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Measurement of the expansion structure of ejecta in supernova remnants (SNRs) provides important information about their explosion mechanism. We combine Chandra proper-motion measurements with XRISM Doppler measurements to derive an approximate, region-averaged three-dimensional velocity map of the Fe-K-emitting ejecta in Cassiopeia A (Cas A), one of the youngest Galactic core-collapse SNRs. Proper motions were measured using the optical flow method. We find that the Fe-K-emitting ejecta expand predominantly outward, except in the western region, with projected velocities of up to ~4580 km/s. The southeastern ejecta exhibit the highest three-dimensional velocity, reaching 5160+-320 km/s, based on the combined proper-motion and Doppler measurements. This velocity exceeds those measured for intermediate-mass elements (IMEs) such as Si and S, and the corresponding ejecta exhibit a relatively small line-of-sight velocity dispersion. These results are broadly consistent with the previously suggested inversion between the Fe-rich and IME-rich ejecta layers. The apparent inward motion or emergence of a new Fe-K emission component in the western region may reflect interaction with dense surrounding material, although the present data do not allow us to distinguish uniquely between these possibilities or to determine their physical origin.

The construction of accretion environments is fundamental to black hole imaging. From a purely geometric perspective, we construct a novel analytic accretion environment comprising a geometrically thick disk, ring-like bumps with a Gaussian profile, and localized compact emission regions modeled by Gaussian distributions. This environment offers high flexibility, enabling independent adjustments of disk thickness, vertical structure, and the positions and morphologies of localized spots, thereby allowing it to qualitatively mimic high-energy astrophysical phenomena. Applying this model to the Kerr spacetime, we investigate the resulting images via radiative transfer and ray-tracing simulations. The results validate the effectiveness of our accretion model and reveal novel observational signatures of Kerr black holes under multi-component illumination, including multiple bright spots and arc-like structures. This work provides a convenient and fully analytic framework for modeling accretion in curved spacetimes, and offers a new perspective on inferring accretion mechanisms and transient high-energy processes from image features.

We investigate strong gravitational lensing by an Ellis-Bronnikov (EB) wormhole in the presence of effective field theory (EFT) corrections to photon propagation. We derive the modified photon propagation law induced by non-minimal couplings between the electromagnetic field and spacetime curvature, which leads to polarization-dependent photon trajectories in the EB wormhole spacetime. Using the strong deflection limit, we derive the corrections to the photon sphere and the logarithmically divergent part of the deflection angle. Although the EFT corrections are parametrically small, their contributions can become appreciable near the photon sphere. In particular, the $R_{μν}F^{μρ}F^ν{}_ρ$ interaction, which is nonvanishing for the EB wormhole but absent in the Schwarzschild spacetime, contributes to the strong-deflection coefficients. This may provide a distinctive lensing signature for observationally distinguishing wormholes from black holes.

We investigate whether squeezing can provide an observable signature of quantum gravitational waves. Because a realistic detector couples only to a particular wave-packet mode, squeezing in global source modes need not remain observable. We show that inflationary two-mode squeezing reduces to an unsqueezed thermal state in the accessible one-mode sector, phase incoherence washes out squeezing in stochastic backgrounds, and the limited coverage of the solid angle of detectors strongly suppresses squeezing from isolated sources. We then show that source squeezing is not essential, {\it i.e.}, a quantized gravitational wave can generate a positive squeezing witness if the detector state is initially prepared in a squeezed state, whereas a classical external gravitational field cannot, producing only a displacement. However, the resulting signal is bounded by the extremely small graviton--detector coupling. Thus, detector squeezing can remove the need for squeezed incident waves, but not the suppression caused by weak gravitational interaction.

We present the most complete modelling of the MOJAVE 1.5 Jansky Quarter Century active galactic nuclei (AGN) jet population, using likelihood-free simulation-based inference. Due to the complex impact of a flux-limit on observed AGN data sets, careful modelling of the parent population is required. In particular, when observing and fitting to multiple data distributions likelihoods become non-intuitive. Parameter degeneracies further complicate the problem and make it suitable for likelihood-free, simulation-based inference. This method relies on a normalising flow learning the likelihood surface or posteriors directly. We extensively validate the flow to show that previous parameter estimates for the AGN jet speed distributions underestimated parameter errors significantly and do not capture the non-gaussianity of the parameter posteriors. The new results enable a better statistical comparison to other AGN population studies, but also a more accurate comparison of supermassive black hole jets with their lower mass counterparts, X-ray binaries (XRB). We find that the AGN follow a Lorentz factor distribution of the shape N(Γ)∝Γ<SUP>b</SUP> with <inline-formula><tex-math id="TM0001" notation="LaTeX">$b= -1.32_{-0.19}^{+0.20}$</tex-math></inline-formula>. This slope is consistent with the XRB Lorentz factor distribution at 2σ. Simulation-based inference as a method is generally well-suited to many astrophysical problems, and this paper shows the convenient applicability of this methodology to parent population studies of jetted AGN with multiple observables specifically.

The vector spherical harmonics (VSH) are crucial ingredients for representation of arbitrary vector fields in curvilinear coordinate systems throughout mathematical physics. Performant, efficient computations of the VSH are essential to high-performance computing applications that utilize spectral analysis, including magnetohydrodynamics (MHD) simulations for geophysics and astrophysics. The free open-source FORTVSH library provides a catalog of numerical routines for evaluating various VSH forms and related mathematical functions. This manuscript establishes the theoretical basis, addresses the computational need for efficient VSH solvers, and presents a comprehensive overview of the FORTVSH routines.

Magnetic reconnection in partially ionized plasmas is an important channel for energy release. While the plasmoid instability is well characterized in 2D fully ionized plasmas, its behavior in the presence of neutral-dominated plasma remains poorly understood in the nonlinear, high-Lundquist-number ($S = 10^5$) regime. We present high-resolution ($16384 \times 4096$ cells) two-dimensional two-fluid (ion $+$ neutral) simulations of Harris-sheet reconnection with upstream plasma beta $β= 2$, comparing fully ionized and partially ionized (ionization fraction $ξ= 10^{-1}$ and $10^{-2}$) regimes. Neutral-ion decoupling accelerates the linear tearing stage and alters the plasmoid hierarchy: the large-scale ``monster'' plasmoid that dominates the fully ionized case is suppressed, and the sheet instead fragments into a dense chain of sub-scale plasmoids. Below the neutral-ion decoupling scale $\ell_{\rm dec}$, ions concentrate into the plasmoids, reaching peak overdensities $ρ_i/ρ_{i,0} \approx 10$ ($ξ= 10^{-1}$) and $3-5\times10^{3}$ ($ξ= 10^{-2}$), while the neutrals remain comparatively smooth. This local pile-up raises the ionization fraction and recouples the two fluids within the plasmoids. Measured from the out-of-plane electric field at the reconnection sites, the reconnection rate in the $ξ= 10^{-2}$ case achieves $R_{\rm rec}\approx0.01$, whereas the $ξ= 10^{-1}$ case rises to a rate $\approx0.02$ and further $0.035$ when apparent coalescence occurs. In the $ξ= 10^{-2}$ case, the ambipolar drift drives a rapid ion inflow $\sim0.5\,v_{A,0}$ into the layer at the same reconnection sites, far above the neutral inflow velocity $\sim0.1\,v_{A,0}$. Here, $v_{A,0}$ is the upstream total Alfvén speed.

We performed numerical simulations to investigate how coronal heating and magnetic field strength regulate the dynamics and morphology of coronal rain in the solar corona. Coronal rain is widely interpreted as a manifestation of thermal non-equilibrium and thermal instability. Many previous studies have investigated condensation under (quasi-)steady heating scenarios, but such heating models do not fully explain the observed properties, including multiple clumps and symmetric drainage toward both loop footpoints. Condensation driven by low-frequency impulsive heating has been much less explored and may produce distinct coronal rain morphology and drainage dynamics. The role of the coronal magnetic field also remains poorly understood. We conducted 1.5-dimensional magnetohydrodynamic simulations along a single strand with self-consistent coronal heating, upon which a single impulsive heating event was imposed. Two key parameters were systematically explored: the duration of localized heating and the strength of the coronal background magnetic field. We find that condensation can be triggered even by a single impulsive heating event, provided that the injected energy is about an order of magnitude larger than that in the steady heating case. Condensation becomes more likely for longer heating durations and weaker magnetic fields. The dynamics of coronal rain depend strongly on the heating duration. When the heating duration is shorter than the radiative cooling timescale, condensations drain toward both loop footpoints. We also find that weaker magnetic fields enhance the nonlinearity of Alfvén waves, promoting fragmented condensations. These results demonstrate that the heating duration and coronal magnetic field strength play key roles in regulating coronal rain formation and dynamics.

Stochastic gravitational-wave backgrounds (SGWBs) provide a unique opportunity to probe both unresolved astrophysical populations and fundamental physics in the early Universe. Future space-based gravitational-wave (GW) detectors, such as LISA and TAIJI, will enable cross-correlation observations that are particularly sensitive to SGWBs and their polarization content. In this work, we investigate the capabilities of two proposed LISA-TAIJI networks, LISA-TAIJIp and LISA-TAIJIm, for detecting and discriminating tensor, vector, and scalar polarization components of isotropic SGWBs. Using the cross-correlation between the two networks, we evaluate their sensitivities and signal-to-noise ratios for SGWBs containing different combinations of polarization sectors. We find that the LISA-TAIJIm configuration, which has a larger relative inclination between the two constellations, exhibits substantially weaker polarization degeneracies and significantly improved performance in separating polarization components, particularly for tensor-vector-scalar backgrounds. Fisher-matrix forecasts for power-law SGWBs show that the two configurations yield broadly comparable constraints on spectral parameters, reflecting the different roles of component separation and parameter estimation. Our results demonstrate that large-angle LISA-TAIJI networks provide a more favorable geometry for model-independent SGWB polarization measurements.

We construct and compare two charged rotating extensions of the Kerr--Levi-Civita geometry. In Einstein--Maxwell theory, a fixed magnetic Kerr--Newman Ernst representative is inverted, and covariance of the coupled Ernst equations, integrability of the dragging and electric-potential quadratures, and the field equations are verified. The resulting local line element agrees with a strong-field representative obtained independently, but no global equivalence of the azimuthal quotients is assumed. In low-energy heterotic string theory, the Hassan--Sen map is instead applied after the vacuum Kerr--Levi-Civita inversion, generating Maxwell, dilaton, and Kalb--Ramond fields. Both branches possess regular local Killing horizons. For the Einstein--Maxwell branch, an exact denominator factorization proves that the subextreme exterior contains neither Ernst zeros nor azimuthal closed timelike curves. The former Kerr ring has finite curvature, although it lies inside an interior region of timelike azimuthal orbits. Its Kretschmann scalar has the form $8\mathcal P_{\rm N}/\mathcal H^6$ and approaches a parameter-independent Levi-Civita law at fixed off-axis latitude. The heterotic branch is qualitatively different: the real component connected to the horizon terminates at a finite-radius $Λ=0$ surface. Exact string-frame slice factorizations and independent Einstein-frame calculations show that this surface is a curvature singularity rather than a conformal-frame artifact. Both families are generically Petrov type I. The results distinguish exact local solution generation from the unresolved construction of complete global spacetimes and the identification of possible distributional sources.

We present a comprehensive study of the ultraviolet (UV)-bright stellar population in the globular cluster NGC 3201 using the Ultraviolet Imaging Telescope (UVIT) onboard AstroSat. By combining far-UV photometry of the UVIT F148W and F169M filters with Gaia and ground-based telescope optical data, we construct UV$-$optical color-magnitude diagrams (CMDs) to identify UV-bright stellar evolutionary phases. Physical parameters of all detected sources are derived by fitting stellar atmosphere models to the observed multi-wavelength spectral energy distributions (SEDs). The effective temperatures of blue horizontal branch stars (BHBs) span the range 7250$-$11250 K. We identify two extreme horizontal-branch stars (EHBs) with effective temperatures of approximately 25,000 K and 17,000 K; one lies within the cluster core radius ($r_\mathrm{c}$=1.3$'$), while the other is located outside. We also detect two candidate ``gap'' objects in the transitional phase between HB stars and white dwarfs, which may represent hot blue hook (BHk) stars. For the blue straggler stars (BSSs), we determine effective temperatures, radii, and luminosities through SED fittings, and estimate their masses and isochrone-equivalent ages using PARSEC isochrones. Finally, we studied the normalized cumulative radial distribution of the observed BHB stars, BSSs, MS stars, RGB stars, and variable stars along with the specific frequency of BSSs across the cluster. These results provide new insights into the late-stage stellar evolution and dynamical processes in NGC 3201.

We implement a novel approach to performing Sun-to-Earth coronal mass ejection (CME) simulations and test it on three geo-effective space weather events. Using a vector magnetogram observed prior to the CME as the boundary condition, we reconstruct non-linear force free field (NLFFF) solutions in solar active regions with an established magneto-frictional method. We find a pre-eruption solar corona containing the NLFFF in the AWSoM model, which then spontaneously erupts. We apply STITCH, a photospheric driving method, when needed, to increase the strength of the CME shock. The eruptions successfully produce magnetic flux ropes (MFRs) that propagate to 1 au in the full MHD simulation. The synthetic white light images of the simulated CMEs share a striking resemblance in shape to observations. The interplanetary MFRs (IMFRs) arrive at 1 au with a 1.5- to 9-hour error. A comparison of simulated solar wind plasma with in-situ measurements shows that IMFR crossing can reproduce a southward $B_z$ and often its magnitudes, which determine the geo-effectiveness of the event.

We present the commissioning and first results of the Thirty and Forty Gigahertz Instrument (TFGI), which observes the sky at 31 and 41 GHz with angular resolutions of 21' and 18' from the second QUIJOTE telescope at the Teide Observatory. Its primary goal is to conduct a deep cosmological survey in selected regions of the Northern sky with high-sensitivity polarization measurements. The commissioning phase covered Nov2021-Oct2022, during which the instrument operated with a configuration of 7 receivers, 4 at 31 GHz and 3 at 41 GHz. Over this period, approximately 1200 h of data were acquired. Of these, 380 h were dedicated to calibration sources, used to characterize the instrumental properties of TFGI, including the pointing model, beam response, gain stability, polarimetric performance, and instantaneous sensitivity. We provide a detailed characterization of these properties and describe how they are being improved for future observing runs. We use 230 h of observations from bright Galactic regions (Cygnus, W43, W44, and W47) to further validate the instrument performance. As an illustrative example, we present the intensity and polarization spectral energy distributions of W44, finding good agreement with existing measurements. From the noise map of these observations, we measure a polarization sensitivity of ~8.3 $μ$K deg$^{-1}$ after an effective observing depth of 0.57 h deg$^{-2}$. This performance, achieved considering only 2 detectors at 31 GHz, is already comparable to that achieved by WMAP (with almost 3 times the integration time per unit area, 1.61 h deg$^{-2}$). Extrapolating these results to the full TFGI array, with up to 29 detectors, we show that the instrument is expected to reach the target sensitivity of ~ 1 $μ$K deg$^{-1}$ at both 31 and 41 GHz over three cosmological fields covering a total area of 3600 deg$^2$ after an effective integration time of 5.7 years.

We present a new avenue to the usual Schwinger effect via the open string pair production for a system of two Dp branes with $1 \le p \le 6$ in Type II superstrings. The two Dp are placed parallel at a separation with one of them carrying the most general constant electromagnetic background allowed for the pair production. By taking the so-called field theory limit, we obtain systematically, from the open string pair production rate, the respective \textit{explicit Lorentz invariant} QED pair production rate in diverse dimensions for a pair of charged/anti-charged massive particles, which can be scalars, spinors or vectors.

We propose an experiment to test Bell nonlocality, a genuine nonclassicality, for the gravitational field. Two masses with embedded entangled spins (e.g., two diamonds with their NV-centre spins entangled) are placed well outside each other's light cones, ensuring a locality-loophole-free scenario. The spins are then coupled to the motion of their respective masses to generate spatial superpositions. Finally, local measurements are performed only on the gravitational fields of the two masses. If gravity is quantum, then the two entangled masses would entangle their gravitational fields, leading to correlations certifying Bell nonlocality of gravity. Trapped and ground-state cooled nano-objects with micron-sized spatial superposition are sufficient for this test. This goes beyond the recent proposals to test nonclassicality of gravity by providing, for the first time in the literature, a minimal tool to (i) create Einstein-Podolsky-Rosen (EPR) state of gravitational curvatures, (ii) witness entangled gravitational curvatures, (iii) rule out any local-realist description of gravity, and (iv) achieve a loophole-free test of gravity's nonclassicality in a fully device-independent way.

Torsional Alfvén waves are a promising mechanism for transporting energy through the solar atmosphere, with implications for coronal heating and solar wind acceleration. Recently, signatures of torsional Alfvén waves have been observed with the Daniel K. Inouye Solar Telescope. We aim to investigate the effects of line of sight integration and plasma conditions on the observable properties of the torsional mode. Here we present three-dimensional magnetohydrodynamic simulations of multiple coronal waveguides, driven by a combination of transverse kink and torsional wave drivers, considering two plasma regimes representative of active region (AR) and quiet Sun (QS) conditions. Synthetic observables of the Fe~\textsc{xiii}~1074.7~nm coronal emission line are produced using the pyCELP forward-modelling framework to enable direct comparison with spectroscopic observations. In the QS regime, red-blue Doppler asymmetries are associated with the driven torsional waves, though their observed amplitudes are substantially reduced by line-of-sight integration. In contrast, the AR regime exhibits red-blue Doppler asymmetries even in the absence of an imposed torsional driver, which may be misidentified as the $m=0$ torsional Alfvén mode. In the AR setup, the red-blue Doppler asymmetries arise from strong phase mixing, generating shear flows and localised vorticity between waveguides where emission is strongest. The differences between the two regimes are governed by the location of peak emission, which is determined by the degree of density inhomogeneity. Our results support the identification of torsional Alfvén wave signatures in the quiet Sun (a weakly inhomogeneous environment), but caution should be exercised when interpreting spectroscopic observations in strongly inhomogeneous environments such as active regions.

We derive an exact radial monotonicity law for static, spherically symmetric spacetimes in metric \(f(R)\) gravity. For \(ds^2=A(r)dt^2-dr^2/B(r)-r^2dΩ^2\), the matter contribution and the scalaron Hessian combine into an effective radial-convergence numerator that fixes the derivative of \(\Q=B/A\). On every connected static interval with \(A>0\), \(B>0\), and \(f_R\equiv df/dR>0\), its sign therefore determines the monotonicity of \(\Q\). The integrated identity retains the finite, generally nonzero value of \(B/A\) at a regular nondegenerate Killing horizon; consequently, a fixed-sign convergence condition orders the horizon endpoint ratios rather than excluding two horizons. A zero-integral obstruction arises for equal endpoint values, including boundaries where \(B\to0\) while \(A\) remains finite and nonzero. Saturation is equivalent to \(B/A=\mathrm{const}\), and in vacuum requires a scalaron profile linear in the areal radius. We illustrate the strict non-saturated branch, within the General-Relativity sector, using the exact constant-density stellar interior, and apply the equality and consistency diagnostics to the constant-\(X\) and power-law solutions of Multamäki and Vilja. In particular, in the Schwarzschild--de Sitter two-horizon parameter range, one constant-\(X\) solution crosses \(f_R=0\) inside the complete static patch, while a direct substitution into the original radial equation exposes an unresolved exponent mismatch in the displayed power-law family. The results provide a model-independent static-sector diagnostic and a precise starting point for a future horizon-regular treatment of black-hole interiors.

We investigate the application of Physics-Informed Neural Networks (PINNs) to the numerical solution of Einstein's vacuum field equations for static spacetimes. We first reproduce the Schwarzschild solution and then extend the method to the axisymmetric $q-$metric, a nontrivial exact solution characterized by a mass quadrupole moment. We analyze the influence of boundary conditions, domain decomposition, and equation redundancy on the convergence and stability of the training process. The proposed framework accurately reconstructs the metric functions in the computational domain while maintaining small residual errors. Our results demonstrate that PINNs provide a robust and flexible approach to solving Einstein's equations and offer a promising foundation for investigating gravitational configurations for which exact analytical solutions are unknown.

Late-time wave tails decay at different rates along future null infinity and along timelike worldlines at finite radius. A compactified numerical evolution must represent both the slower decay at null infinity and the faster interior decay, producing an increasingly sharp transition between the two regimes. We address this difficulty for semilinear wave equations in Minkowski spacetime using homothetic hyperboloidal coordinates adapted to the scaling structure of the tail. In these coordinates, the tail approaches a smooth radial profile with the same decay rate at every compactified radius. The formulation therefore avoids the steepening of the radial profile seen in stationary hyperboloidal evolutions, and it reaches late times in a number of steps that grows only logarithmically with retarded time. We demonstrate this approach using pseudospectral simulations in 3+1 dimensions and reproduce the generic decay rates conjectured by Rinne. We also provide numerical evidence consistent with a nongeneric codimension-one cancellation of the leading tail coefficient at null infinity, resulting in a faster decay rate.

A natural extension of the successful $D3$-${\bar D}3$-brane inflation model in string theory is one with two $D3$-${\bar D}3$-brane pairs, giving rise to a novel feature in string-theory. When the first pair annihilates, $D1$-strings and $F1$-strings are produced. Following the non-commutative geometric properties in string theory, it is shown by Myers that the presence of the $D3$-charge field strength of the remaining $D3$-${\bar D}3$-brane pair leads to the formation of dielectric $3$-branes -- neutral, finite-size bound states of $3$-branes with $D1$-strings. They behave as matter with little or no pressure. Spanning a wide range of masses, they seed primordial black holes in the early universe.

One of the scenarios used to explain the dipper phenomenon in stars is having the innermost disk regions being close to the line of sight to the star - inclined greater than 60 degrees - regardless of the inclination of the outermost disk ("mis-aligned" or "broken" disks). If the dust and gas in these disks sample the same material, edge-on disks will be viewed through larger column densities of gas and dust than the average disk. We have examined inter-night variability of eight Kepler-discovered dipper stars using the SpeX spectrograph on NASA's Infrared Telescope facility at a spectral resolving power of R~750. The He I line at 1.083 microns exhibits a wide variety of profiles, which change from night to night. In the majority of cases, an inverse P Cygni profile, indicative of inflowing gas, is present. The line profile (and continuum flux level) often change on time scales of 1 day. The Paschen and Brackett lines also change on similar time scales. In one object, V935 Sco, Pa beta and Br gamma went from being in emission to being in absorption over the course of 5 weeks. In another, EPIC 203850058, Pa beta went from being in emission in 2017 to vanishing altogether in 2018. The accretion rates determined using Pa beta for these stars tend to be smaller than those using Br gamma, indicating that the former line is more susceptible to self-absorption than the latter, as would be expected for a highly inclined disk.

One of the fundamental questions we ask in cosmology is what the spatial curvature of the universe is. Experimental observations show that the universe is very close to being spatially flat. However, new results, mainly from measurements by the Planck satellite, indicate that the curvature parameter might be positive. We will, however, examine completely new theoretical results that suggest that we actually live in a closed Universe.

In the present paper, we derive a new charged black hole solution embedded in a quintessence field within a framework combining the Rastall and massive gravity in an asymptotically flat spacetime. We interpret the integration constant associated with the structural properties of the quintessence field as a thermodynamical variable. In this context, several limiting cases have been explored with the aim to shed light on the role of the model parameters. Then, we analyse the thermodynamic properties and stability of the charged black holes in flat spacetime, where the quintessence field is characterized by the state parameter \( ω_q = -2/3 \). Detailed studies are perfomed to see how the various parameters affect phase transitions between small and large black holes. In particular, we establish a connection between the impact parameter and unstable circular photon orbits with the thermodynamic phase transitions. Our analysis suggest that the photon sphere observables could remarkably provide an indirect observational signature of black hole thermodynamic critical transitions. Generally, thanks to the effective contribution of the quintessence parameter, we found that the overall thermodynamic behavior basically resembles that of the charged RN--AdS black hole with thermal features analogous to of a Van der Waals fluid ones. Finally, we have checked these findings using thermodynamic geometry with the Quevedo metric. Our results showed that the critical behaviour derived from the standard thermodynamics is fully consistent with geothermodynamics description, where the phase structure and divergence points exactly coincide with those identified from heat capacity and Gibbs free energy analyses.