High Energy Astrophysical Phenomena Papers

ALT At high redshift, the cosmic microwave background (CMB) both cools relativistic leptons and provides seed photons for inverse-Compton (IC) X-ray emission. These two effects link the particle content of a bubble directly to the soft X-ray depression used to identify a cavity. We derive a physical criterion connecting lepton cooling, bubble volume, enthalpy evolution, and 0.5–2 keV X-ray visibility, while separating intrinsic filling of the X-ray depression from detector-dependent cavity recovery. For uncooled electron populations described by a power law with spectral index s=2.5, minimum Lorentz factor 100≤γₘᵢₙ≤1000, and maximum Lorentz factor γₘₐₓ=10⁵, the measured atmosphere of the testbed high-redshift cluster SPT-CL J2215-3537 requires 0.6–1.2% of the pressure in radiating leptons to fill the soft-band deficit. Across the full tested ranges of s, γₘᵢₙ, and atmospheric properties, the required fraction is 0.2–3.7%. Complete loss of this component reduces the enthalpy by less than 3%

ALT Accreting black holes follow a Fundamental Plane (FP) linking radio and X-ray luminosities to black hole mass, but whether this relation changes in the ultramassive regime, above \sim10¹0 Msun, remains unresolved. Brightest cluster galaxies (BCGs) provide the most direct access to this regime, although earlier claims of enhanced radio emission relied largely on heterogeneous nuclear measurements and black hole masses inferred from host-galaxy proxies. We recalibrate the forward FP using galaxies with direct dynamical mass measurements and reassess the evidence for a high-mass radio excess using the available BCG data. We find that the archival proxy-mass sample cannot uniquely establish such an excess because the inferred signal remains sensitive to the mass calibration and radio-selection properties of the sample. Using mock observing programs, we further show that the predictive width measured from existing observations makes a high-mass transition difficult to both detect and locali

ALT The composition of dense matter in neutron star interiors remains uncertain, with the possible appearance of the non-strange Δ resonances providing an important source of theoretical uncertainty. In this work, we investigate the gravitational wave signatures of hyperonic neutron stars containing Δ baryons within a density-dependent relativistic mean-field framework based on the DDME2 parametrization. Within this framework, the baryons interact among themselves via mesons. For Δ baryons, the interaction is uncertain to date. We investigate how variations of the scalar and vector Δ-meson couplings, subject to xσΔ-xωΔ=0.2, modify the tidal properties and inspiral gravitational-wave signals. We find that the allowed variation in the Δ-σ coupling leads to substantial changes in the tidal properties of the neutron stars, which in turn leave distinct imprints on the gravitational-wave waveforms, particularly in their frequency evolution, accumulated phase, and strain amplitude. These waveform

ALT We report polarimetric observations of the repeating fast radio burst (FRB) source FRB 20240209A with the Green Bank Telescope at L band and 800 MHz. This source has previously been associated with a quiescent elliptical galaxy with a mass-weighted stellar age of approximately 11 Gyr. We detected three bursts, and the brightest burst resolves into eight components that drift toward lower frequencies. The bursts have mutually consistent rotation measures (RMs), and the small extragalactic RM agrees with the low RM predicted from the small host-galaxy dispersion measure (DMₕost). The linear polarization fraction decreases toward lower frequencies within the brightest burst, consistent with depolarization by an RM scatter of σ_RM≃3.5 rad m⁻². This RM scatter is comparable to those of several repeaters in star-forming galaxies. Therefore, FRBs in both star-forming and old, quiescent hosts can reside in relatively complex magneto-ionic environments, which suggests that FRB sources may form

ALT The April 2018 gamma-ray flare of the flat-spectrum radio quasar 3C 279 exhibited sharp peaks within a broader outburst. Its rapid substructure was previously attributed to magnetic reconnection. Motivated by its distinctive temporal profile within our comparison sample of bright blazar flares and its resemblance to caustic-crossing features in optical microlensing events, we investigate a finite-source binary-microlensing interpretation. A jointly fitted model of binary-lens magnification, a smooth intrinsic flare, and Gaussian-process variability reproduces the principal double-peaked structure of the GeV light curve. The adopted local solution has a binary mass ratio of q≃0.098 and an Einstein timescale of t_E≃0.327 d. The absolute lens mass and projected source radius depend on the lens distance and relative proper motion. The fitted geometry can correspond to a Galactic planetary-mass binary or an intervening stellar or substellar system, with different physical source sizes in ea

ALT In recent years, several astrophysical objects have been identified as potential multi-messenger photon-neutrino emitters. New instruments dedicated to the study of these two messengers have been and are still being built, achieving unprecedented sensitivity. This observational improvement increases the need for more sophisticated and precise analysis tools. Multi-wavelength and multi-messenger data fitting is crucial to constrain astrophysical models of particle acceleration and radiation mechanisms at work in these extreme environments. In this contribution, we present a novel framework dedicated to multi-wavelength and multi-messenger modeling, built within the open-source software Gammapy. The framework enables data analysis from optical up to very-high-energy gamma rays, and fitting physical models directly to instrumental counts in these wavebands, without the need of computing flux points. Focusing on blazars as a first example of a multi-messenger candidate, we showcase the per

ALT High-energy neutrinos provide a unique probe of the most extreme non-thermal processes in the Universe. The detection of the ultra-high-energy neutrino event KM3-230213A by the KM3NeT neutrino telescope, with an inferred parent-neutrino energy of O(10¹7 eV), represents a major step forward in neutrino astronomy. In this review, we summarize the main properties of the event and discuss its interpretation within different astrophysical and cosmological scenarios, including active galactic nuclei, transient sources, and cosmogenic neutrinos, highlighting the role of multi-messenger constraints. Despite extensive follow-up observations, no unambiguous electromagnetic counterpart has been identified, and the event remains in tension with the absence of comparable detections in other experiments. This situation leaves open the possibility of more complex source scenarios or contributions from physics beyond the Standard Model. We also discuss the implications for the ultra-high-energy neutri

ALT Well-motivated models of particle dark matter predict its self-annihilation into Standard Model end states, which may be detected by particle detectors or telescopes. We conduct an indirect dark matter search using gamma rays detected by the VERITAS telescope from within 2° of the Galactic Center. A template-based approach is used for background estimation, increasing our sensitivity to sources of gamma-ray emission with spatial extents comparable to the 3.5° VERITAS field of view. The analysis benefits from data taken exclusively at large zenith angles, which results in an increased effective area at energies above 1 TeV, compared to small zenith angle observations. No significant gamma-ray signal is observed in the search region, resulting in upper limits on the end-state-dependent dark matter annihilation cross sections. Cross sections as low as langle σ_ann vrangle = 1.77× 10^-25 cm³ s^-1 are excluded at 95% confidence assuming complete annihilation into τ^+ τ^-, an Einasto profile

ALT The outcome of black hole–neutron star (BHNS) mergers is studied using a general-relativistic mass-shedding condition in the Kerr spacetime, which determines the tidal-disruption radius from the mass ratio, the black hole (BH) spin, the neutron star (NS) compactness, and the ratio of central to average density, η=ρᶜ⁄ρ̄. Comparing this radius with the innermost stable circular orbit, we map the boundary between tidal disruption and plunge for two relativistic mean-field equations of state (EOSs) and for an ensemble of about 10⁴ agnostic EOSs spanning the current uncertainty in the NS radius. We find that the disruption threshold is set mainly by the compactness, but NSs with the same compactness and different η can have different outcomes. Using the tidal disruption radius, we construct a remnant-mass model calibrated against 70 numerical-relativity simulations, which reproduces the remnant mass with an accuracy comparable to existing fits. Applying the disruption criterion and the remn

ALT We show that the jittering-jets explosion mechanism (JJEM) can qualitatively reproduce the shape of the black hole (BH) mass distribution observed by LIGO-Virgo-KAGRA and quantitatively reproduce the peak around Mₚ₁ ≃ 10 M⊙. To show this, we construct a toy model that captures the JJEM physics that explodes all massive stars, even when BHs form. According to the JJEM, BHs form when the star's binding energy is large, implying a massive progenitor with initial mass Mᵢ ≳ 25 M⊙, and when jets are launched along a fixed axis, implying a rapidly rotating pre-collapse stellar core. In this case, the jets are launched along the core's angular momentum and are inefficient at removing mass near the equatorial plane; when the stellar binding energy is large, a large fraction of the equatorial mass is accreted to form the BH. The toy model includes two parameters: the threshold binding energy in the core for BH formation and the jet opening angle along the poles. Although our finding is encouragi

ALT Binary neutron star (BNS) and black hole–neutron star mergers are among the most important multimessenger sources. Observations of these events could provide valuable insights into several unresolved phenomena, ranging from the efficiency of magnetic field amplification during and after mergers to the formation of magnetars and jets. Understanding these phenomena requires accurate modelling of magnetic field evolution, which remains a challenge for state-of-the-art general relativistic magnetohydrodynamics (GRMHD) simulations. In this work, we present RAKI (Relativistic Astrophysics Code Implementation of the Alternative Finite Difference Approach for GRMHD), the first GRMHD code to simulate BNS mergers using high-order alternative finite-difference methods with adaptive-order WENO (AFD-WENO-AO) reconstruction and a fully non-staggered, node-centred vector-potential formulation. The code achieves up to ninth-order convergence in smooth test problems while accurately capturing discontin

ALT After decades of only failed transitions, the black hole X-ray binary GS 1354-64 has undergone a complete outburst again. We present its first X-ray polarimetric investigation in the fading steep-power-law (SPL) state, following an earlier intermediate-state IXPE campaign during the same outburst. The 2-8 keV emission has a polarization degree (PD) of 2.3 +/- 0.2% and polarization angle (PA) of 5 +/- 3 degrees. Compared with the intermediate state, where PD=3.9 +0.3/-0.2% and PA=-2 +/-2 degrees, the broadband PD is lower while the PA remains consistent. The intermediate-state type-C quasi-periodic oscillation state has also disappeared, marking a major reconfiguration of the accretion flow. Assuming a distance of 7 kpc, the observed the intensity drop from 2.5% to 1.6% of the Eddington luminosity over 10.2 days is punctuated by several broad hour-to-day-scale troughs, tracing two distinct tracks in hardness-intensity space. Simultaneous spectroscopy shows the troughs' spectra contains

ALT Superluminous supernovae, among the brightest stellar explosions, are thought to be powered by newborn magnetars. One such event, SN 2024afav, “chirped”, displaying sinusoidal, increasing-frequency (P(99 d)∼39±8 d to P(166 d)∼17±5 d) light-curve modulations. The model explaining this chirp, a precessing disk falling onto the dimming magnetar, generalizes to a normalization-independent universal relation between the period derivative and the dimming rate of the light curve. Applied to the published chirp alone, with the spin-down time measured from the unmodulated light curve, the relation predicts that the chirp must halt near t ≈ 150 d and reverse. With continued follow-up of SN 2024afav, we report the discovery of five new modulations in which the chirp reverses (P(177 d)∼24±4 d to P(208 d)∼38±2 d) at t=147±3 d, as predicted. Applied to chirping events from upcoming surveys, the relation could identify the precession mechanism and disk physics of each engine.

ALT Resolve, the microcalorimeter array aboard the X-Ray Imaging and Spectroscopy Mission (XRISM), provides an unprecedented spectral view of Tycho's Supernova Remnant (SNR). We fit the X-ray spectra of the five public archival XRISM observations of the SNR with a multi-component non-equilibrium ionization plasma model assuming spherical expansion and a uniform abundance pattern. For pointings away from the center of the object, we divide each into three spectra based on angular distance from the remnant's geometric center and fit all three simultaneously with the center-pointing observation. We tie elemental abundances together across all model components. We find that pseudo-spherical expansion fits well the Tycho SNR in the regions where Resolve has observed, although iron appears slower on the approaching side than the receding side. We also find that a close-to-uniform abundance pattern across the SNR is consistent with these data.

ALT Kilohertz quasi-periodic oscillations (QPOs) have been observed in a few rare gamma-ray bursts (GRBs), but their physical origin remains uncertain. We reanalyze the two short GRBs with reported kilohertz QPOs, GRB 910711 and GRB 200415A, and find evidence for possible harmonic-like organization around a characteristic frequency scale near 360 hertz. The similar harmonic-like pattern in GRB 910711 and in the confirmed magnetar giant flare (MGF) event GRB 200415A, including features extending above 4 kilohertz, points to timing phenomenology on frequency scales relevant to magnetar oscillation modes. The temporal and spectral similarities between the two events and the difficulty of explaining these features in merger-driven scenarios favor an MGF-like interpretation of GRB 910711. This timing phenomenology may therefore provide an additional observational handle, complementary to spectral, temporal, and localization information, for distinguishing rare MGF-like transients from merger-dr

ALT We present the results from a high-cadence, multi-channel time-domain survey targeting the Coma galaxy cluster, utilizing the newly commissioned 1.6-m Multi-channel Photometric Survey Telescope (Mephisto). Mephisto's pioneering mosaic focal-plane design enables strictly simultaneous imaging in three distinct optical bands (u/v, g/r, i/z). This unique capability allows for the extraction of true, instantaneous spectral energy distributions (SEDs) of fast-evolving transients. Our survey continuously monitors a 20-square-degree footprint centered on the Coma cluster. By strategically targeting this massive local structure, we exploit cosmic anisotropy to significantly amplify the volumetric discovery rate of stellar explosions. During the initial six-month pilot operation, we successfully detected and tracked nine new supernovae (SNe). Remarkably, our high-cadence, multi-band strategy allowed us to capture ultra-early rising light curves and pre-discovery signals (e.g., SN 2026dyr) hours

ALT Black hole (BH) spin grows as accretion supplies angular momentum and decreases as magnetically driven jets remove it, driving the BH toward an equilibrium spin, a⋆;ₑq. Existing estimates of a⋆;ₑq rely on general-relativistic magnetohydrodynamic (GRMHD) simulations with fixed BH mass and spin, and cover only the extreme limits of horizon magnetic flux φ→0 and magnetically arrested disks (MADs), φ≈φ_MAD. We evolve the BH mass and spin in long 3D GRMHD simulations of radiatively inefficient accretion flows aligned or anti-aligned with the BH spin. We find the spin-up parameter as a function of BH spin and φ/φ_MAD, and derive the equilibrium spin as a continuous function of flux, a⋆;ₑq(φ/φ_MAD). The equilibrium spin falls steeply with flux, from a⋆;ₑq≈0.9 in weakly magnetized flows through a⋆;ₑq(φ/φ_MAD=0.5)≈0.5 to a⋆;ₑq(φ/φ_MAD=1)≈0.06 in sustained MADs. In weakly magnetized flows, a slowly spinning BH must accrete at least its initial mass to reach equilibrium, whereas MADs spin down ra

ALT Stellar-mass black holes (sBHs) embedded in active galactic nucleus (AGN) disks may affect X-ray variability through magnetic reconnection. As sBHs migrate and accumulate near migration traps, they disturb the disk magnetic field and trigger reconnection events that heat the disk and eject plasma into the overlying corona. The injected plasma dissipates kinetic energy and can trigger the release of magnetic energy already stored in the corona. We calculate the resulting coronal X-ray emission and the disk ultraviolet (UV) variability, including the disk thermal response and reprocessing of incident X-rays. We find that embedded sBHs can initially enhance short-timescale X-ray variability, but the variability becomes suppressed as more sBHs accumulate in the disk. At higher mass accretion rates through AGN disks, this suppression appears earlier. As sBH-associated reconnection events become more frequent, their active regions increasingly overlap, reducing the magnetic energy available

ALT The interaction between winds from tidal disruption event (TDE) and the circumnuclear medium (CNM) around supermassive black holes is believed to contribute to the radio emission of non-jetted TDEs. This model has been studied both analytically and numerically. In previous works, the properties of wind are usually set to be angle-independent. It is not clear to what extent the results deviate from those in the case where TDE wind properties are angle-dependent. We perform numerical simulations to investigate the effects of the angular distribution of wind properties on the radio light curve in the wind-CNM interaction model. We find that given that the wind mass and kinetic energy fixed, the radio light curve in the optically thin waveband significantly depends on the angle-dependence of wind properties. Even when the swept-up CNM mass is still less than the wind mass, a slight deceleration can affect the light curve. Changing the angular dependence of the wind properties causes the de

ALT We introduce a spectral-model-independent spectral-shape metric, δᵣₘₛ, to probe phase-resolved spectral evolution in the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124. The δᵣₘₛ–luminosity relation reveals a reproducible convergence–redivergence behavior: spectra from two geometrically opposite pulse phases converge to near identity around Lₜ∗∼8.8×10³⁷ erg s⁻¹, with a mean shape difference of only ∼2%, and rediverge at higher luminosities, most prominently at higher energies. This strong phase-resolved spectral convergence coincides with a pulse-profile reversal and traces the same transition luminosities previously identified from broadband continuum evolution, including Lₜ∼4.5×10³⁷ erg s⁻¹. These results suggest a possible luminosity-driven reweighting between a broadly distributed surface-reprocessed/reflected component and an increasingly important direct accretion-column contribution, while establishing δᵣₘₛ as a robust, spectral-model-independent diagnostic of radiation-g