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Physics Astrophysics High Energy Astrophysical Phenomena

astro-ph.HE Watch High Energy Astrophysical Phenomena

Thursday 17 September

High Energy Astrophysical Phenomena

Evolution and mergers of eccentric white dwarf binaries encountering intermediate-mass black holes

Adarsh Pandey, Aryabrat Mahapatra and Tapobrata Sarkar

We study how a close passage past an intermediate-mass black hole (IMBH) changes the evolution of an eccentric white dwarf (WD) binary that has nominal Roche lobe overflow at internal pericenter. Using three-dimensional smoothed particle hydrodynamics, we follow binaries with eccentricity $e_{\rm in}=0.6$ and mass ratios $q=0.2$ – $0.7$ on parabolic, retrograde orbits around a Schwarzschild IMBH at two encounter strengths, $\beta^b=0.5$ and $1.5$. We compare them with the same binaries evolved in isolation. The IMBH greatly speeds up the merger. Every IMBH-perturbed binary that merges does so during or shortly after its second internal pericenter passage, which is always less than the time taken to merge in isolation. The merger outcome is not monotonic with encounter strength. The $q=0.2$ binary retains two bound cores for $\beta^b=1.5$ but merges at $\beta^b=0.5$. In the deeper encounter it settles onto a less eccentric post-encounter orbit, showing that the timing of the encounter relative to the inner orbit plays an important role. Deep encounters also produce transient reversals of the internal orbital angular momentum for $q=0.2$ – $0.4$, while the material bound to the accretor gains more spin angular momentum than that bound to the donor. The specific orbital angular momentum of the binary CM around the IMBH changes by only a few $\times10^{-4}$ of its initial value. We also identify He-origin material that can burn helium on a dynamical timescale, amounting to $\sim15$ – $17\%$ of the He-origin mass for $q=0.7$. IMBH encounters can therefore quantitatively change the evolution of eccentric WD binaries.
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High Energy Astrophysical Phenomena

The 2025.6 X-ray Minimum of Eta Carinae

Amit Kashi and Noam Soker

We present observations of the recent 2025.6 X-ray minimum and recovery of $\eta$~Carinae, obtained with NICER/XTI, Swift/XRT, XRISM/Xtend, and Chandra/HRC-I. We construct light curves in the soft (0.5–2 keV) and hard (2–10 keV) bands and derive hardness ratios. The X-ray emission reached its minimum about 10 days earlier than in the previous five cycles. As in previous events, the spectrum softened during the decline, while the X-ray luminosity varied on short timescales. These findings strengthen the conclusion that enhanced line-of-sight absorption, such as that expected during an eclipse, does not have a significant role in producing the spectroscopic event. Instead, they indicate that the X-ray decline is caused by stochastic variations and instabilities in the two stellar winds and their interaction, destruction of the colliding winds region, and accretion of primary wind onto the secondary. The main process behind the deep X-ray minimum near periastron is the accretion of mass lost by the primary star onto the secondary star. Accretion at periastron passages in $\eta$ Carinae implies that accretion occurred at much higher rates during the nineteenth-century Great and Lesser Eruptions, when the primary star lost mass at much higher rates. Such accretion likely launches jets. The broader implication is that accretion events in binary systems, possibly with jet launching, power other luminous blue variable giant eruptions and intermediate-luminosity optical transients.
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High Energy Astrophysical Phenomena

A 32-day Quasi-periodic Modulation in the Post-peak Light Curve of the Superluminous Supernova SN 2018bsz

Aiswarya Sankar.K, Ting-Wan Chen, Seán J. Brennan et al. (+8 more)

Recurrent structure in supernova (SN) light curves may reveal physical clocks associated with central-engine dynamics, binary orbital motion, or interaction with structured circumstellar material (CSM). We present a multiband photometric reanalysis of the nearby hydrogen-poor superluminous SN (SLSN-I) 2018bsz (z=0.0267), using Swift/UVOT and GROND observations spanning approximately 110 rest-frame days after maximum light. After independently modelling and subtracting the smooth decline in each band, Generalized Lomb-Scargle periodograms reveal recurrent residual variations at broadly consistent phases across ten bands from u to Ks. The strongest individual-band detections, in the g, r, i, z, and J bands, yield periods of 31.5-31.8 days, while the joint multiband analysis gives a rest-frame period of P=31.61 (+/-0.03) days. The modulation is traced over approximately three cycles and is therefore described as quasi-periodic. The signal is robust to the adopted detrending procedure, while comparison-star and lunar-cycle tests disfavour an observational systematic. The modulation is broadly phase-coherent from the optical to the near-infrared, but its amplitude increases towards shorter wavelengths; lower-significance bands also show possible wavelength-dependent shifts in the best-fitting period. The present observations do not uniquely determine the physical origin. Lense-Thirring precession, interaction with CSM structured before the explosion, and post-SN interaction with a surviving companion remain possible. SN 2018bsz provides one of the clearest examples of coherent, month-scale photometric modulation in an SLSN-I.
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All authors Aiswarya Sankar.K, Ting-Wan Chen, Seán J. Brennan, Morgan Fraser, Keiichi Maeda, Thomas Moore, Sheng Yang, Amar Aryan, Dietrich Baade, Ping Chen, Chow-Choong Ngeow

High Energy Astrophysical Phenomena

Investigating the influence of the full-kinematics treatment for charged-current processes in binary neutron star mergers

Ramon Jaeger, Henrique Gieg and Tim Dietrich

Weak interactions are fundamental in the merger of binary neutron stars (BNS). They provide cooling, change the composition of matter outflows, and can lead to significant secular outflows through neutrino-driven winds. Hence, the inclusion of neutrino transport has become standard in numerical-relativity simulations of mergers involving neutron stars. In more detail, the interactions of neutrinos with matter in a truncated two-moments scheme (M1) are described through interaction rates, calculated from a set of standard interactions. Among those are charged-current reactions, for which the elastic approximation is commonly assumed. More sophisticated alternatives are provided by, e.g., the publicly available NuLib library, or rates computed within a full kinematics approach. In this work, we compare merger simulations of a BNS systems employing each of these three prescriptions for the computation of charged-current weak rates, included in the M1 equations with an implicit-explicit time integrator. We find that the elastic approximation slightly overestimates the dynamical ejecta mass, while underestimating the amount of high electron fraction material therein. Additionally, it overestimates the neutrino luminosities at early times. The effects of full kinematics in comparison to the NuLib rates appear negligible within the uncertainties of the simulation data. Similarly, the post-merger gravitational-wave spectra for the three prescriptions were found to be indistinguishable within the numerical uncertainties. Nucleosynthetic yields, computed with a nuclear network, are broadly consistent, except in the atomic mass range $A\in[90,125]$ where the elastic approximation shows larger abundances. Finally, the different interaction rates seem to affect the remnant disk properties, hinting at the importance of more accurate interaction rates for simulations spanning secular timescales.
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High Energy Astrophysical Phenomena

The rise and decline of a transient obscuration event in a moderately distant type-I quasar

A. Akylas, A. Georgakakis, E. Pouliasis et al. (+5 more)

Outflows of ionised material, often dubbed warm absorbers (WAs), are common in nearby Seyferts but sparse in distant and more energetic quasars (QSOs). We present multi-epoch X-ray and optical observations of the serendipitously identified type-I QSO 2MASS J14302580+4159572 at a redshift of $z=0.35$, revealing the rare rise and decline of a transient ionised absorber over a 22-year timescale. The discovery observations were obtained with XMM-Newton between 2002 and 2005. The first two observations show unremarkable X-ray spectra typical of unobscured type-I QSOs, whereas the 2005 observation reveals a pronounced suppression of the spectrum below $\sim3\,\mathrm{keV}$. Spectral analysis shows that this event is produced by a moderately ionised absorber with column density $N_{\rm H}\simeq5\times10^{22}\,\mathrm{cm^{-2}}$ and ionisation parameter $\log\xi\simeq1.4$. Joint analysis of XMM-Newton, NuSTAR, and Swift/XRT observations spanning 22 years indicates that the absorber remained stable for nearly 18 years before weakening in the 2023–2024 Swift/XRT data. Throughout this period, the intrinsic 3–10 keV continuum remained remarkably stable, indicating that the observed variability is driven by changes in line-of-sight ionised absorption rather than intrinsic luminosity fluctuations. We interpret the event as a thermally driven ionised outflow located outside the broad-line region (BLR), at sub-parsec to parsec scales, with a density of order $10^{6}\,\mathrm{cm^{-3}}$. Supporting this scenario, the high-ionisation [O\,III] $\lambda4363$ emission line varies in tandem with the X-ray absorption, suggesting that the outflow progressively modifies the ionisation state of the inner narrow-line region (NLR) on decadal timescales. These observations provide a rare view of the long-term evolution of a quasar warm absorber and its impact on the circumnuclear environment.
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All authors A. Akylas, A. Georgakakis, E. Pouliasis, I. Georgantopoulos, C. Ricci, M. Chira, A. Ruiz, E. M. Xilouris

High Energy Astrophysical Phenomena

Polarization of Synchrotron Emission from Rotation Powered Pulsars

Seima Saeki, Shota Kisaka and Jumpei Takata

With the launch of the Imaging X-ray Polarimeter Explorer in 2021, recent observations have provided X-ray polarization data for several rotation-powered pulsars. To fully exploit these data, it is important to better understand the relationship between polarization properties and the magnetospheric structure. In this paper, we investigate polarization properties in the outer gap model, incorporating both outward emission and inward emission toward the rotation axis, which has not been considered previously. We find that the polarization position angle varies at the intensity peaks, typically accompanied by a high polarization degree, as is in the case for outward emission. Moreover, the variation in the polarization position angle tends to exceed 50 degrees when the intensity peaks of outward and inward emissions overlap, compared to cases with only outward emission. We compare the polarization model including inward emission with IXPE observations. For example, for PSR B1509-58 where inward emission is proposed to be dominant in the outer gap model, we find that observed PA swing prefers a line of sight from the southern hemisphere rather than the northern hemisphere.
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Wednesday 16 September

High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Relativistic Stellar Oscillations from Ultralight Dark Matter

Shanae Oishi, Christopher Reyes and Jeremy Sakstein

Ultra-light dark matter, composed of bosons behaving as classical waves, can resonantly excite stellar oscillations. In this work, we investigate this phenomenon in relativistic stars, showing that $\ell=0$ fluid modes can be excited. We derive a framework for calculating the mode amplitude and apply it to neutron stars to predict the associated surface temperature fluctuations; the resulting signal lies beyond current observational sensitivities. We discuss prospects for extending this approach to other compact objects.
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High Energy Astrophysical Phenomena

Lepto-hadronic modeling of blazars associated with well-reconstructed high-energy neutrino events

Leonard Pfeiffer, Margot Boughelilba, Simone Garrappa et al. (+3 more)

The association of active galactic nuclei (AGN) with high-energy neutrinos has been studied through population and individual source analyzes, but whether these associations reflect physical processes or chance coincidences remains open. Blazars, AGN with relativistic jets aligned with the line of sight, spatially consistent with IceCube high-energy events, offer a natural framework to address this question. Combining multifrequency observations with physically motivated emission models allows us to assess whether these sources can plausibly account for the detected neutrinos. We study six blazars with single gamma-ray counterparts to well-reconstructed IceCAT-1 high-energy events, accounting for the improved localization expected from IceCAT-2, and compile their multifrequency spectral energy distributions (SEDs). We model these sources within a lepto-hadronic framework, exploring their parameter space to reproduce the observed emission while favoring solutions that maximize neutrino production. The SEDs can be reproduced with compact emission regions close to the broad-line region with moderate Doppler factors. The emission is generally leptonic-dominated, with subdominant contributions from hadronic cascades, except for one source where the gamma-rays are hadronic-dominated. For four, the energy density of the relativistic particles is close to equipartition with the magnetic field. The predicted neutrino detection rates reach up to $~5x10^{-2}$ per year, mostly compatible with one observed IceCube event under Poisson statistics. While these blazars are unlikely to significantly contribute to the diffuse neutrino flux, their properties are consistent with being possible counterparts to individual neutrino events, driven by contributions of external photon fields to the hadronic interaction rate, even for sources where these fields have previously been considered as non-significant.
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All authors Leonard Pfeiffer, Margot Boughelilba, Simone Garrappa, Sara Buson, Massimiliano Lincetto, Annette Bremer

High Energy Astrophysical Phenomena

Hydrodynamic Simulations of Eccentric Black Hole Encounters in AGN Discs

Andrew Imai, Henry Whitehead and Bence Kocsis

We investigate close encounters between stellar-mass black holes (BHs) in the gaseous discs of active galactic nuclei (AGNs), during which binary black holes (BBHs) may form. We perform a suite of 483 2D adiabatic viscous hydrodynamic simulations within a shearing box prescription using the Eulerian grid code Athena++. We co-evolve the two embedded BHs with the gas. To probe the dependence of capture on non-circular initial conditions, we vary the initial radial separation $b$, the eccentricity $e$ of one of the stellar BHs around the central supermassive black hole, and the eccentric phase angle $\phi_e$. We consider eccentricities from $e=0$ to $e=0.1$ and compare them with the local disc aspect ratio $h=H/R_0\simeq0.005$. We find that small eccentricities shift the capture window in the parameter space. Eccentricities of order the disc aspect ratio, in particular $e\sim h$–$2h$, produce successful captures at initial separations that do not capture in the circular models. By contrast, systems with $e \gg h$ retain less gas before encounter and have lower capture fractions across the sampled parameter grid. We find that the first periapsis distance is a useful predictor of direct capture. Predicting direct capture for $r_{\rm p}<0.1r_H$ correctly classifies $92.7\%$ of Hill sphere encounters. The Hill sphere gas mass helps identify gas-poor failures, but provides no clear additional capture boundary within this suite, in which the initial disc density, temperature, and viscosity parameter are held fixed. Pre-encounter eccentricity modifies gas-assisted BBH formation through its combined effects on the first-encounter geometry and the gas reservoir available for orbital energy dissipation.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR

Coronal Mass Ejections from Young Suns: An Observational View through the Solar-Stellar Connection

Kosuke Namekata

Kepler and TESS have revealed that magnetically active stars frequently produce flares and superflares, raising the possibility that frequent, fast, and massive coronal mass ejections (CMEs) may affect planetary space weather environments. However, observational constraints on stellar CMEs remain extremely limited. Several diagnostics, including Doppler-shifted spectral lines, coronal dimming, and radio bursts, have been explored mainly for cool dwarfs, while recent observations have begun to extend these searches to young solar-type stars, which serve as important benchmarks for the solar-stellar connection. The goal of this review is to summarize the current understanding and recent observational progress on CME signatures from young solar-type stars, with a primary focus on Doppler-shift methods. It examines how these signatures resemble solar observations and models, what the inferred physical properties can tell us about the occurrence of stellar CMEs and their contribution to stellar mass loss, and how Doppler-shift signatures can be connected with multi-wavelength diagnostics. Comparisons with CME candidates from cool dwarfs and close binaries further place these signatures in the broader context of stellar CMEs in different stellar environments. These results have important implications for the space weather environments of young planets, while also pointing to key directions for future observations and modeling.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR

A census of massive eclipsing binaries in the Whirlpool Galaxy

Cheyanne Shariat, Kareem El-Badry, Maude Gull et al. (+3 more)

We present a census of massive eclipsing binary (EB) candidates in the Whirlpool Galaxy (M51), a face-on spiral galaxy at 7.5 Mpc with $Z\sim Z_\odot$. Using 34 epochs of red-optical Hubble Space Telescope (HST) photometry and a single epoch of broadband UV-optical photometry, we identify 173 massive EB candidates ($>5~{\rm M_\odot}$) with orbital periods $P_{\rm orb}$=1-30 days. The sample traces young stellar populations across the spiral arms of M51. Joint light-curve and spectral energy distribution fits for 136 systems yield binaries spanning $T_{\rm eff}=10{,}000$-$50{,}000$ K, $R=7$-$66~{\rm R_\odot}$, and masses of $5$-$120~{\rm M_\odot}$. We use detailed injection-recovery simulations to quantify the survey's completeness and infer intrinsic properties of the massive binary population. At the population level, we infer a close-binary fraction of $f_{\rm close}=27.4\pm7.1\%$ for $2\leq P_{\rm orb}/{\rm d}<20$ and $0.1\leq q\leq1$. Over the same period range, the intrinsic period distribution is consistent with being log-uniform, ${\rm d}N/{\rm d}\log P_{\rm orb}\propto P_{\rm orb}^{\pi}$ with $\pi=0.29\pm0.32$. These demographics broadly match those of massive-star populations in the Milky Way, LMC, and SMC, with tentative evidence for a lower close-binary fraction at lower metallicity. Across the $\sim0.2$-$1.1~Z_\odot$ range spanned by these galaxies, the overall similarities suggest that the formation of close massive binaries depends only weakly on metallicity. Our survey establishes EBs as a powerful tool for measuring massive-binary demographics beyond the Local Group and provides a rich panchromatic census of $\sim$100,000 luminous stars and variables in M51.
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All authors Cheyanne Shariat, Kareem El-Badry, Maude Gull, Daniel R. Weisz, Charlie Conroy, Alessandro Savino

High Energy Astrophysical Phenomena

A Significant Dust Reservoir Uncovered with JWST in the Type Ic SN 1983V More Than 40 Years Post-Explosion

Sophia S. Kressy, Thomas Moore, Ori D. Fox et al. (+26 more)

Searching for late-time emission from supernovae (SNe) is an active field. Infrared (IR) wavelengths are sensitive to thermal emission from dust, which can be used to probe SN contributions to the cosmic dust budget and progenitor mass-loss histories. The older an SN, the more likely it is for any existing dust to have cooled below the detection threshold of most observatories, even JWST. Decades-old IR observations of SNe are therefore exceedingly rare. Here we present fortuitous and serendipitous JWST IR observations that detect a point source at the position of the Type Ic SN 1983V more than 40 yr post-explosion. We demonstrate that the source is unlike nearby HII regions and likely to be the dusty SN. We further show evidence from archival HST data of a plausible H/alpha counterpart associated with ongoing SN shock interaction that collisionally heats the dust. In this scenario, the dust is distributed in a torus, more consistent with mass loss from binary interaction than a spherical wind. While not the oldest SN detected by JWST (SN 1980K), SN 1983V is a close second. Moreover, it has a relatively large dust mass (~7.7 x 10^-3 M_solar), particularly for a stripped-envelope SN. Although the dust is not likely newly formed, it does suggest such systems may contribute to dust production, particularly in the early Universe where massive stars and binary systems were more common. Spectroscopic observations can ultimately confirm the SN nature of this source.
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All authors Sophia S. Kressy, Thomas Moore, Ori D. Fox, Nathan Smith, Raphael Baer-Way, Luc Dessart, Michael J. Dulude, W. V. Jacobson-Galán, Melissa Shahbandeh, Tea Temim, Chris Ashall, David A. Coulter, Alexei V. Filippenko, Sebastian Gomez, Jacob Jencson, Kyle Medler, Daniel Patnaude, Tamás Szalai, Schuyler D. Van Dyk, Aiden Agostinelli, Sarah Healy, Zachary G. Lane, Dan Milisavljevic, Shazrene Mohamed, Benjamin Radmore, Armin Rest, Arkaprabha Sarangi, Yossef Zenati, Noah Zimmer

High Energy Astrophysical Phenomena · Cross-listed from astro-ph.CO

On the Clustering Bias of Unresolved Gamma-Ray Sources

Bhashin A. Thakore, Marco Regis, Michela Negro et al. (+4 more)

The Unresolved $\gamma$-Ray Background (UGRB) encodes the collective emission of source populations that are too faint to be detected individually, and its cross-correlation with tracers of the large-scale structure has emerged as a powerful tool to characterize those populations. In this work, we analyze the weak-lensing–UGRB cross-correlation using a reference blazar model. By modeling the relation between blazar $\gamma$-ray luminosity and host-halo mass, we infer the average linear bias $\langle b(z) \rangle$ and host-halo mass $\langle M(z) \rangle$ of the unresolved blazar population. We find that the signal is dominated by moderately biased sources, with $\langle b \rangle \gtrsim 2$ at $z\simeq 1$, hosted by halos with $\langle M \rangle \sim \mathcal{O}(10^{13}\,M_\odot)$. Repeating the analysis with a misaligned-AGN model yields consistent bias values, indicating that the result is determined by the clustering properties of the UGRB rather than by the details of the $\gamma$-ray source model.
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All authors Bhashin A. Thakore, Marco Regis, Michela Negro, Stefano Camera, Daniel Gruen, Nicolao Fornengo, Aaron J. Roodman

High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Collective flavor conversion in dense neutrino plasmas

Damiano F. G. Fiorillo

Compact transient sources, such as supernovae (SNe) and neutron star mergers (NSMs), host a population of thermal neutrinos in their inner cores, which decouple from the dense matter as they stream out. Yet, collisionless does not mean free; their density is large enough to mediate collective waves driven by the coherent weak neutrino–neutrino interaction. These collective waves carry little energy, but crucially they transport flavor. Neutrinos thus form a collisionless plasma with flavor transport driven by flavor waves, whose quanta are called flavomons $\psi$. The wavelength of these flavomons is much shorter than the characteristic scales of SNe and NSMs, so that a brute-force numerical treatment of the neutrino plasma is beyond our capabilities. Yet flavomons cannot be neglected, as the stimulated neutrino decays, schematically $\overline{\nu}_e\to \overline{\nu}_\mu\psi$, cause a rapid buildup in their population, more conventionally called a flavor instability. We review the theory of the neutrino plasma, and systematically discuss the instabilities leading to flavomon growth, and connect this emerging quasiparticle description with recent advances in numerical neutrino flavor kinetics.
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High Energy Astrophysical Phenomena

Radiation-Driven Magnetic Fields in Sub-Keplerian Accretion Flows: An Alternative to MRI

Mukesh Kumar Vyas and Asaf Pe'er

Large scale magnetic fields play a crucial role in shaping the dynamics and structure of the inner parts of accretion disks and the resulting jets from the vicinity of black holes and neutron stars. Currently, the primary mechanism considered for generating and amplifying large scale magnetic fields in accretion disks is the magneto-rotational instability (MRI). Here we show that non-conservative radiation fields provide a rapid and independent mechanism for generating and amplifying magnetic fields in accretion flows. A luminous, compact corona generates a radiation-driven poloidal field that is subsequently amplified by differential rotation, yielding quadratic magnetic-field growth and, over a broad range of coronal luminosities and sizes and MRI amplification locations, magnetization on timescales comparable to or shorter than those of the MRI. The mechanism therefore provides not merely an alternative to MRI, but an additional and, over a substantial region of parameter space, dominant channel for magnetic-field generation from initial conditions with zero magnetic field. Since the radiation-driven source arises directly from the non-conservative nature of the radiation field, such magnetic-field generation is an inevitable consequence of sufficiently strong and anisotropic radiation sources in black-hole accretion flows. More generally, the mechanism requires only a non-conservative radiation field and differential plasma motion, and is therefore expected to operate in a broad range of luminous astrophysical systems, including active galactic nuclei, gamma-ray bursts, and tidal disruption events.
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High Energy Astrophysical Phenomena

Studying the Intermittency of Magnetized Interstellar Medium by Synchrotron Polarization Intensity and its Gradient

Ru-Yue Wang and Jian-Fu Zhang

Based on synthetic polarization observations, we qualitatively investigate the intermittency of the magnetized interstellar medium (ISM) through synchrotron polarization intensity (SPI) and synchrotron polarization gradient (SPG). Three principal findings emerge from our analysis: (1) The intermittency of the magnetized ISM is strongly modulated by magnetization and compressibility, with pronounced modulation effects predominantly observed in regimes characterized by strong magnetization and high compressibility. (2) The characterized intermittency of the magnetized ISM demonstrates a dependence on the electron spectral index under conditions where the random magnetic field dominates. (3) SPG enables the identification of finer three-dimensional fractal structures in the case of strong Faraday depolarization. The synergistic application of SPI and SPG substantially enhances the reliability of probing magnetized ISM intermittency from real observations.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Shadow and Polarized Images of Schwarzschild-MOG Black Hole Illuminated by Thick Accretion Disk

Cheng Hu, Huan Ye, Hong Lei et al. (+1 more)

Recent EHT polarimetric observations provide a powerful probe of near-horizon physics and modified gravity. We investigate the shadow and polarization images of a static Schwarzschild-MOG black hole illuminated by geometrically thick, optically thin accretion flows. Adopting a phenomenological model and the Hou disk model, and solving null geodesics and relativistic radiative transfer, we study how the MOG parameter, inclination angle, anisotropic synchrotron emission, and flow geometry affect the total intensity and linear polarization. It is found that the MOG parameter mainly enlarges and brightens higher-order photon rings and broadens the shadow, while the inclination angle breaks axial symmetry and enhances polarized emission inside the shadow. The Hou disk model suppresses off-equatorial radiative interference and localizes polarization signals within higher-order rings. Electric vector position angle (EVPA) patterns are mostly azimuthal but vary rapidly near lensed images. These results provide theoretical diagnostics for constraining MOG with EHT/ngEHT polarimetric observations.
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All authors Cheng Hu, Huan Ye, Hong Lei, Zi-Chao Lin

High Energy Astrophysical Phenomena

WFST Follow-up of S251112cm: Searching for an Optical Counterpart to a Subsolar-mass Compact-binary Merger Candidate

Zhengyan Liu, Zelin Xu, Ji-an Jiang et al. (+20 more)

Electromagnetic counterparts to gravitational-wave (GW) sources probe the properties, environments, and evolution of compact-object mergers. S251112cm belongs to an emerging class of GW candidates with possible subsolar-mass components. We present an optical counterpart search for S251112cm with the Wide Field Survey Telescope (WFST). The observations began 20.2 hr after the GW trigger and continued for three nights, covering approximately $780~\mathrm{deg}^{2}$ and $51\%$ of the localization probability in the updated skymap. We searched for newly emerging, rapidly evolving, off-nuclear optical transients and identified four candidates whose host-galaxy distances are broadly consistent with the GW distance estimate. However, all four evolve substantially more slowly than AT 2017gfo, ruling out an AT 2017gfo-like origin and disfavoring their association with S251112cm as rapidly evolving kilonova counterparts. Combining WFST and DECam observations increases the covered localization probability to approximately 67%. Conditional on the counterpart lying within this footprint, we constrain a grid of binary-neutron-star kilonova models. For viewing angles $\theta_{\rm obs}<60^{\circ}$, with the other parameters fixed to the best-fitting values for AT 2017gfo, models with dynamical ejecta mass $M_{\rm dyn}\gtrsim0.01\,M_{\odot}$ or wind ejecta mass $M_{\rm wind}\gtrsim0.05\,M_{\odot}$ are disfavored over most of the covered GW probability. Beyond kilonova models, our phenomenological analysis constrains rapidly evolving optical transients with $T_{\rm rise,1/2}\leq2$ days to peak absolute magnitudes fainter than approximately -13 mag. To our knowledge, the coordinated WFST and DECam observations provide the strongest optical constraints to date on possible rapidly evolving counterparts to this new class of GW candidates involving subsolar-mass compact objects.
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All authors Zhengyan Liu, Zelin Xu, Ji-an Jiang, Wen Zhao, Zhiping Jin, Zigao Dai, Dezheng Meng, Yefei Yuan, Xuefeng Wu, Lulu Fan, Xu Kong, Xander J. Hall, Brendan O'Connor, Feng Li, Ming Liang, Binyang Liu, Zhen Wan, Hairen Wang, Jian Wang, Tinggui Wang, Hongfei Zhang, Xianzhong Zheng, Qingfeng Zhu

High Energy Astrophysical Phenomena

The unidentified PeV source LHAASO J2108+5157: A microquasar remnant?

Josep Martí, Pedro L. Luque-Escamilla, Leandro Abaroa et al. (+4 more)

Context. LHAASO~J2108+5157 is one of the most intriguing Galactic ultra-high-energy $\gamma$-ray sources. It is detected up to hundreds of TeV but lacks a clear counterpart at lower frequencies. Fermi-LAT observations have revealed a compact GeV source spatially coincident with the PeV emission, while VERITAS and LST-1 impose stringent upper limits in the intermediate TeV band, producing a pronounced spectral suppression between the GeV and TeV domains. Aims. We investigate a time-dependent hadronic scenario in which a past episode of microquasar jet activity injected relativistic protons that are now observed in a relic phase. Methods. Energy-dependent diffusion naturally segregates the particle population: Lower-energy protons remain confined near dense molecular gas and account for the GeV emission, whereas higher-energy particles reach more distant material, producing the hard TeV–PeV component. Results. For representative interstellar conditions, the model is simultaneously consistent with the broadband spectral energy distribution, the spatial compact–extended dichotomy, and the lack of detectable radio or X-ray counterparts. Conclusions. The observed bimodal $\gamma$-ray phenomenology thus emerges as a transport effect of a single injected proton population, supporting the interpretation of LHAASO~J2108+5157 as a fossil microquasar remnant observed at an intermediate evolutionary stage and highlighting relic jet systems as a viable class of Galactic PeVatrons.
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All authors Josep Martí, Pedro L. Luque-Escamilla, Leandro Abaroa, Gustavo E. Romero, Valentí Bosch-Ramon, Josep M. Paredes, Cintia S. Peri

High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Probing Feebly Interacting Particles with 511 keV Line from Circumstellar Medium of Supernovae

Yu Cheng, Chui-Fan Kong, Yen-Hsun Lin et al. (+2 more)

Core-collapse supernovae (CCSNe) are intense sources of feebly interacting particles (FIPs), whose visible decays beyond the stellar envelope can generate distinct electromagnetic signals. We propose the circumstellar medium (CSM) surrounding the CCSNe progenitor as a new target for 511 keV line searches. FIPs escaping the stellar interior may decay into electron-positron pairs and inject positrons into the CSM, where they slow down and subsequently annihilate, giving rise to a 511 keV line as an electromagnetic precursor to the supernova shock breakout. Using dark photons as a benchmark, we calculate the time-dependent signal for CSM profiles motivated by SN 2023ixf and SN 2024ggi. For a Galactic supernova at 10 kpc, the projected reach of COSI and AMEGO extends beyond existing supernova limits over part of the MeV-scale dark-photon parameter space. The same FIP decays can simultaneously heat the CSM and sublimate circumstellar dust, producing a rapid spectral transition from infrared excess to optical or ultraviolet emission [Y. Cheng et al., Phys. Rev. Lett. 137, 111005 (2026)]. The coincident appearance of this transition and the 511 keV line would provide a distinctive correlated multiwavelength signature of FIP decays in the dense presupernova environment.
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All authors Yu Cheng, Chui-Fan Kong, Yen-Hsun Lin, Meng-Ru Wu, Seokhoon Yun

High Energy Astrophysical Phenomena

Toward Autonomous Radio Follow-up of Multi-messenger Transients with RADAR: From Alert Parsing to Inference and Observation Scheduling

Mihael Hategan-Marandiuc, Tanner O'Dwyer, Alessandra Corsi et al. (+6 more)

Multi-messenger astronomy (MMA), the joint study of cosmic sources through gravitational waves (GWs), electromagnetic (EM) radiation, neutrinos, and cosmic rays, is rapidly reshaping time-domain astrophysics. Realizing the promise of MMA will require coordinating heterogeneous observing resources and automating the chain from alert to analysis to follow-up. RADAR (Radio Afterglow Detection and AI-driven Response) is a federated, privacy-enhancing framework for the radio follow-up of GW events, previously validated on GW170817. Here, we extend it along three axes. First, we benchmark three large language models (LLMs; GPT-5.5, Claude-Opus-4.7, and Gemini-3.5-Flash) against the GW170817 radio light curve dataset. GPT-5.5 attains the highest event-level $F_1$ score, the harmonic mean of precision and recall, at $0.893 \pm 0.010$, and the highest GCN-level recall, $0.794 \pm 0.013$, improving on previous GPT-4.1 results by 16% and 10%, respectively, while Claude-Opus-4.7 achieves the highest precision, $0.978 \pm 0.014$. Second, we introduce concurrent likelihood evaluation, which speeds up the MCMC computation by a factor of $40\times$ over our previous results. Third, we present an LLM-driven scheme that converts natural-language observing requests into submission-ready scheduling blocks for the Karl G. Jansky Very Large Array. Together, these developments advance RADAR toward a scalable, largely autonomous system for GW radio follow up.
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All authors Mihael Hategan-Marandiuc, Tanner O'Dwyer, Alessandra Corsi, Eliu Huerta, Zilinghan Li, Ian T. Foster, Kyle Chard, Ryan Chard, Amal Gueroudji

High Energy Astrophysical Phenomena

Detectability of 100 TeV Gamma Rays from PeV Cosmic Rays Accelerated in the Early Phase of a Supernova Shock Wave

Tomotaka Nishikawa, Tsuyoshi Inoue and Alexandre Marcowith

Galactic cosmic rays (CRs) are thought to be accelerated by diffusive shock acceleration in supernova remnants (SNRs), but observations of SNRs aged ~100-1000 yr suggest maximum CR energies below the PeV level. Recently, Inoue et al. (2021) showed with kinetic-MHD simulations that CRs can be accelerated to ~3 PeV when a blast wave shock propagates through dense circumstellar material (CSM) within tens of days after explosion. Such CSM can be produced by a red-supergiant (RSG) wind with an observationally motivated mass-loss rate of ~10^-3 Ms/yr. 100 TeV gamma-rays from neutral pion decay can test PeV CR acceleration. However, hadronic gamma-rays from very young SNRs can be significantly attenuated by supernova photospheric photons and the cosmic background radiation. Using the CR distribution functions and CSM density profiles from Inoue et al. (2021), we calculate time-dependent gamma-ray fluxes from Type II-P SNe, including both attenuation processes. We find that photospheric photons attenuate the intrinsic 1-100 TeV gamma-ray flux by about one order of magnitude. For the dense CSM case with dot{M}_RSG ~ 10^-3 Ms/yr and an outer CSM radius of ~ 3x10^15 cm, CTA can detect 100 TeV gamma-rays with a 50 h observation out to ~ 4 Mpc (this is reduced to 3 Mpc if the spatial extent of the CSM is shrunk to 2x10^15 cm). These results suggest that early 100 TeV observations of nearby Type II-P SNe can probe PeV CR acceleration in sufficiently dense and extended CSM.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Gravitational lensing of neutrinos through the lens of decoherence

Bikash Kumar Acharya and Ujjal Kumar Dey

We propose a new source of neutrino decoherence arising from stochastic gravitational lensing by unresolved mass substructure. Unlike traditional models that treat the lensing potential as smooth and deterministic, we decompose the lensing potential into a smooth component and a stochastic population of compact clumps. The substructure induced fluctuations of the lensing potential generate stochastic corrections to the neutrino time delay, thereby producing random relative phases between different neutrino mass eigenstates. Averaging over the clump ensemble converts these phase fluctuations into an effective dampening of flavour coherence. This framework connects stochastic gravitational lensing with neutrino phenomenology and offers a novel route to probe dark matter substructure through astrophysical neutrino flavour transitions.
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High Energy Astrophysical Phenomena

RadioAstron space-VLBI imaging of the jet in M87: II. The parsec-scale structure at 4.8 GHz

Evgeniya V. Kravchenko, Tuomas Savolainen, Alexey S. Nikonov et al. (+9 more)

We present RadioAstron observations of the M87 jet obtained on 2014 February 4-5 at 4.8 GHz ($\lambda=6.2$ cm). The correlated signals between the 10-m space antenna and ground-based radio telescopes were detected up to projected baselines of $\thicksim3$\,Earth diameters (0.65~G$\lambda$). The orientation of the jet downstream of the core shifts northward, consistent with the long-term oscillating trend reported in other studies. An elliptical body mode of the Kelvin-Helmholtz instability can create the observed helical shape of the jet. The estimated wavelength of this instability mode suggests that its origin may be related to some processes in the accretion disc. We present 1.7 - 4.8 GHz and 4.8 - 15.4 GHz spectral index maps showing flattening of the spectrum along the outer streamlines and along the jet spine. Simulations and analysis show that this spectral flattening is partially artificial and is a common systematic effect of the CLEAN procedure of VLBI imaging. The spectral index of the M87 jet is consistent with the optically thin emission, while its steepening can be produced by the propagation of the instability modes causing higher pressure regions near the jet boundary and in its interior.
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All authors Evgeniya V. Kravchenko, Tuomas Savolainen, Alexey S. Nikonov, Ilya N. Pashchenko, Elena E. Nokhrina, Petr A. Voitsik, Leonid I. Gurvits, Yuri Y. Kovalev, Jae-Young Kim, Mikhail M. Lisakov, Andrei P. Lobanov, Kirill V. Sokolovsky

High Energy Astrophysical Phenomena

FRB 20250607A: Weirder Than Fiction

J. C. F. Balzan, A. Bera, C. W. James et al. (+4 more)

We present a detailed analysis of FRB 20250607A, a two-component FRB detected by the CRAFT survey which exhibits several unusual component-dependent properties, including apparent differential DM, circular polarisation, and time-resolved RM, together with a polarisation-angle (PA) swing that persists through the scattering tail. The scattering timescale and scintillation bandwidth are inconsistent with a single thin-screen origin, placing the dominant scattering screen within $\lesssim61$ kpc of the FRB source. The lack of PA flattening in the scattering tail is a behaviour not previously reported in an FRB or pulsar and difficult to reconcile with a simple propagation origin. We find no statistically significant differential DM between the two components despite an apparent, and visually obvious, difference in their dispersion, indicating that current uncertainties may be overly conservative. If this apparent differential DM is true, it implies a substantial difference in electron column density between the component sightlines. Two further CRAFT FRBs, FRB 20190611B and FRB 20210407E, show similarly large component-dependent electron-column differences, corresponding to electron-density contrasts consistent with plasma local to a neutron-star magnetosphere. We find no evidence for generalised Faraday rotation, and FRB 20210407E shows component-dependent PA structure. Taken together, these results suggest that the observed DM, circular polarisation, and PA behaviour may arise from differing sightlines through an inhomogeneous or evolving magnetosphere rather than from independent phenomena. Multi-component FRBs therefore provide a sensitive probe of plasma structure local to their progenitors.
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All authors J. C. F. Balzan, A. Bera, C. W. James, B. W. Meyers, A. T. Deller, R. M. Shannon, Z. Wang

High Energy Astrophysical Phenomena · Cross-listed from astro-ph.GA

A Possible Hidden Hot Halo Component Revealed by Joint X-ray and Sunyaev–Zel'dovich Observations

Jiang-Tao Li, Rui Huang, Joel N. Bregman et al. (+2 more)

Massive galaxies are expected to host extended halos of hot gas that contain a substantial fraction of their baryons and feedback energy. X-ray observations probe the dense, X-ray-bright component of this circumgalactic medium (CGM), while the thermal Sunyaev-Zel'dovich (SZ) effect is also sensitive to lower-density gas through its integrated thermal pressure. We present the first spatially resolved joint X-ray and SZ analysis of a massive isolated disk galaxy, using deep XMM-Newton observations of NGC 4594 from the X-raying the Accretion Reservoir Transferred to the ATmosphere Orbiting a Massive Spiral (XART-ATOMS) program, together with multiple independent Planck-based SZ reconstructions. The measured SZ signal in the inner halo appears to exceed the value predicted from the X-ray-derived gas properties, even after accounting for major systematic uncertainties. Our preferred interpretation is that an additional lower-density CGM component contributes little to the observed soft X-ray emission but carries substantial thermal pressure. Under a simple two-phase model in approximate pressure equilibrium, the most probable X-ray-to-SZ ratio implies that the detected X-ray-emitting gas occupies only a small fraction of the halo volume, with a characteristic filling factor of f_X ~ (4-6)x10^-3 near r~50 kpc. Such a small filling factor implies that single-phase X-ray analyses can underestimate the baryon mass contained in the hot CGM by a factor of ~2.5, while the thermal energy content of the CGM is underestimated by about one order of magnitude. The non-detection of the additional component in the X-ray spectra requires it to be very hot and/or spatially extended, so weak in the soft X-ray emissions. These results favor the presence of a "hidden" X-ray-faint hotter halo component, while alternative interpretations, including a nonthermal contribution, are not excluded.
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All authors Jiang-Tao Li, Rui Huang, Joel N. Bregman, Q. Daniel Wang, Cameron Pratt

Tuesday 15 September

High Energy Astrophysical Phenomena · Cross-listed from gr-qc

An Operative Viability Boundary for Relaxed Single-Particle Penrose Extraction in Kerr-Vaidya Spacetimes

Fabio Buffoli

We present a corrected numerical investigation of the Penrose process in the Kerr-Vaidya metric for a rotating black hole losing mass at constant rate. We fix two errors from an earlier version: an incorrect metric component g_r phi (verified by transformation from Boyer-Lindquist coordinates) and the omission of the Wald/Christodoulou area theorem in the split optimization. Using the standard single-particle treatment (energy and angular momentum conserved at the split, but not the escaping fragment's radial momentum), we compute optimal trajectories and the bounded energy gain Delta E = E3 - E1 versus spin a/M and mass-loss rate alpha. For static Kerr holes, Delta E rises with spin from about 0.106 M at a/M = 0.8 to 0.245 M at a/M = 0.99. In the dynamic case, holding a fixed while m(v) decreases causes the effective spin a/m(v) to drift strongly toward extremality (from 0.81 to 0.9999), a geometric effect that makes static-dynamic comparisons at equal nominal a/M misleading. We map the critical mass-loss rate alpha_crit(a) above which no viable extraction is found (using m >= 0), obtaining alpha_crit(0.90) ~ 0.036, alpha_crit(0.95) ~ 0.019 and alpha_crit(0.99) ~ 0.0038, decreasing with spin. A check with full four-momentum conservation recovers the Bardeen-Press-Teukolsky bound near the horizon, but escaping solutions are rare away from it; thus our main results refer to the relaxed single-particle formulation common in the literature.
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High Energy Astrophysical Phenomena

Updated estimates of post-merger gravitational wave energy released in GW170817 and the maximum neutron star mass

Yong Chen, Bo Gao, Yong-Jia Huang et al. (+2 more)

In this work, benefiting from the increased sample of neutron stars with measured masses and radii as well as the incorporation of the chiral effective field theory and perturbative QCD constraints, the tidal parameter $\kappa_2^T$ is constrained to be $78^{+17}_{-11}$ (68.3% credible interval, mainly adopted in this work unless mentioned specifically) for the binary neutron stars involved in GW170817. Such a $\kappa_2^T$ is in favor of strong gravitational wave radiation in the post-merger phase and the corresponding energy is estimated to be $E_{\rm GW,p} \simeq 0.051^{+0.022}_{-0.017}\,M_\odot c^2$. Assuming the remnant from binary neutron star merger is a supramassive neutron star, as suggested by the modeling of the electromagnetic counterparts of GW170817, we examine the maximum mass of nonrotating neutron stars ($M_{\rm TOV}$) while accounting for the uncertainty in the remnant's lifetime ($t_{\rm c}$). Our results show that $M_{\rm TOV}$ varies from $2.09^{+0.11}_{-0.09}\,M_\odot$ for $t_{\rm c}=0.1$ s to $2.18^{+0.10}_{-0.09}\,M_\odot$ for $t_{\rm c}=1$ s. This result is consistent with that independently inferred from the re-construction of the equation of state of neutron star matter, i.e., $M_{\rm TOV,exc}=2.16^{+0.10}_{-0.07}M_\odot$, particularly if the very massive neutron stars with masses measured indirectly have been removed in constructing the prior distribution of $M_{\rm TOV}$. The consistency of the maximum mass of nonrotating neutron stars found in different approaches suggests a reasonable understanding of this key parameter for dense-matter physics.
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All authors Yong Chen, Bo Gao, Yong-Jia Huang, Shao-Peng Tang, Yi-Zhong Fan

High Energy Astrophysical Phenomena

Radiation Magneto-hydrodynamic Simulations of MRI with Zero-Net-Vertical-Flux: Necessity of Resolving the Thermal Scale For Strongly Magnetized Initial Conditions

Yi-Xian Chen, Yan-Fei Jiang and Jeremy Goodman

We perform 3D radiation magneto-hydrodynamic (RMHD) shearing-box simulations of local patches of optically thick accretion disks, applicable to sub-Eddington active galactic nuclei (AGN) at $\sim 1000$ gravitational radii around a supermassive black hole (SMBH) of $10^7-10^8M_\odot$. In particular, we set up zero-net-vertical-flux (ZNVF) simulations with strong net azimuthal fields ($B_y$) characterized by initial gas-to-magnetic pressure ratio $\beta_0$. We find that $\beta_0 \sim 1$ simulations relax to the classical MRI state with steady-state $\beta\sim 10-20$ and slow periodic reversals of the mean azimuthal field (dynamo cycles), losing memory of their initial $B_y$ configuration. The outcome of simulations starting from $\beta_0=0.1$ (superthermal magnetic pressures) depends upon the numerical resolution as quantified by the number of grid cells per thermal scale height $H_\mathrm{th}$. Resolved simulations ($\Delta z\le H_\mathrm{th}/5$) settle into final states similar to the larger-$\beta_0$ runs, exhibiting thermally dominated midplanes heated by MRI turbulence and undergoing dynamo cycles. In contrast, lower resolution $\beta_0 = 0.1$ runs evolve towards states dominated by a coherent mean field, similar to what is observed in recent isothermal MHD simulations. However, those disks fail to sustain sufficient turbulent heating near the midplane and undergo runaway cooling and contraction. While such states might be sustained in global models with even lower initial $\beta_0$ and/or continuous $B_y$ injection, our results indicate that they could arise purely from under-resolving the midplane MRI dynamo that would otherwise be able to generate and emanate randomized fields. We highlight the need to resolve a fraction of the thermal scale height (the classical MRI wavelengths) for strongly magnetized initial conditions to obtain converged outcomes in RMHD simulations.
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High Energy Astrophysical Phenomena

The Hidden Evolution of Gamma-Ray Burst Jets Revealed by Gravitational Waves

Gerardo Urrutia, Fabio De Colle, Agnieszka Janiuk et al. (+2 more)

Gamma-ray burst (GRB) jets can propagate through massive stars, where the high opacity of the stellar envelope traps electromagnetic radiation and limits observations of the jet at early times. Gravitational waves (GWs), in contrast, can escape these dense environments and provide direct information of the jet dynamics. Previous studies have shown that GRB jets can emit GWs at low frequencies, while high-frequency emission may arise from jet-driven cocoons. However, the cocoon contribution typically requires energies much larger than those inferred from observations and produces stochastic fluctuations that can resemble noise. In this work, we show that high-frequency GW signals, can instead be generated by variability in the central engine. We consider jet models with different luminosity histories. We find that the GW emission depends on both the central engine history and the interaction of the jet with the stellar envelope. Rapid luminosity fluctuations are progressively suppressed as the jet propagates through the star, while the GW signal preserves information about the early jet evolution. Jets powered by strongly variable engines produce a GW signal peaking at tens of Hz, whereas smoothly varying engines are dominated by low-frequency emission. For a source at 36 Mpc, comparable to the distance of the closest GRB observed to date, the high-frequency signal from our most energetic variable models can reach amplitudes detectable by current ground-based interferometers. Our results suggest that the detection or non-detection of GWs from nearby GRBs and Type Ic broad-lined SNe could help constrain the mechanisms operating in the central engine.
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All authors Gerardo Urrutia, Fabio De Colle, Agnieszka Janiuk, Claudia Moreno, Enrico Ramirez-Ruiz