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Physics Astrophysics High Energy Astrophysical Phenomena

astro-ph.HE Watch High Energy Astrophysical Phenomena

Wednesday 23 September

High Energy Astrophysical Phenomena

Broad-band spectral-timing: simultaneous NICER and HXMT observations reveal an anticorrelation between the softest and hardest X-ray fluxes in MAXI J1820+070

Niek Bollemeijer, Phil Uttley and Bei You

Black hole X-ray binaries (BHXRBs) in the hard state show variable X-ray emission across a wide range of energies. Hard X-rays are thought to be produced through Compton-upscattering of seed photons from the accretion disc by the hot X-ray corona, leading to a spectral component roughly shaped like a power-law with a high-energy cut-off. A complete study of the coronal variability requires a full view of the spectrum, from the low-energy cut-off at the seed photon energy to the high energy cut-off, which may be related to the coronal temperature. We present a detailed spectral-timing study of strictly simultaneous observations by NICER and Insight-HXMT of the bright BHXRB MAXI~J1820+070, covering the 0.3-250 keV energy range. The high data quality allows us to study the lags and coherence across the full X-ray band. We find that at low Fourier frequencies ($\lesssim0.02$ Hz), the soft X-ray flux below 1 keV and hard X-ray flux above 100 keV are anticorrelated, i.e. phase lags of $|\pi|$ rad. No anticorrelation is observed when comparing energy bands above 1 keV. We consider several explanations for the observed complex behaviour, such as a modulation of the coronal cut-off energy by the disc, power-law pivoting and a contribution from non-thermal electrons. Future, more detailed modelling efforts based on the observed effects may place new constraints on the dynamics of the X-ray emitting regions around accreting black holes.
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High Energy Astrophysical Phenomena

Relativistic accretion-disc models of Cyg X-1 in the soft state: a case study

Luciano Ye and Luigi Foschini

The thermal continuum of Cygnus X-1 in the soft state is often modelled with a multicolour disc in a Newtonian spacetime approximation. That choice is convenient for estimating the inner radius, but it neglects relativistic effects near the black hole; these are usually restored through colour-correction and boundary-condition factors. Using Suzaku data from one exemplary observation, we compare this Newtonian description with some fully relativistic disc models, keeping the same treatment of absorption, ionised winds, Comptonization and reflection. With the mass, distance and inclination fixed to the best current dynamical values, the Newtonian inner radius is consistent with a high-spin Kerr innermost stable circular orbit. The relativistic models do not provide a uniformly better description of the spectrum: some worsen the fit or leave spin, inclination and hardening unconstrained, while the formally best model requires an inclination much larger than the dynamical one or a smaller spin. Apparent tensions with the models arise from data degeneracies and from the limited energy band, not from a failure of general relativity.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR

White Dwarf Pulsars: an emerging class of compact binaries

David A.H. Buckley

I summarise the expanding class of white dwarf pulsars for which the prototype, AR Scorpii, was only discovered in 2016. At the time of this meeting, two further systems had been found (eRASSU J191213.9-441044 and SDSS J230641.47+244055.8). These are thought to be detached (or nearly detached) white-dwarf–red-dwarf binaries, where the white dwarf's magnetic field interacts with the M-star's corona, resulting in the capture and acceleration of electrons to relativistic velocities in the magnetosphere. This gives rise to non-thermal synchrotron emission across the electromagnetic spectrum. The first discovered, and brightest, system, AR Sco (which is the focus of this paper), shows a white dwarf spin-down consistent with magnetic dipole emission in a strongly magnetic white dwarf. The polarisation properties of the two systems for which polarimetry has been obtained (AR Sco and eRASSU J191213.9-441044) show spin-modulated polarisation, primarily linear, which remarkably reaches 40% in AR Sco. Furthermore, the Stokes Q and U parameters vary in the same manner as the optical polarisation of the Crab pulsar, consistent with a rotating magnetic dipole. These systems are intriguing in that each one seems to show different behaviours (e.g. pulse profiles and light-curve properties), presumably mainly due to different viewing aspect angles. It has recently been postulated that these systems represent a short-lived phase of binary evolution linking asynchronously rotating intermediate polars and synchronous polars.
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High Energy Astrophysical Phenomena

PANGU I: A Parthenon-Based Framework for Fixed-Spacetime GRMHD

Yuehang Li, Fan Zhou, Mingyang Luan et al. (+2 more)

Existing general-relativistic magnetohydrodynamics (GRMHD) codes offer complementary capabilities, but these capabilities remain distributed across different frameworks. To unify them, we introduce PANGU–Parthenon-Based Astrophysics for Numerical Relativity and GRMHD in a Unified Framework–a modular computational framework that integrates a shared solver across physical regimes, flexible spacetime geometry, and extensible computational infrastructure within a single architecture. In the first paper of this series, we present and validate the fixed-spacetime GRMHD component of PANGU. The shared solver combines finite-volume Godunov methods with constrained transport on uniform and block-structured, statically refined meshes, and covers hydrodynamics and magnetohydrodynamics in Newtonian, special-relativistic, and general-relativistic regimes. PANGU provides a flexible geometry interface supporting coordinate selection and either on-the-fly or precomputed evaluation of prescribed stationary spacetimes, demonstrated here in Kerr spacetime with Cartesian Kerr-Schild and modified Kerr-Schild coordinates; additional components can be incorporated without restructuring the core solver. Verification covers all three regimes on uniform and statically refined meshes: smooth Newtonian and special-relativistic problems approach second order against their analytic solutions, with deliberately matched double-precision trajectories agreeing with AthenaK to roundoff; in fixed-spacetime GRMHD, the stationary analytic solutions are preserved to second order, and the turbulent accretion reproduces the same flow and flux scales as BHAC. Together, these results establish the fixed-spacetime matter-sector foundation for the broader PANGU framework. Numerical-relativity and coupled dynamical-spacetime GRMHD capabilities will be presented in subsequent work.
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All authors Yuehang Li, Fan Zhou, Mingyang Luan, Minyong Guo, Bin Chen

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

H_alpha observations of the Be/X-ray binary MWC 656

R. Zamanov, V. Marchev, K. A. Stoyanov et al. (+8 more)

Context. MWC 656 is a Be/X-ray binary identified as an emission-line object, X-ray and radio source and might be a weak gamma-ray source, harbouring an enigmatic secondary component. Aims. Our aim here is to study the orbital modulation of the H-alpha emission line, disc truncation and to evaluate the binary parameters. Methods. During the period 2021-2026, we obtained high resolution (30000) optical spectra of MWC 656 and analysed them together with the published data. Results. We find an orbital modulation of the H_alpha emission line with period P orb = 60.43 +/- 0.03 d. By assuming that the Be circumstellar disc is truncated by the companion, we estimate orbital eccentricity e = 0.11 +/- 0.01, semimajor axis a=160 R_sun, and mass of the binary M_1 + M_2 = 15.1 M_sun. Conclusions. The orbital modulation of the H_alpha emission suggests that (1) in this object we have a new type of cyclic variability of a Be disc; (2) the orbit is mildly eccentric; and (3) the secondary component of MWC 656 is a massive object 5 M_sun, more massive than the neutron stars - a stellar-mass black hole or a massive stripped star.
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All authors R. Zamanov, V. Marchev, K. A. Stoyanov, M. D. Christova, M. Minev, M. Moyseev, D. Marchev, J. Marti, M. F. Bode, P. Markishki, S. Stefanov

High Energy Astrophysical Phenomena

Origin of the high-energy spectral cutoff in gamma-ray pulsar halos

Qi-Ling Chen, Ming-Ming Kang, Kun Fang et al. (+2 more)

The spectra of gamma-ray pulsar halos exhibit a clear cutoff feature at tens of TeV. This work investigates the origin of the spectral cutoff by interpreting the HAWC data of the Geminga halo. The most straightforward interpretation is that the cutoff reflects the maximum electron acceleration capability of the central PWN. We model this scenario using spherically symmetric diffusive shock acceleration, where the upstream scale of the PWN termination shock limits the maximum energy. We also propose an alternative interpretation of the cutoff: if the electron diffusion coefficient in the surrounding interstellar medium rises rapidly at high energies, a substantial fraction of high-energy gamma-ray flux from the target source can leak outside the analysis region, producing an observed spectral cutoff effect. Both explanations successfully reproduce the spectral cutoff of the Geminga halo. Meanwhile, the two scenarios make distinct predictions on the statistical properties of pulsar halo spectral cutoffs and the X-ray spectral features of the corresponding PWNe, which may be tested through further observations.
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All authors Qi-Ling Chen, Ming-Ming Kang, Kun Fang, Yi-Qing Guo, Chao-Wen Yang

High Energy Astrophysical Phenomena

On the rates of binary star collisions around hypermassive black holes

Megha Sharma, Alexander Heger, Evgeni Grishin et al. (+1 more)

Observations of the Milky Way show that galaxies have binary stars in their centres. Using Monte Carlo simulations, we model the population statistics of encounters of such binaries with galactic central black holes (BHs) of mass 10^5-10^{10} Msun (hypermassive BHs >= 10^8 Msun HMBHs). Whereas main-sequence single stars cannot undergo tidal disruption events around HMBHs, stellar collisions of main-sequence binary stars may produce observable transients. We use the post-Newtonian Einstein-Infeld-Hoffmann (EIH) equations as implemented in the three-body code Multistar. We simulate 100,000 models each for six BH masses and 10 different binary parameter distributions. We find that stellar collisions occur in all models, with collision fractions ranging from ~7% for circular, equal-mass binaries to ~0.2% for the observationally motivated Moe2017 binary distribution. Most collisions occur for prograde, tight binaries at low impact parameters beta = r_t/r_p, where the tidal force of the BH drives up the binary eccentricity during pericentre passage. Collision velocities are close to the binary escape velocity for lower-mass BHs (<= 10^8 Msun), but span a wider range for higher-mass BHs, implying an increasing fraction of stellar mergers. Using empirical loss-cone rates and the local BH mass function, we estimate detectable LSST collision rates of ~8,600 and ~3,000 events per year for our Kroupa mass function, Opik's law, inclined orbit model and our MS17 model, respectively, around 10^6 Msun BHs; ~330 and ~150 events per year around 10^8 Msun BHs; and ~3.5 and ~2.8 events per year around 10^9 Msun BHs. These results suggest that binary stellar collisions around HMBHs represent a potentially observable population of nuclear transients that may contribute to the growing sample of ambiguous nuclear transients detected by wide-field optical surveys.
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All authors Megha Sharma, Alexander Heger, Evgeni Grishin, Daniel J. Price

High Energy Astrophysical Phenomena

Estimating redshift distributions of Fermi blazars with multiple machine-learning models

Kerui Zhu, Jiaming Chen, Yonggang Zheng et al. (+4 more)

Approximately half of the Fermi-LAT-detected blazars lack spectroscopic redshift measurements, which limits population studies and investigations of the cosmological evolution of the gamma-ray blazar population. We aim to develop a multi-model ensemble framework to systematically evaluate the performance of machine-learning models for blazar redshift estimation and to provide predictive redshift probability distributions with quantified uncertainties for all blazars without measured redshifts. We construct an ensemble of 35 machine-learning regression models in the $1/(1+z)$ space. Predictions from individual models are combined to build non-parametric redshift probability density functions (PDFs) for each source, allowing an explicit characterization of predictive uncertainty. The source-level PDFs are subsequently stacked to derive population-level redshift distributions. Across the diverse models within our framework, we find comparable predictive performance, indicating a feature-limited performance plateau. We compile a catalog of predictive redshift distributions for 1,590 blazars without measured redshifts. The stacked PDFs recover the characteristic double-peaked redshift structure of blazars without using source class labels. For blazar candidates of uncertain type, the predicted redshift distribution exhibits a bimodal structure, suggesting a slightly larger contribution from FSRQ-like sources. Our multi-model statistical framework achieves competitive predictive performance while retaining faint sources. We provide predicted redshift distributions with quantified uncertainties for 1,590 blazars without measured redshifts, offering improved support for population and evolutionary studies. More broadly, this approach highlights the practical value of uncertainty-aware multi-model regression ensembling in astronomical data analysis.
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All authors Kerui Zhu, Jiaming Chen, Yonggang Zheng, Ruixin Zhou, Shan Chang, Li Zhang, Niansheng Tang

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

Quasinormal-mode redshifts of black holes in galactic halos with radial pressures

Che-Yu Chen, Hassan Hassanabadi and Soroush Zare

We investigate the black-hole quasinormal mode redshifts induced by a galactic dark-matter halo with nonvanishing radial pressure. The halo is described by an anisotropic fluid with a generalized density profile and a constant radial equation-of-state parameter $w$. We construct the corresponding static, spherically symmetric spacetime and study the resulting corrections to the unstable light ring and quasinormal-mode spectrum. For dilute halos and $w\neq -1$, the light-ring angular frequency and Lyapunov exponent acquire the same leading-order gravitational redshift, implying a universal shift of the real and imaginary parts of the quasinormal frequencies. The magnitude of this effect depends on both the halo structure and the radial pressure. This universal redshift relation holds not only in the case of $w=0$ that has been discovered in the literature but also in a much wider scenario, supporting its importance in black hole spectroscopy. On the other hand, the case $w=-1$ is qualitatively different: the factor that corresponds to the leading common redshift vanishes, the corrections are further suppressed by the hierarchy between the black-hole and halo scales, and the universal relation is broken, with the dominant contribution controlled by the inner halo profile. Our numerical computations of scalar-field quasinormal mode spectra confirm the analytic eikonal results.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR

Orbital and spin periods of the intermediate polar 2PBC J0800.5-4306

Yabing Zhao, Chunhua Zhu, Sufen Guo et al. (+3 more)

2PBC J0800.5$-$4306 (Swift J0800.7$-$4309; optical counterpart Gaia DR3 5533020382479508736) is a hard-X-ray-selected cataclysmic variable whose magnetic subtype had remained uncertain. We analyse TESS photometry from Sectors 88 and 89 and archival XMM-Newton timing data. The TESS data show signals at the orbital frequency $\Omega$, its first harmonic $2\Omega$, the white-dwarf spin frequency $\omega$, and the spin–orbit beat frequency $\omega-\Omega$. We measure $P_{\rm orb}=5.38540^{+0.00027}_{-0.00028}$ h and $P_{\rm spin}=0.7015570^{+0.0000059}_{-0.0000062}$ h. The XMM-Newton EPIC-pn, MOS1, and MOS2 periodograms show broad peaks near the 2525.6-s (0.701557-h) optical spin period. Together with the previously published cataclysmic variable classification and Swift/BAT hard-X-ray detection, these timing results identify 2PBC J0800.5$-$4306 as an intermediate polar. We report the first published measurements of its orbital and white-dwarf spin periods, while SPOC difference-image diagnostics and pixel-level timing tests support that the measured TESS modulations are associated with the Gaia counterpart despite the crowded field.
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All authors Yabing Zhao, Chunhua Zhu, Sufen Guo, Guoliang Lü, Xizhen Lu, Xinyu Liu

High Energy Astrophysical Phenomena

Is accretion flow magnetically arrested in M87?

Yuao Ma, Xinwu Cao, Jia-Wen Li et al. (+2 more)

It is still debating on whether the disc is magnetically arrested in M87. We assume that a weak external magnetic field is dragged inwards by the accretion disc, which is substantially enhanced to drive strong jets near the black hole horizon via Blandford-Znajek mechanism. The jet power of M87 has been well constrained with the observational data, while the accretion rate in the inner region of the accretion flow in M87 is estimated by fitting the multi-waveband continuum spectrum and the data of the Faraday rotation measurement, with which the surface density of the disc is derived. Our calculations show that, in order to produce the observed jet power $P_{\rm jet}=3.2\times 10^{43}~\rm erg~s^{-1}$, an external field with several hundred $\mu \rm G$ at the outer edge of the disc is required to be amplified in the disc to hundreds G at the BH horizon, and the accretion flow in M87 must be magnetically arrested. A standard and normal evolution (SANE) disc is allowed in M87, only if the jet power is significantly lower than $\sim 1.75 \times 10^{43} \rm erg~ s^{-1}$,
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All authors Yuao Ma, Xinwu Cao, Jia-Wen Li, Jianchao Feng, Qingwen Wu

High Energy Astrophysical Phenomena

A multi-telescope fit to the FRB population, allowing for FRB repetition

Jordan L. Hoffmann, Clancy W. James, Jason X. Prochaska et al. (+3 more)

Fast radio bursts (FRBs) are millisecond-duration radio pulses with extragalactic origins. A majority of FRBs have only had a single pulse detected from them; however, an increasing number have been identified as repeaters. Here we present a full redshift – dispersion measure ($z$-DM) analysis including a simple power-law model of repetition rates. We assume all FRBs are drawn from the same intrinsic population and fit these FRB population parameters using data from ASKAP, Murriyang (Parkes), DSA, FAST, MeerKAT and CHIME catalogue 1. We do not utilise data from CHIME catalogue 2 yet, as we cannot robustly identify all repeaters in the sample. We constrain the minimum repetition rate above $10^{39}$ erg to $R_{\mathrm{min}}=-4.23^{+1.10}_{-1.74}$ d$^{-1}$, the maximum repetition rate to $R_{\mathrm{max}} \gtrsim 1$ d$^{-1}$ and the repetition rate index to $R_{\gamma}=-2.20^{+0.12}_{-0.22}$. These parameters do not show significant correlation with any other FRB parameters. Our values are an improvement on existing results and are self-consistent with the DM distribution of CHIME and observed ratios of repeaters to non-repeaters across the sky. We also provide updated estimates of the FRB energy function and host galaxy parameters.
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All authors Jordan L. Hoffmann, Clancy W. James, Jason X. Prochaska, Ines Pastor-Marazuela, Kritti Sharma, Liam Connor

Tuesday 22 September

High Energy Astrophysical Phenomena

EP260119a: A High-Redshift Gamma-Ray Quiet Fast X-ray Transient Probing a Potentially Hidden Population of Relativistic Explosions

Camila Angulo-Valdez, Asuka Kuwata, Edilberto Aguilar-Ruiz et al. (+41 more)

Gamma-ray quiet fast X-ray transients (FXTs) provide a new approach for studying relativistic explosions that evade traditional gamma-ray triggers. In this work, we present multi-wavelength observations and analysis of EP260119a, a high-$z$ FXT detected by Einstein Probe/WXT and followed up with COLIBRÍ, the Liverpool Telescope, and other facilities. Spectroscopy yields a redshift of $z = 5.47$, making EP260119a the most distant FXT detected by Einstein Probe to date. Despite its luminous X-ray and optical emission, no prompt gamma-ray counterpart was detected by SVOM/ECLAIRs, SVOM/GRM, Swift/BAT, or Konus/Wind, despite contemporaneous coverage. The broadband afterglow is well described by synchrotron emission from a uniform relativistic jet propagating into a shallowly stratified external medium close to the constant-density limit, indicating a standard relativistic explosion despite the absence of detectable gamma rays. Occupying the extreme high-$z$ end of the growing Einstein Probe FXT sample, EP260119a supports the possibility that soft X-ray surveys are uncovering relativistic transients that remain undetected by current gamma-ray instruments. Our results indicate that some explosions in the early Universe escape conventional gamma-ray surveys and demonstrate the value of combining sensitive soft X-ray discovery with rapid optical follow-up to obtain a more complete view of the high-$z$ transient population.
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All authors Camila Angulo-Valdez, Asuka Kuwata, Edilberto Aguilar-Ruiz, Rosa L. Becerra, Ramandeep Gill, Miguel Angel Aloy, Antonio de Ugarte Postigo, Alan M. Watson, Chen-Wei Wang, Diego López-Cámara, Daniel A. Perley, Aleksandra Bochenek, Simone Dichiara, Enrique Moreno Méndez, Nathaniel R. Butler, Benjamin Schneider, Jean-Luc Atteia, Stéphane Basa, William H. Lee, Dalya Akl, Sarah Antier, Alexis Coleiro, James DeLaunay, Damien Dornic, Jean-Grégoire Ducoin, Francis Fortin, Dmitry Frederiks, Leonardo García-García, Noémie Globus, Olivier Godet, Jamie Kennea, Gianluca Lombardi, Alexandra L. Lysenko, Francesco Magnani, Tyler Parsotan, Margarita Pereyra, Anna Ridnaia, Samuele Ronchini, Fredd Sánchez-Álvarez, Dmitry Svinkin, Anastasia Tsvetkova, Mikhail Ulanov, Peter Veres, Hui Yang

High Energy Astrophysical Phenomena

Disk truncation triggers relativistic jet launching of a highly accreting supermassive black hole

Hirofumi Noda, Kazuhiro Hada, Hiroto Maruyama et al. (+10 more)

Relativistic jets are among the most energetic phenomena in the Universe and influence galaxy evolution through energetic feedback, yet the physical connection between accretion flows and jet launching remains unresolved. Jet production is widely attributed to the Blandford-Znajek mechanism, which extracts black hole rotational energy via large-scale magnetic fields sustained by an accretion flow. Although this mechanism is observationally well supported for hot, geometrically-thick accretion flows, highly accreting environments with strong jets present a paradox, as they are typically dominated by cold, geometrically-thin disks incapable of sustaining large-scale magnetic fields. Here, we report coordinated X-ray microcalorimeter spectroscopy achieved by X-ray Imaging and Spectroscopy Mission (XRISM) and millimeter/radio interferometric observations by Global Millimeter VLBI Array (GMVA), Very Long Baseline Array (VLBA), and East Asian VLBI Network (EAVN) on the broad-line radio galaxy 3C120. High-resolution X-ray spectroscopy reveals a relativistically-broadened Fe-K$\alpha$ line, indicating that the cold, geometrically-thin disk is truncated at $\sim 20~R_{\rm g}$ ($R_{\rm g}$ is the gravitational radius), with the inner region replaced by a hot, geometrically-thick flow. Concurrently, radio imaging indicates that jet profile extrapolation toward the black hole horizon yields a radius comparable to or even narrower than the disk truncation radius. These results demonstrate that, even at high accretion rates, jet launching is linked to a hot inner flow, possibly via the Blandford-Znajek process, providing evidence for a universal disk-jet connection via geometric transition.
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All authors Hirofumi Noda, Kazuhiro Hada, Hiroto Maruyama, Taishu Kayanoki, Kenji Toma, Taiki Kawamuro, Yoshiyuki Inoue, Tomohisa Kawashima, Soyeon Yeo, Ilje Cho, Keiichi Asada, Yasushi Fukazawa, Yoshihiro Ueda

High Energy Astrophysical Phenomena

In-Situ Star Formation in a Thick Disk: A Model for Metal Enrichment of Mrk 573

Hannalore J. Gerling-Dunsmore, Mitchell C. Begelman, Jared A. Goldberg et al. (+2 more)

Recent observations of the nearby Seyfert galaxy Mrk 573 found that the galactic nucleus and bicone regions surrounding the jet are significantly metal-enriched. In this paper, we develop a simple model for the observed metal enrichment, assuming a population of stars in the $10 - 30 \, M_{\odot}$ range formed in-situ in the accretion disk. We assume the stars form according to a Salpeter initial mass function and generate the typical metal yields expected of field stars of the same mass. We find that in the thick disk ($H/r \sim 0.1$) case, a Salpeter population of stars can easily explain the observed metal enrichment and provide limits on the formation model; we also argue that a thin disk ($H/r \lesssim 0.01$) is incompatible with the observed metal enrichment. In our model, AGN metallicity scales most strongly with disk thickness, implying a potentially powerful observable diagnostic.
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All authors Hannalore J. Gerling-Dunsmore, Mitchell C. Begelman, Jared A. Goldberg, Taeho Ryu, Dominika Ł. Król

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

Light-curve analysis of the classical nova YZ Ret: revisit with fully self-consistent nova outburst models

Izumi Hachisu and Mariko Kato

YZ Ret is a well observed nova from the first detection of an X-ray flash phase in classical novae until the end of a supersoft X-ray source phase. We have reanalyzed multiwavelength light curves of YZ Ret over the total duration of the outburst. Our fully self-consistent nova outburst model, a 1.25 $M_\odot$ white dwarf (WD) and a mass accretion rate of $1\times 10^{-9} ~M_\odot$ yr$^{-1}$ on to the WD, reasonably reproduced $V$/$g$ light curves as well as the very early X-ray flash and late supersoft X-ray light curves. This model explains the emergence epoch of the GeV gamma-ray emission because a shock naturally arises far outside the nova (WD) photosphere just after the optical maximum. In our model, before optical maximum, later ejected matter has a smaller velocity than that of earlier ejecta and therefore the ejecta expands while, after optical maximum, later ejected matter has a larger velocity than that of earlier ejecta and collision with the earlier ejecta makes a shock.
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High Energy Astrophysical Phenomena

SN2025aico: An Interesting Case Of $^{56}$Ni Mixing, Ejecta Asymmetries, and Dust Formation in a Type IIb Supernova

Grace Davis, Kyle Medler, Chris Ashall et al. (+8 more)

Stripped-envelope supernovae provide a window into how massive stars lose their layers, mix radioactive material, produce dust, and explode asymmetrically. We present optical, near-infrared (NIR), and mid-infrared (MIR) observations of SN~2025aico, a Type~IIb supernova in LEDA~35384. Our spectroscopic sequence, spanning $+1$ to $+167$ d after explosion, follows its evolution from a photospheric phase exhibiting both hydrogen and helium features to prominent helium emission and, ultimately, nebular-phase ejecta. Using He I $1.083$ and $2.0581\,\mu$m transitions, we investigated the kinematics and geometry of the helium-rich material. Both transitions exhibit a three-phase, non-monotonic velocity evolution: an initial rapid decline, a subsequent increase, and an eventual plateau. We interpret this behavior as evidence for limited outward mixing of $^{56}$Ni, such that radioactive energy deposition reaches the outer helium-rich ejecta progressively as the ejecta expand, producing the non-thermal electrons responsible for helium excitation. After $\sim100$ d, both NIR He I transitions develop double-peaked emission profiles. Similar structure in the oxygen emission indicates ejecta asymmetry. Comparison with other supernovae suggests a tentative connection between this structure and explosion energy, potentially linked to the delay between core collapse and explosion. Serendipitous JWST observations at $+124.4$d reveal an infrared excess. Modeling favors warm ($\sim800$–$1500$K) carbon dust newly formed in the ejecta, together with cooler carbon or silicate dust likely associated with pre-existing circumstellar material. SN~2025aico demonstrates how continuous optical-to-MIR observations can connect progenitor evolution, explosion physics, ejecta geometry, and dust production.
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All authors Grace Davis, Kyle Medler, Chris Ashall, Willem B. Hoogendam, Cameron Pfeffer, David O. Jones, Shunsaku Horiuchi, Tyco Mera, Benjamin Shappee, Charlotte Ward, Grace Showerman

High Energy Astrophysical Phenomena

Waiting in the wings: extended emission GRBs from high latitude emission and jet expansion

Isabelle Worssam, Benjamin P. Gompertz and Hendrik J. van Eerten

The process powering extended emission (EE) Gamma Ray-Bursts (GRBs) from compact object mergers is a debated problem due to their observed prompt emission timescales of 10 to 100s, far in excess of the expected accretion timescales following a binary neutron star (BNS) merger. Here, we present an analytical thin shell structured jet model designed to alleviate this tension. We show that a changing jet structure in which early emission is produced by a high Lorentz factor narrow, structured jet and later emission comes from a wider, lower Lorentz factor tophat-like jet can successfully reproduce the EE GRB phenomenology. All emitting shells are powered by a $0.1$ $M_{\odot}$ accretion torus and launched within a time window of 2s after merger. Spatially-resolved opacity checks confirm that our emission sites remain optically thin, in part due to the spectral softness associated with EE. We present light curves from our model compared to representative EE bursts detected by Swift: GRBs 211211A, 211227A and 060614. While our light curves provide good matches to the data, our spectra do not evolve fast enough when compared to the well-measured spectral evolution observed in GRB 211211A. We discuss an update to the model incorporating inherent spectral evolution in the shells as future work. We also show a parameter exploration of the model and identify a parameter range within which standard short GRB-like light curves could be produced, showcasing the ability of the model to produce short bursts with and without EE.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Supernova Gamma-Ray Echo as an Astrophysical Near Detector for Galactic Neutrino Propagation

Garv Chauhan and Yago Porto

Core-collapse supernova neutrinos offer a unique probe of physics beyond the Standard Model over Galactic baselines. Yet even a high-statistics nearby burst may be limited by source uncertainties, especially for propagation effects that average over energy and appear only as an overall flux suppression. Inspired by near–far comparisons in terrestrial oscillation experiments, we show that next-generation MeV observations of the supernova gamma-ray echo—the delayed 511 keV signal induced by the neutrino burst in the stellar envelope—can serve as an astrophysical near detector for the emitted $\bar\nu_e$ fluence. Comparing this echo-inferred source fluence with the surviving flux measured at Earth gives a direct test of neutrino propagation between the stellar surface and the detector. For pseudo-Dirac neutrinos, this turns the fully averaged active–sterile oscillation regime from an unobservable source-normalization ambiguity into a measurable near–far deficit, with sensitivity for nearby progenitors to splittings $\delta m^2\gtrsim 10^{-18}~{\rm eV}^2(0.22~{\rm kpc}/D)(E_\star/15~{\rm MeV})$. The same near–far logic also applies to other propagation-induced modifications of the observed flux, including neutrino decay and neutrino interactions with Galactic dark matter.
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High Energy Astrophysical Phenomena

A Coherent Search for New Galactic Magnetars using Fermi/GBM and follow-up Swift/BAT: 7 Candidates and a Large Burst Catalog

Ariel Perera, Barak Zackay and Tejaswi Venumadhav

Galactic magnetars are a rare class of neutron stars with extreme magnetic fields, with only about 30 known sources. Their short hard X-ray and soft gamma-ray bursts provide an important channel for discovering magnetar activity. Wide-field instruments such as the Fermi Gamma-ray Burst Monitor (GBM) are well suited to detecting these bursts, but their broad localization uncertainties make it difficult to associate individual bursts with specific sources. Coded-aperture telescopes, in contrast, provide arcminute-scale localizations but have narrower fields of view. We search for previously unidentified magnetars and construct a magnetar burst catalog by combining a broad sample of short, soft triggers from our coherent GBM detection pipeline over 2013-2025 with targeted searches of archival Swift/BAT images. We develop a statistical association framework that uses BAT detections to localize GBM bursts to arcminute precision, together with a spatio-temporal clustering framework that associates poorly localized bursts with common sources. Together, these methods identify 1720 magnetar bursts, of which 1170 are confidently associated with 11 known magnetars. The remaining 550 bursts could not be assigned to specific sources. We identify seven candidate new magnetars through three complementary discovery channels. The catalog, composed of the associated bursts, extends to the lowest fluences observed by GBM. We also provide spectral and temporal analyses of the catalog. The new candidate sources provide targets for X-ray follow-up that could reveal previously unidentified Galactic magnetars. More broadly, this work demonstrates how combining instruments with complementary capabilities can maximize the scientific return of high-energy transient observations.
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High Energy Astrophysical Phenomena

Compact Object Mergers and Micro-Tidal Disruption Events: A Multi-Messenger Probe of Dense Stellar Environments

Konstantinos Kritos, Francesco Iacovelli, Rosalba Perna et al. (+1 more)

Dense stellar environments such as globular clusters and young massive clusters produce both compact-object mergers detectable in gravitational waves and tidal disruption events observable electromagnetically. We investigate compact-object mergers, tidal disruption events, and micro-tidal disruption events ($\mu$TDEs) in these environments using rapid Monte Carlo star-cluster simulations with the Rapster code, extended with a unified treatment of neutron-star (NS) and black-hole (BH) spin evolution under disk accretion across the NS-to-BH transition, as well as new dynamical channels for disruptions by single compact objects and by BH-BH binaries. Simulating $10^5$ clusters across ten model variations and two formation histories, we find a BH-BH merger-rate density peaking at $z\approx2$-$3$ at several tens ${\rm Gpc}^{-3}\,{\rm yr}^{-1}$, consistent with the local LVK rate, while the cumulative number of $\mu$TDEs exceeds that of mergers by a factor of a few. Since we include only dynamically formed binaries, $\approx90\%$ of $\mu$TDEs proceed through single compact-object-star encounters, and our rates are conservative lower limits. Prograde accretion during $\mu$TDEs spins up compact objects, providing a purely dynamical, randomly oriented pathway for producing high-spin BHs, and can drive accretion-induced collapse of NSs to BHs that populate the lower mass gap.
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All authors Konstantinos Kritos, Francesco Iacovelli, Rosalba Perna, Emanuele Berti

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

Precise QCD Predictions for the Scotogenic Model at Colliders

Mohammed Boukidi, Camila Ramos and Richard Ruiz

Motivated by the anticipated data rates of the high-luminosity phase of the Large Hadron Collider, we report the first predictions up to next-to-leading order in quantum chromodynamics with parton-shower matching for the production and decay of scalars and heavy neutrinos from the Scotogenic model for neutrino masses and dark matter at $\sqrt{s}=14$ TeV and $\sqrt{s}=100$ TeV. We carry out our simulations using the FeynRules+MadGraph5\_aMC@NLO+Pythia8 pipeline. As a byproduct, we report the public release of the SM\_Scoto Universal FeynRules Object libraries for modeling the Scotogenic model in high-energy processes. We comment on prospects of observing new rare $Z$ decays at future $e^+e^-$ collider facilities as well as on the production of new particles at a multi-TeV $\mu^+\mu^-$ collider.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Resonant neutrino flavor conversion within dark matter spikes

P. S. Bhupal Dev, Elisa Gaido, Alejandro Ibarra et al. (+1 more)

We investigate how neutrino–dark matter (DM) interactions modify the flavor composition of high-energy neutrinos from active galactic nuclei (AGNs). Coherent forward scattering in a DM spike around the central supermassive black hole can generate a flavor-dependent potential, inducing flavor conversion as the neutrinos escape. We calculate the flavor composition at Earth for pion-decay and muon-damped sources and find substantial departures from vacuum-oscillation expectations. At neutrino energies around $100\,\mathrm{TeV}$, the matter potential begins to compete with vacuum oscillations when the coupling-weighted net DM number density reaches $|\epsilon_\alpha(n_\chi-n_{\bar\chi})|\sim10^{18}\,\mathrm{cm}^{-3}$. We compare our predictions with IceCube flavor triangle measurements under the illustrative assumption that the diffuse flux originates from AGNs with common neutrino-production and DM-spike properties. Under this assumption, when the DM-induced potential acts on the muon flavor and dominates the vacuum terms at production, the $p\gamma$ muon-damped predictions lie outside the 95% MESE contour. High-energy neutrino flavor measurements can therefore provide a novel probe of neutrino-DM interactions in astrophysical environments.
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All authors P. S. Bhupal Dev, Elisa Gaido, Alejandro Ibarra, Yago Porto

High Energy Astrophysical Phenomena

Dark-matter relay for ultra-high-energy cosmic rays

Mikhail Sekretov

Astrophysical sources of ultra-high-energy cosmic rays (UHECRs) remain unidentified, and no nearby cosmic accelerators are established even for the highest-energy events. In addition, puzzling correlations of a fraction of UHECR flux with distant blazars were found by HiRes and, recently, Telescope Array experiments. Here we propose that dark matter can provide a mechanism to effectively transfer UHECRs from distant sources: dark-matter particles are boosted by UHECRs near the source and pass their momentum to baryons close to the observer. The probability of this relay is not negligible because of the growth of dark-matter-baryon cross section with energy. We demonstrate that the effect is observable for fermionic dark matter of mass $m_{\mathrm{DM}} \sim 25$ MeV and discuss possible observational signatures of this mechanism.
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High Energy Astrophysical Phenomena

Self-consistent modeling of energy-dependent synchrotron polarization in blazars with application to Mrk 421

Frederike Apel, Motseothata Tisang, Victor Barbosa Martins et al. (+3 more)

Observations of black hole jets closely aligned with our line of sight, known as blazars, have revealed that their X-ray emission can be significantly more polarized than in the optical bands. This chromatic polarization is often attributed to an energy-stratified jet. In blazars with X-ray emission dominated by electron synchrotron, the pitch angle between the magnetic field lines and electron velocity might also affect the frequency dependence of the polarization degree, but this effect is generally neglected in state-of-the-art models. In this work, we model the polarization and spectral energy distribution of blazars, accounting for the system's temporal evolution and pitch-angle-dependent electron synchrotron cooling. Our analysis investigates the impact of the magnetic field structure and electron distribution on polarization and tests whether this can help explain its observed frequency dependence. By extending the numerical simulation code AM3, we incorporate a self-consistent calculation of electron synchrotron emission and simulate various magnetic-field structures. We apply these models to Mrk 421 and compare them with observations. Finally, we present a time-dependent shock-in-jet model to investigate frequency-dependent polarization angle swings. Our results show how different magnetic-field structures affect energy-dependent polarization signatures. In the case of Mrk 421, the data can be explained by a helical field in the jet. In the shock-in-jet scenario, the X-ray polarization angle changes by 70 degrees, while remaining almost constant in the optical/infrared. Our results demonstrate that a self-consistent treatment of the temporal evolution of electrons is essential to model the polarized synchrotron emission from blazars and can naturally explain chromatic polarization signatures without requiring an energy-stratified jet.
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All authors Frederike Apel, Motseothata Tisang, Victor Barbosa Martins, Xavier Rodrigues, Markus Böttcher, Anna Franckowiak

High Energy Astrophysical Phenomena

Neutrino Astronomy at High Energies - An Experimental Review

Christian Spiering

Neutrino astronomy at high energies is an emerging field but still in a state of infancy. It is barely a dozen years ago that a diffuse flux of extraterrestrial neutrinos with energies in the TeV and PeV ranges was detected and even fewer that first individual sources could be identified. A new window to the universe has been opened after four decades of efforts to realize the gigantic instruments that made that breakthrough possible. This review describes the road towards the present, reviews the actual status of the field and sketches future developments.
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High Energy Astrophysical Phenomena

Inelastic scattering effects on polarization predictions for cold and warm atmospheres in the X-rays

J. Podgorný, L. Marra, M. Dovčiak et al. (+8 more)

X-ray polarization models of accreting compact objects often assume the so-called Chandrasekhar-Sobolev polarization prescription for elastic Thomson scattering in electron-scattering semi-infinite atmospheres. In this letter, we examine the effects of inelastic Compton scattering. We assume homogeneous slabs of cold and warm (up to 1 keV) electrons. We use a Monte Carlo approach. We discuss the polarization difference arising in the X-rays between the elastic and inelastic scattering assumptions for plane-parallel transmission and reflection, and the role it can play in the interpretation of X-ray polarimetric observations of accreting compact objects.
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All authors J. Podgorný, L. Marra, M. Dovčiak, R. Taverna, R. W. Goosmann, M. Gupta, F. Marin, G. Matt, J. Poutanen, A. Różańska, A. Veledina

High Energy Astrophysical Phenomena

Dense Dump Codec: Error-Controlled, Random-Access Compression of GRMHD Time Series for Slow-Light Radiative Transfer

Zelin Zhang, Zhenyu Zhang and Bin Chen

Black-hole movies provide a unique venue to study the time evolution of accreting plasma. Modeling this evolution with slow-light radiative transfer requires closely spaced simulation outputs, because the plasma changes as light propagates through it. Saving these outputs in full is costly, whereas sparse sampling introduces time-interpolation errors. We present the Dense Dump Codec (DDC), a compression method for dense GRMHD time series. DDC stores exact anchor states and compactly represents the intermediate evolution, while allowing direct access to selected times and variables. Applied to eight production sequences spanning SANE and MAD simulations at several black-hole spins with 0.1M output, DDC archives are about 10 times smaller than the original dense data and even about 50% smaller than the original 0.5M output in aggregate. We further integrate a native DDC reader into polarized slow-light radiative transfer. Tests at 86, 230, and 345 GHz show that DDC may reduce aggregate Stokes-image errors by 54-79% relative to slow-light calculations using original 0.5M input. These results indicate that DDC enables high-cadence slow-light calculations without the prohibitive cost of saving every dense GRMHD state in its original form.
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High Energy Astrophysical Phenomena

KPolaris: GPU-accelerated Polarized Radiative Transfer in General Relativity

Zelin Zhang and Bin Chen

Polarized black-hole images carry information about both the magnetized plasma and the curved spacetime through which the radiation propagates. We present KPolaris, an open-source GPU-accelerated general-relativistic polarized radiative-transfer code built on the Kokkos framework. It supports analytic plasma models and GRMHD simulation data, with both fast-light and slow-light radiative transfer. The code also provides diagnostics that connect image structure to the emitting plasma and to propagation effects along the rays. These include emissivity-weighted plasma maps, emission contributions from selected regions, selected-ray histories, and sensitivities of the Stokes parameters to plasma parameters. Numerical convergence tests and comparisons with an independent radiative-transfer code assess the accuracy of the calculations. GPU acceleration reduces the computational cost of image generation, facilitating multi-frequency and time-dependent studies of polarized black-hole images.
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High Energy Astrophysical Phenomena

Einstein Probe discovery of the magnetar EP J223759.5+531421

N. Rea, F. Coti Zelati, A. Borghese et al. (+44 more)

We report the discovery and early outburst evolution of the new Galactic magnetar EP J223759.5+531421, detected by the Einstein Probe Wide-field X-ray Telescope on 2026 June 28. Follow-up observations with Einstein Probe, XMM–Newton, NuSTAR, SVOM, and IXPE revealed coherent X-ray pulsations at P ~ 6s and an average period derivative of Pdot ~ 2.8x10^{-12} s/s. These values imply a nominal polar dipolar magnetic field of B_dip ~ 2.6x10^{14} Gauss and a characteristic age of tau_c ~ 34 kyr, although the structured timing residuals suggest possible torque variability. The rms pulsed fraction is approximately 20-25% below ~7 keV and decreases at higher energies. The broadband spectra require multiple thermal components and a hard power-law tail. Adopting a distance of 3.3 kpc, the 0.5-30 keV luminosity declined from 1.2x10^{35} to 5.7x10^{34} erg/s during the first month, accompanied by a decrease in the inferred thermal-emitting areas. At this distance, the Galactic latitude b=-4.56 deg corresponds to a height of approximately 0.26 kpc below the Galactic plane, which is difficult to reconcile with the nominal characteristic age under a simple midplane-birth scenario even for an extreme proper motion velocity. Short X-ray bursts independently confirm the magnetar nature of the source. No near-infrared or radio counterpart were detected by GTC, Medicina or FAST, respectively, down to K_s>21 mag and S_{1.25 GHz} <2.3 microJy. These observations demonstrate the potential of the Einstein Probe WXT monitoring to uncover previously quiescent Galactic magnetars and follow their outbursts from their earliest observed stages.
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All authors N. Rea, F. Coti Zelati, A. Borghese, Y.L. Wang, H. Yang, X. Mao, C.-Y. Dai, E. Arrigoni, J. Bai, G. Bernardi, M. Burgay, S. Cao, A. Coleiro, D. De Grandis, P. Esposito, H. Feng, Y.-C. Fu, A. Geminardi, D. Götz, S. Guillot, Y. Huang, M. Imbrogno, G. L. Israel, C. Jin, A. Kong, Y. Li, L. Lin, C.-S. Liu, C. Maitra, A. Marino, S. Mereghetti, J.-U. Ness, S. Ng, M. Pilia, A. Possenti, H. Sun, L. Tao, R. Taverna, A. Tiengo, R. Turolla, W. Yuan, S. Zane, B. Zhang, L.-X. Zhang, Z. Zhang, W.-W. Zhu, Y.-C. Zou