ALT We analyse what physical quantities of SN ejecta can be obtained from a single epoch of nebular JWST spectroscopy. We provide an analytical framework, which together with a grid of helium star SUMO spectral synthesis models allows the determination of these quantities for any stripped-envelope SN. Using the framework, we derive critical electron densities for major emission lines, finding values in the range 10⁵-10⁶ cm⁻³. This means formation under close to LTE conditions for 200 – 300 days. We obtain ionisation fractions of Ni and Co, as well as the temperature and filling factor of the Fe/He zone, the ⁵⁶Ni and stable nickel masses, the Ni/Fe ratio, and the optical depths τ[NᵢII]₆.₆₃₆μₘ and τ[CₒII]₁₀.₅₂μₘ. Besides these constraints from iron group elements, one can also obtain the filling factor of the carbon burning (O/Ne/Mg) zone from [Ne II] 12.81 μm, and the masses of Ar II and Cl I. We apply our methodology to a publicly available JWST spectrum of Type Ic SN 2024aecx. We find tha

ALT The recent discovery that microquasars operate as efficient particle accelerators brings back into focus the question as to whether their larger cousins, radio galaxies, also operate as efficient accelerators. We here discuss what we presently know about particle acceleration in radio galaxies, focusing particularly on the nearest such object, Centaurus A.

ALT Late light curves of neutron star mergers can contain substantial fission power while remaining compatible with different radioactive inventories. We develop a unified source attribution model in which a common bounded response formulation links nuclear heating, radiation transport and isotope identification. Within particle dependent deposition and gray photon escape, matched luminosity comparisons show how long alpha and fission responses become aligned. Explicit competing inventories then reproduce a light curve with a substantial ²⁵⁴Cf contribution, with smooth evolution of the beta luminosity response providing an important source of compensation. We derive optimization certificates and collective likelihood bounds that quantify the resulting limits on identification under specified source and ejecta constraints. We also introduce an energy conserving model with two radiation frequency groups to resolve the information carried by color at fixed total luminosity precision. Its exac

ALT We study the dynamical response of a hydrodynamic disk to an impulsive kick imparted to a central massive object, focusing on the global properties of the material that remains gravitationally bound. We construct a simple analytic model and show that the bound material can acquire a significant tilt—or even a retrograde rotation—if the kick is directed close to the disk midplane. We perform 3D numerical simulations and show that (1) the bound material relaxes to a torus-like circularized flow of which total mass and angular momentum is in fair agreement with the analytic estimate, and (2) in spite of nonlinear hydrodynamic effects, a highly tilted or retrograde disk can be sufficiently formed. The simulations additionally reveal a transient warp in the kicked disk, suggesting a kick as a formation mechanism of tilted or warped disks around compact objects. About 10% of the kinetic energy the bound material acquired from the kick is converted to the internal energy, while the exact conv

ALT Collapsar disks are proposed sources of r-process elements, but neutron-rich disk ejecta require gas to be both neutronized in the disk and ejected before accretion. The magnetic field that enables ejection also thickens the disk and accelerates the inflow, lowering both the density and the time available for weak processing. To compute the resulting neutronization, we evolve the electron fraction of the inflowing gas, adopting at each radius the temperature that maximizes the net neutronization rate for the local density and instantaneous composition. This yields a lower limit on Yₑ that is independent of the disk's thermal history. We adopt an optimistic unbinding radius of R_unb=10 rg for all magnetic fluxes capable of launching outflows. Combining this ejection condition with favorable assumptions of low black hole mass, ejection of midplane gas, and the upper end of the γ-ray burst (GRB) engine-power scale, which limits the accretion rate to Ṁ_BH≲0.4 M⊙ s⁻¹, gives a fiducial lower

ALT Horizon scale images probe the plasma dynamics and spacetime in the vicinity of accreting black holes. Interpreting these images requires models that connect image morphology to the structure of the underlying accretion flow. Disk misalignment is expected in many systems, yet most horizon scale image analyses assume that the emitting flow is aligned with the black hole spin. We investigate whether tilted accretion flows produce image features distinguishable from axisymmetric flows and whether their effects can be absorbed by changes in aligned model parameters. Using general relativistic magnetohydrodynamic simulations of tilted magnetically arrested disks spanning black hole spins a*=-0.9375 to 0.9375 and initial disk tilts 0° to 30°, we generate synthetic images and quantify their morphology through fits to an aligned, semi-analytic emission model. Using this approach, we show that high spin prograde models show little sensitivity to disk tilt, as the inner emitting flow aligns with

ALT We present a pedagogical introduction to the I-Love-Q relations within Newtonian gravity using an enthalpy-based formulation of stellar structure. Introducing the specific enthalpy converts Euler's equation into a first integral of hydrostatic equilibrium and recasts the equation of state as a relation between the mass density and the enthalpy. Together with Poisson's equation, these relations provide a unified description of spherical equilibrium and deformations induced by rotation and tides. We illustrate the formalism with polytropic equations of state and a relativistic free Fermi gas. In Newtonian gravity, slow uniform rotation and a weak tidal field excite the same linear quadrupolar response, yielding the exact identity Ī²Q̄/λ̄≡1 for the dimensionless (“bar”) observables—independently of the equation of state. This exact identity is distinguished from the approximate universality of the pairwise I-Love-Q relations, whose residual dependence on stellar structure is illustrated b

ALT Recently, the Five-hundred-meter-Aperture-Spherical-radio-Telescope (FAST) Galactic Plane Pulsar Snapshot (GPPS) survey discovers an extraordinary binary neutron star (BNS) system, PSR J0653+0443, possessing the highest known orbital eccentricity (e≈0.992) among Galactic BNSs (Yang et al., 2026), which provides a unique probe into the formation and evolution of BNSs. In this Letter, we perform compact binary population synthesis calculations with self-consistent neutron star spin evolution to model extremely eccentric BNS population detectable by the FAST GPPS survey, explicitly incorporating observational selection effects. We find that BNSs with e>0.99 constitute only ∼0.6% of the detectable population, with an expected detection number of λ_GPPS≈0.31 over the full planned footprint. Thus, although rare, systems as eccentric as PSR J0653+0443 are compatible with standard binary evolution. We further demonstrate that extremely eccentric BNSs arise preferentially from core-collapse sup

ALT We study toroidal structures formed by a charged, non-self-gravitating perfect fluid orbiting a Schwarzschild black hole immersed in a poloidal magnetic field. To obtain analytic and semi-analytic solutions, we assume a cold, rest-mass-dominated fluid with uniform specific charge and zero conductivity, so the current is purely convective. This regime is formally opposite to the ideal GRMHD limit. The total magnetic field comprises a prescribed asymptotically uniform external component and an approximate self-field associated with the azimuthal current in the torus. We represent the latter by a regularized dipole approximation to the field of an effective circular current loop. Neglecting the self-field, the method of characteristics yields a general implicit solution of the integrability condition for a function parametrizing the specific-angular-momentum profile. A separately obtained cylindrically dependent class recovers a solution from our earlier work. For the radially dependent c

ALT GRB 220706A exhibits X-ray flaring activity extending to 51 days after the trigger, along with a luminous optical thermal component emerging on a timescale of 10⁶ s. Such prolonged central-engine activity could be powered by a collapsar fed by an extended stellar envelope. However, a relativistic jet may struggle to break out of the extended envelope. These challenges motivate a tidal disruption event (TDE), although explaining both the early relativistic jet and the luminous late optical thermal emission within one TDE framework remains difficult. We investigate a TDE scenario in which shocks between stellar debris streams drive the early inflow of some debris, potentially carrying preexisting fossil magnetic flux toward the black hole (BH) and powering an initial relativistic jet via the Blandford-Znajek mechanism. The remaining debris circularizes into an accretion disk, whose thermal emission accounts for the late optical thermal component. For GRB 220706A, the optical timescale, l

ALT Population studies of fast radio burst (FRB) repetition typically model sources using a single distribution of steady, time-averaged burst rates, yet individual sources show significant departures from this assumption. Here we present FRB 20210601A, a source discovered by the Canadian Hydrogen Intensity Mapping Experiment FRB project (CHIME/FRB) that represents the most extreme such departure yet identified. The source produced at least nine bursts within a single 106.5 s interval – the highest instantaneous burst rate yet reported from a repeating FRB source at gtrsim 2 Jy ms fluences – yet remained otherwise undetected despite \sim 320 hr of exposure spanning 7 yr. The active interval occurred while the source was in a side-lobe of the CHIME/FRB digitized beams, implying an instantaneous burst rate gtrsim2.3× higher (gtrsim 21 bursts) at main-lobe sensitivity. The source's intense, brief burst storm flanked by apparent quiescence despite substantial exposure is unique among the CHIME

ALT We conducted a systematic spectral analysis of the X-ray flares of gamma-ray bursts (GRBs) observed by Swift/X-ray Telescope from 2005 April to 2015 March. The sample includes 521 flares from 206 GRBs. The GRBs cover the three subtypes: X-ray flashes (XRFs), X-ray-rich GRBs (XRRs), and classical GRBs (C-GRBs). We used a simple power-law model to fit the spectra during the flares, as well as to fit the preflare and postflare afterglow emissions, and obtained the parameters such as photon index and intrinsic hydrogen column density (N_H). By comparing the spectral characteristics of flares, prompt emission, and afterglow emission, and analyzing the temporal evolution of the X-ray flare spectra, we obtained the following main conclusions. (1) The spectrum during the flare phase is significantly harder than that of the preflare and postflare phases, while there is no obvious difference in the spectrum between the preflare and postflare phases. (2) The intrinsic N_H shows no significant cha

ALT Compton scattering exchanges energy between radiation and hot gas and changes the spectrum in high-energy astrophysical systems such as tidal disruption events (TDEs). Because it couples all photon energies and can act faster than the hydrodynamic time step, many radiation-hydrodynamics (RHD) simulations omit its spectral redistribution or treat its energy exchange in the gray approximation. We extend the multigroup flux-limited diffusion module of the moving-mesh RHD code RICH to include Compton scattering with the exact thermal Compton kernel. In our semi-implicit scheme for the thermal radiative transfer equation, the temperature dependence of the kernel enters a generalized Fleck factor, and the group Compton operator conserves photon number and satisfies detailed balance. In equilibration tests, the scheme agrees with independent solutions, relaxes without absorption and emission to a Bose-Einstein spectrum, and remains stable for time steps that do not resolve the Compton equilib

ALT When modeling the emission from an X-ray binary system, we often assume that the rotational axis of the compact object is aligned with that of the binary system. While often considered irrelevant for spectroscopic observations, the orientation of the binary system is crucial for polarimetric properties. With the recent advent of X-ray polarimetric observations enabled by the Imaging X-ray Polarimetry Explorer (IXPE), indications of a possible misalignment of the neutron star rotational axis with respect to the binary axis have been reported. Following these indications, we present an analytical model for the polarimetric properties of emission arising from a spreading layer on an inclined weakly magnetized neutron star (WMNS). We discuss the expected polarimetric properties and study the impact of geometrical parameters of the system on the polarization of WMNS emission. We compare our results with those for aligned systems. Finally, we discuss applications of the model to recent and u

ALT Fast X-ray Transients (FXTs) are brief, intense X-ray flashes unassociated with persistent X-ray sources or known stellar objects, typically lasting from seconds to hours. With the launch of the Einstein Probe in January 2024, the study of FXTs has entered a new phase. Its large field of view significantly increases the discovery rate, enabling more robust statistics. The leading discoveries have revealed a fraction of gamma-ray bursts, while the rest remain unexplained. We propose a methodology to search for possible relations between gravitational waves (GWs) and FXTs, as it may serve as a discrepancy among proposed theoretical models. To achieve this, we intend to utilize X-Pipeline, an unmodeled search algorithm, to identify excess coherent signals in LIGO-Virgo-KAGRA (LVK) O4b observations. Out of 64 FXTs events, we obtained 19 FXTs for constructing robust searches. Given the absence of any detectable GW signals, we subsequently calculated upper limits and estimated exclusion dist

ALT Shock breakout (SBO) occurs when a radiation-mediated shock wave, formed during a core-collapse supernova (SN) explosion, breaks out from the stellar surface of the progenitor or from the circumstellar material (CSM), if the CSM is optically thick. The X-ray outburst XRO 080109, associated with a Type Ib/c SN 2008D, has been suggested to be an SBO event from a regular supernova. The recent detection of EP260321a, associated with a broad-lined Type Ic SN 2026gzf, by the Einstein Probe adds a new SBO event. However, the two SBO events differ markedly in their spectra, energetics, late-time radio emission and early optical emission within the first day. We show that the non-detections in radio follow-up observations of EP260321a imply a shock velocity ≤sssim0.1c, whereas the radio data of XRO 080109 suggest a shock velocity of simeq0.2c. These velocities are consistent with those inferred from the properties of the X-ray outbursts and explain the difference in their X-ray spectra. The X-r

ALT Ultra-relativistic magnetic monopoles traversing the atmosphere are expected to initiate extensive air showers characterized by a muon deficit and a compact lateral profile. Using 1438.81 live days of data from the LHAASO-KM2A array, we present a search for these cosmic particles within the Lorentz factor range of 10⁵ to 10⁷. A machine-learning classifier utilizing lateral shower shape observables from the electromagnetic detectors is deployed to separate the signal from the hadronic background. No statistical excess above the background expectation is observed, and upper limits at the 90% confidence level are established on the isotropic monopole flux. The resulting upper limit is 2.6 × 10⁻¹⁶ cm⁻² s⁻¹ sr⁻¹ at γ=10⁵ and reaches 8.8 × 10⁻¹⁹ cm⁻² s⁻¹ sr⁻¹ at γ=10⁷, which improves upon the astrophysical Parker bound by over three orders of magnitude. This work establishes constraints on the monopole flux within a previously unconstrained velocity window.

ALT We develop a relativistic dynamical model for the thermal evolution of expanding kilonova ejecta, in which the effective temperature is determined by the ejecta mass, opacity, velocity, and radioactive heating rate. The resulting thermal evolution follows a broken power law as the ejecta transition from the optically thick to the optically thin regime. We incorporate this thermal history into a relativistic spectral calculation that accounts for the equal-arrival-time surface geometry and Doppler boosting, enabling us to model the observed kilonova spectrum and constrain the underlying ejecta properties. We investigate the effects of the model parameters and relativistic corrections on the observed temperature and spectral shape, and find that light-travel-time delays and Doppler shifts can produce significant spectral distortions at high ejecta velocities. We apply the model to the early-time X-shooter spectrum of the kilonova AT2017gfo and infer the ejecta mass, opacity, and velocity

ALT We perform a time-integrated search for high energy neutrinos from 27 fast X-ray transients (FXTs) detected by Einstein Probe, using 14 years of publicly available IceCube muon-track data. We employ the unbinned maximum likelihood method which is used for IceCube point source searches. We do not find any statistically significant excess from any of the FXTs. The largest TS value is obtained for EP240801a (TS=4.77), corresponding to a local p-value of 0.04, which is not significant. We therefore calculate 90% confidence level upper limit on the differential muon neutrino flux. The most stringent upper limit is obtained for EP241021a, with E² Φ(E)=2.1× 10⁻¹⁰ (GeV cm⁻² s⁻¹), for a E⁻² spectrum at a pivot energy of 100 TeV.

ALT Faint Type I supernovae blur the boundaries between spectroscopic classes, obscuring their progenitors and explosion mechanisms. One proposed origin is a double detonation, in which ignition of an accreted helium shell triggers the explosion of a carbon oxygen white dwarf. Surviving helium is a direct prediction of this mechanism, but its strongest near-infrared feature overlaps with carbon and is ambiguous in isolation. Here we present SN 2024kce, observed with simultaneous optical and near-infrared spectroscopy eleven days before maximum light. We identify He I absorption at three wavelengths, all consistent with a velocity of 16,400 km/s, well above the photosphere. This is the first detection of helium before maximum light in a thermonuclear supernova. Helium at these velocities requires nearby radioactive material for non-thermal excitation, pointing to ash from a detonated surface helium shell. A double detonation model reproduces the optical spectral evolution, suppressed blue f