigid flux tubes sourced by an electric charge localized at its center and a uniform distribution of opposite charge along its outer surface. This configuration is a solution of the effective action that describes the end point of tachyon condensation in string. The rigidity of the flux tubes implies that the star is ultrastable under linear perturbations of the geometry and matter. The star can be effectively deformed, or ``defrosted'', allowing internal pulsations, as expected from regular, horizonless astrophysical black holes. To describe the defrosted star, we need to include a perturbative amount of magnetic-monopole charge and tachyon kinetic energy. This recasts the star as a macroscopic ``DIon'', whose flux tubes can stretch and contract, having both electric and magnetic charges localized at its center and distributed uniformly along its outer surface. Previously, the non-radial oscillations of the defrosted star were analyzed to leading order in a perturbative ``defrosting'' parameter $γ$, by describing the energy-momentum tensor as that of a fluid. Here, we derive the same spectrum in a much simpler way, using the Born-Infeld Lagrangian. Thus, we verify that the sound velocities scale as $γ$ and the parametrically long lifetimes scale as $1/γ^2$. Because the perturbation equations are derived from a Lagrangian, their consistency is guaranteed for arbitrary deviations away from spherical symmetry, facilitating numerical studies on frozen stars away from equilibrium.

We report on a stationary solution of the four-dimensional vacuum Einstein equations, which describes a Lorentzian wormhole harboring two antipodal co-rotating black holes. These are hung from the ``other'' spacelike infinity by two semi-infinite cosmic strings with positive or negative tension. The metric also admits a naked ring singularity for almost all values of the parameters.

Large catalogs of blue horizontal-branch (BHB) stars are essential for studying substructures and kinematics of the Galactic halo. And accurate determination of atmospheric parameters for BHB stars provides insight into stellar evolution. In this work, we perform a systematic search for BHB stars based on LAMOST DR11, and identify $13\,988$ BHB spectra, corresponding to $10\,236$ unique BHB stars. We estimate an identification rate of $\sim80\%-90\%$, and a contamination rate of $\lesssim10\%$ for our sample. Atmospheric parameters for these BHB stars are estimated via the data-driven method named the Stellar LAbel Machine (SLAM). We demonstrate the necessity of including color indices in the spectral labeling to effectively break the degeneracy between effective temperature and surface gravity. We note a bump in the distribution of [Fe/H], and most of these metal-rich BHB stars belong to the disk population. We also provide a list of 4282 blue straggler (BS) stars with determined atmospheric parameters.

We prove a strict dominant energy deformation theorem for asymptotically flat initial data with corners along a hypersurface $Σ$. The deformation preserves a corner condition on the Bartnik data across $Σ$. We show that if the dominant energy condition holds on each side of $Σ$ and the Bartnik data satisfy this corner condition, then the exterior end satisfies $E \ge |P|$ in every dimension $n \ge 3$.

For mid-latitude Rossby waves (RWs) in the atmosphere, the expression for the energy flux for use in a model diagnosis, and without relying on a Fourier analysis or a ray theory, has previously been derived using quasi-geostrophic equations and is singular at the equator. By investigating the analytical solution of both equatorial and mid-latitude waves, the authors derive an exact universal expression for the energy flux which is able to indicate the direction of the group velocity at all latitudes for linear shallow water waves. This is achieved by introducing a streamfunction as given by the inversion equation of Ertel's potential vorticity, a novel aspect for considering the energy flux. For ease of diagnosis from a model, an approximate version of the universal expression is explored and illustrated for a forced/dissipative equatorial basin mode simulated by a single-layer oceanic model that includes both mid-latitude RWs and equatorial waves. Equatorial Kelvin Waves (KWs) propagate eastward along the equator, are partially redirected poleward as coastal KWs at the eastern boundary of the basin, and then shed mid-latitude RWs that propagate westward into the basin interior. The connection of the equatorial and coastal waveguides has been successfully illustrated by the approximate expression of the group-velocity-based energy flux of the present study. This will allow for tropical-extratropical interactions in oceanic and atmospheric model outputs to be diagnosed in terms of an energy cycle in a future study.

Stellar clusters preserve crucial information on the formation and evolutionary processes that shaped the Milky Way (MW) as we see it today. However, several MW clusters still lack sufficient data to constrain their metallicity, ages, and, in some cases, even their basic kinematic properties. We present low-resolution MODS at LBT spectroscopy for five such systems (i.e. Koposov 1, Koposov 2, Muñoz 1, Pfleiderer 2, and RLGC2) from which we derive systemic heliocentric radial velocities (V<SUB>sys</SUB>) with typical uncertainties of ≤10 km s<SUP>−1</SUP>, and metallicities based on the equivalent widths of the infrared Ca II triplet measured in red giant branch members. For Pfleiderer 2 and RLGC2, we provide the first spectroscopic determinations of their systemic velocities and metallicities, for which we find V<SUB>sys</SUB> = 6 ± 5 km s<SUP>−1</SUP> and −313 ± 6 km s<SUP>−1</SUP>, and [Fe/H] = −0.75 ± 0.09 dex and −2.33 ± 0.13 dex, respectively. For the other three clusters, we find results that are consistent with the existing literature. Thanks to our new spectroscopic measurements, we were able to perform an orbital analysis to investigate their origin. We find that Pfleiderer 2 likely formed within the MW, RLGC2 is dynamically associated to the Gaia─Sausage─Enceladus accretion event, and Koposov 1 was likely stripped from the Sagittarius dwarf spheroidal while Muñoz 1 is only tentatively associated with the latter system. Koposov 2, at a high orbital energy, does not show a clear association with any known progenitor system.

Magnetars are a subclass of neutron stars with ultrastrong surface magnetic fields. Some magnetars exhibit persistent hard X-ray emission, characterized by power-law tails with photon indices around 1─1.5, extending from ∼10 keV to several hundred kiloelectron volts. The leading explanation for this hard X-ray component is resonant Compton scattering, in which the thermal seed photons are upscattered by relativistic electron─positron pairs flowing along magnetic field lines in the magnetosphere. In this work, we adopt the pair outflow framework of the magnetar magnetosphere and calculate the resonant Compton scattering opacity, as well as the spectrum and polarization of the upscattered emission. We find that resonant cooling can substantially modify the magnetospheric plasma density and impose strong optical depth constraints on the hard X-ray emission regions. Under the viewing geometry inferred from the Imaging X-ray Polarimetry Explorer, an equatorial twist near the stellar surface provides a viable configuration for the NuSTAR hard X-ray spectrum of 4U 0142+61, while a polar-twist geometry is disfavored. Joint spectral, timing, and polarimetric modeling will be essential for distinguishing between the magnetospheric scattering geometries and understanding the physical properties of the pair plasma.

The behaviour of the pressure drag generated by trapped lee waves and upward-propagating internal waves in non-hydrostatic, stratified flow over a mountain ridge is investigated as a function of nonlinearity. A two-layer atmosphere is adopted, with piecewise-constant static stability and a uniform wind profile. The lower layer, between the surface and <mml:math><mml:mrow><mml:mi>z</mml:mi><mml:mo>=</mml:mo><mml:mi>H</mml:mi></mml:mrow></mml:math>, has stability <mml:math><mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msub></mml:math>, and the upper layer extends indefinitely above with stability <mml:math><mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>, where <mml:math><mml:mrow><mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>&lt;</mml:mo><mml:msub><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>. Simulations are performed with a numerical model suitable for flows ranging from the microscale to the mesoscale, and nonlinearity is varied solely by increasing the mountain height. Linear reference values are obtained from the theoretical results reported by Teixeira et al., (2013). Two configurations are considered: (i) one in which trapped-lee-wave drag dominates over the drag due to propagating waves, and (ii) another in which the two components are of comparable magnitude. A set of diagnostics is introduced to clarify the physical processes associated with increasing nonlinearity. The results show that, as nonlinearity increases, the evolution of the total drag and its components is governed not only by the amplitude of the trapped lee waves, but primarily by the waveguide efficiency, which depends on the upstream Scorer parameter profile and the stable layer depth. This efficiency dictates whether the energy extracted from the incident flow through its interaction with the orography is largely retained and recycled within the trapped mode, or is instead transferred to propagating components and to processes associated with detuning and saturation. These findings may have implications for drag parametrisation in global climate and weather-prediction models.

Sterile neutrinos are well-motivated extensions of the Standard Model, introduced to address fundamental questions such as the origin of neutrino masses and the nature of dark matter. Exploiting the precise data reconstruction achieved by the CUPID-0 experiment, we searched for spectral distortions in the double β-decay of <SUP>82</SUP>Se compatible with the emission of a sterile neutrino. The analysis relies on the construction of a detailed background model down to 200 keV, enabling an accurate characterization of the main sources of contamination. Using a Zn<SUP>82</SUP>Se exposure of 9.95 kg·yr, we explored sterile neutrino mass hypotheses between 0.5 MeV and 1.5 MeV. No evidence for a signal was observed in any scenario; therefore, we derived 90% C.I. upper limits on the active-sterile mixing probability sin<SUP>2</SUP> θ, obtaining the most stringent bound, sin<SUP>2</SUP> θ &lt; 8 × 10<SUP>-3</SUP>, for a sterile neutrino mass of 0.7 MeV.

We report observations of faint (10<SUP>−6</SUP>), concentric, planet-wide rings in the polarized flux of sunlight that is reflected by Venus, obtained during a serendipitous, 36-minute run in 2010, with the highly sensitive Extreme Polarimeter (ExPo) on the William Herschel Telescope. The rings appear to be centered slightly downwind of the subsolar point, are visible in different filters across the visible, and are not obvious in the simultaneous total flux observations. ExPo's dual-beam exchange and double-differencing design strongly suppresses first-order instrumental polarization, and we could not identify an instrumental cause of the observed pattern. Because ExPo was dismantled before the rings were identified in the data, this is the only set of observations of these rings. We are therefore careful in claiming the detection of a new atmospheric phenomenon on Venus. However, numerical radiative transfer simulations show that planet-wide rings in polarization can arise owing to density variations of 5%─10% in the gas above the clouds, consistent with a gravity wave. Our simulations also show that such density variations would not show up in total flux observations. By presenting our observations and numerical simulations, we hope to motivate new polarimetric observations of Venus that could confirm or refute the presence of such planet-wide waves.

We evaluate the Baryon Acoustic Oscillation (BAO) signal in the bispectrum as a tool to detect and characterize the relative velocity effect. We extend the existing framework by presenting an updated model for the redshift-space tree-level bispectrum that comprehensively incorporates all relative velocity terms. We introduce a novel, unbiased technique to extract the isotropic BAO dilation parameter ($α_{\rm iso}$) solely from the bispectrum monopole. Validated against N-body simulations, this template-based extraction successfully recovers the acoustic scale and enhances the statistical constraining power by $\sim30\%$, when analyzed in tandem with the pre-reconstruction power spectrum, offering a powerful complement to standard post-reconstruction pipelines. We quantify how individual relative velocity components distort both two- and three-point statistics. We find that these effects induce distinct systematic shifts in the extracted $α_{\rm iso}$ between the two probes. We find systematic discrepancies of up to $2\%$ for $b_{v^2}=\pm0.05$ and up to $20\%$ for $b_{δ^{bc}}\le-2$. This differences demonstrate that a direct comparison of independent power spectrum and bispectrum BAO measurements can break parameter degeneracies and isolate the amplitude of these biases. Finally, we provide a concrete prescription to detect and constrain the three associated relative velocity bias parameters, showing that a joint analysis is highly sensitive to the $b_{v^2}$ and $b_{δ^{bc}}$ amplitudes. This establishes the bispectrum BAO as a robust cosmological probe for current and next-generation galaxy surveys, serving both as a cross-check for standard analyses and a crucial diagnostic tool against systematic biases.

Quantum gravity impacts Standard Model fields through effective interactions generated by renormalization. These appear as Standard Model Effective Field Theory (SMEFT) operators, where the effects of new physics are parametrized by the values of the higher-order SMEFT coefficients. As a step towards predicting these SMEFT coefficients from the asymptotically safe Standard Model with quantum gravity, we investigate the flow of a representative set of four-fermion operators under gravitational fluctuations. We strengthen the evidence for the near-perturbative nature of asymptotic safety by finding that these dimension-six-interactions remain irrelevant, resulting in the prediction of specific values for the dimensionless ratios of SMEFT coefficients in the IR. We also find a mechanism that can make a specific subset of these operators relevant. This subset is determined by symmetry considerations. On this basis, we provide a categorization of dimension-six-SMEFT interactions into two categories. Interactions in the first category are predicted to be non-zero, but Planck-scale suppressed. Interactions in the second category are predicted to be zero at small gravitational coupling, but may become free parameters of the theory at larger gravitational couplings.

We construct a locally scale-invariant formulation of Schrödinger geometry in symmetric teleparallel gravity. The unique scale transformation preserving autoparallelism, torsionlessness and the Schrödinger affine structure is first identified, from which a quadratic scale-invariant action is obtained. Using the Palatini formalism, we show that the conditions required for scale invariance are exactly those ensuring that the affine connection is dynamically reduced to the Schrödinger form. Our results establish a direct correspondence between local scale symmetry and length-preserving affine geometry, providing a new geometric framework for scale-invariant metric-affine gravity.

We investigate the role of large separation, rotational correction order, structural resolution, and non-adiabatic effects in modelling the rotating $δ$ Scuti star V1790 Ori. From TESS data, we extract 69 frequencies and determine $Δν\simeq 82$ $μ$Hz. Rotating MESA models are computed at low and high resolution; their pulsation frequencies are calculated with GYRE (adiabatic/non-adiabatic, first-order rotation) and FILOU (adiabatic, second-order rotation). Using $Δν$ as a structural constraint is necessary to reduce model degeneracy. For the selected minimum-misfit reference model, considering only the 40 modes with consistent $(n,\ell,m)$ labels, the RMS$_{40}$ theoretical frequency differences are 0.442 $μ$Hz (resolution), 0.062 $μ$Hz (non-adiabatic), and 2.962 $μ$Hz (GYRE vs FILOU); including all 48 frequencies gives RMS$_{48}$ values of 1.033, 2.326, and 3.931 $μ$Hz. Relative to observations, higher resolution reduces residuals from 4.457 to 4.387 $μ$Hz (RMS$_{40}$) and from 4.715 to 4.682 $μ$Hz (RMS$_{48}$); non-adiabatic effects change them marginally to 4.381 and 4.673 $μ$Hz. FILOU gives the largest residuals: 5.331 $μ$Hz (RMS$_{40}$) and 5.270 $μ$Hz (RMS$_{48}$). Second-order rotation produces the largest frequency shifts, but improving agreement with observations requires denser grids and self-consistent FILOU optimisation. The 260.672 $μ$Hz peak -- previously identified as the fundamental radial mode -- shows uncertain identification. The results should be interpreted as diagnostics of modelling systematics and mode-identification robustness.

Whether the principles of general relativity extend to a quantum regime remains one of the open questions in modern physics. In classical general relativity, Einstein's equivalence principle underpins the interpretation of gravity as spacetime geometry. However, it is not known whether the principle remains valid when the source of gravity could be quantum. Here we show that gravity-induced entanglement (GIE) of quantum clocks provides a framework to characterize local position invariance (LPI) in such a regime, which constitutes one of the subprinciples of the equivalence principle. By adapting the existing formulation of quantum LPI to the context of GIE, we analyze two schemes that address complementary aspects of LPI and differ only in the choice of the initial clock state: one in which entanglement will be generated if and only if there is a genuinely quantum violation of LPI, and the other where classical-like LPI violation manifests in the frequency of entanglement oscillation. Our results suggest that GIE of quantum clocks offers an approach to investigating fundamental principles of general relativity in the quantum regime, shedding new light on the interplay between quantum mechanics and the theory of gravity.