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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

Dissecting the variability of Sagittarius A* in the orbiting hotspot framework

Maciek Wielgus and Aristomenis Yfantis

Sagittarius A* (Sgr A*) exhibits intermittent flaring behavior, characterized by strong enhancements in high-energy emission (infrared and X-ray). These events have been interpreted as related to the coherent orbital motion of localized regions of energized plasma - hotspots. While this framework has been quite fruitful in constraining properties of Sgr A*, it is subject to important challenges and limitations. In this brief review we discuss the recent observational and theoretical developments related to this line of research, commenting on the concerns regarding the hotspot model validity and the future perspectives.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Heavy Inert Doublet Reconciles LZ and IceCube

Hitoshi Murayama and Bea Noether

We show that the inert doublet dark matter can reconcile the purported LZ direct detection signal and IceCube constraints. The preferred dark matter mass is 11.3–20.3~TeV, an order of magnitude heavier than the standard higgsino target. The smaller number density at a high mass can reproduce the LZ event given the exponential sensitivity to the velocity, while suppressing the capture rate in the Sun because of the higher infall velocity. On the other hand, the high mass requires a large coupling between the inert doublet and Higgs boson to have the correct annihilation cross section for the freeze-out. There is about an order of magnitude of validity range below the Landau pole, beyond which a UV completion is needed. Electroweak precision observables are below the experimental limits despite the large coupling.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR

A comprehensive multi-wavelength study of two X-ray emitting Be stars

Hema Anilkumar, Blesson Mathew, Savithri H. Ezhikode et al. (+4 more)

We present a multi-wavelength study of two X-ray-emitting Be stars, CXOU J052218.4+332820 (Star 1) and CXOU J052304.2+332846 (Star 2), identified from the LAMOST DR5 Hot emission-line stars catalog. Our spectroscopic and photometric analyses indicate that Star 1 is a main-sequence B5e star, whereas Star 2 is a pre-main-sequence B7e star. The LAMOST spectrum of Star 1 exhibits shell-type Feii, Paschen, and Oi emission from circumstellar regions extending to ~ 3 - 5Rstar. TESS period analysis reveals a rapid rotational period of ~ 0.355 d and an inclination angle of ~ 68 deg, confirming the Classical Be nature of Star 1. In contrast, Star 2 shows strong accretion signatures, including inverse P-Cygni Feii and redshifted Hei absorption features. Its ZTF light curves display irregular dimming events of up to ~ 3 mag and quasi-periodic variability on timescales of ~ 50 - 110 d, consistent with circumstellar obscuration in an accreting Herbig Be star. To further investigate the high-energy properties of the two systems, we analyzed their Chandra X-ray spectra. Chandra spectroscopy yields moderate plasma temperatures (kT ~ 1.7-1.8 keV) and X-ray luminosities of logL_X ~ 30.4-30.6 erg/s, comparable to those observed in magnetically active coronae. Furthermore, the positional agreement between the Chandra and Gaia coordinates indicates that the X-ray emission is spatially associated with the optical systems rather than a nearby resolved companion. These results suggest that the observed X-ray emission originates from magnetically influenced circumstellar activity, consistent with magnetically confined wind-shock (MCWS) or magnetically torqued disk scenarios.
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All authors Hema Anilkumar, Blesson Mathew, Savithri H. Ezhikode, Sreeja S. Kartha, Suman Bhattacharyya, Sneha Nedhath, Ajith N

High Energy Astrophysical Phenomena

Tomography analysis of the intermediate-luminosity Type Iax SN 2024pxl

Barnabás Barna, Mridweeka Singh, Lindsey A. Kwok et al. (+90 more)

We present an abundance tomography analysis of SN 2024pxl, an intermediate luminosity Type Iax supernova ($M_{r}=-16.82 \pm 0.19$ mag), with the most-detailed follow-up in the Type Iax subclass to-date. As one of the few intermediate luminosity Type Iax objects, SN 2024pxl may link the two extremes of the peculiar thermonuclear supernova subclass. To test this hypothesis, we analyze its spectral evolution through the first 100 days after the explosion and aim to probe the structure of its ejecta. We conduct an abundance tomography analysis using synthetic spectra produced with the one dimensional radiative transfer code TARDIS. The fit of the spectral time series provides a radial scan of physical properties and probes the stratification of chemical elements throughout most of the SN ejecta. The observed spectral evolution is well fit with the final model, similar to the general predictions of the pure deflagration scenarios, but significant modifications are required in the density function of the inner ejecta and in the chemical profiles of the outermost regions. The constrained physical characteristics, such as the photospheric velocities and the time of maximum light, are also consistent with other SNe~Iax. Despite their small number, intermediate luminosity Type Iax SNe are not outliers in the subclass but demonstrate the continuous nature of SNe Iax through their luminosity range. Following this observation, we argue that all SNe~Iax share the same progenitor and explosion origin.
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All authors Barnabás Barna, Mridweeka Singh, Lindsey A. Kwok, Saurabh W. Jha, R. Dastidar, Conor Larison, Alexei V. Filippenko, J. P. Anderson, Jennifer E. Andrews, Moira Andrews, G. C. Anupama, Prasiddha Arunachalam, Katie Auchettl, Dominik Bánhidi, K. Azalee Bostroem, Thomas G. Brink, Régis Cartier, Ping Chen, T.-W. Chen, Collin T. Christy, David A. Coulter, Sofia Covarrubias, Kyle W. Davis, Connor B. Dickinson, Yize Dong, Joseph Farah, Andreas Flörs, Ryan J. Foley, Noah Franz, Christoffer Fremling, Lluís Galbany, Anjasha Gangopadhyay, Aarna Garg, Elinor L. Gates, Or Graur, Mariusz Gromadzki, Daichi Hiramatsu, Emily Hoang, D. Andrew Howell, Brian Hsu, Joel Johansson, Arti Joshi, Lordrick A. Kahinga, Ravjit Kaur, Sahana Kumar, Piramon Kumnurdmanee, Hanindyo Kuncarayakti, Chang Liu, Keiichi Maeda, Kate Maguire, Bailey Martin, Curtis McCully, Darshana Mehta, Luca M. Menotti, Anne J. Metevier, A. A. Miller, Kuntal Misra, Erika Mochnács, T. E. Müller-Bravo, Megan Newsome, Estefania Padilla Gonzalez, Kishore C. Patra, Jeniveve Pearson, Anthony L. Piro, Abigail Polin, Aravind P. Ravi, Armin Rest, Nicolas Meza Retamal, O. M. Robinson, César Rojas-Bravo, Devendra K. Sahu, David J. Sand, Brian P. Schmidt, Steve Schulze, Michaela Schwab, Manisha Shrestha, Matthew R. Siebert, Sunil Simha, Nathan Smith, Jesper Sollerman, Shubham Srivastav, Bhagya M. Subrayan, Tamás Szalai, Kirsty Taggart, Rishabh Singh Teja, Jacco H. Terwel, Samaporn Tinyanont, Stefano Valenti, József Vinkó, Aya L. Westerling, J. Craig Wheeler, Yi Yang, WeiKang Zheng

High Energy Astrophysical Phenomena

Stability of hybrid stars via catastrophe theory

Eduardo S. Fraga and Sergio E. Jorás

We discuss the possible topologies of the mass-radius relation for hybrid stars in light of catastrophe theory. We show that different branches of compact stars emerge or disappear depending on the bifurcation structure of the underlying catastrophe potential function, the mathematical analog of an effective potential. For that we assume a single first-order phase transition between nuclear and quark matter. After a warm up with incompressible fluids, we present a qualitative discussion explaining the (dis)appearance of (un)stable branches for hybrid stars as the control parameters change.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Plasma instability splits axion-photon conversion lines

Elizabeth A. Tolman

The axion coupling to electromagnetism may leave signatures of uncertain nature in astrophysical spectra; one is thought to be a single line centered at the axion frequency $\omega = \sqrt{\left(m_a c^2/\hbar\right)^2 + c^2 k_a^2}$, with $m_a c^2/\hbar$ the axion Compton frequency and $\hbar k_a$ its momentum. If a parametric plasma instability is excited, this signature is transformed into a set of lines, which may be proportionally spaced and linearly polarized. A Crab pulsar signal at a frequency near recent cosmological predictions for the axion Compton frequency has some qualitative similarity to this signature, though this similarity is not evidence that the signal is caused by axions.
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High Energy Astrophysical Phenomena

Composition-dependent acceleration of UHE Cosmic Rays in AGN electric fields

E. Koutsoumpou, I. Contopoulos, A. Nathanail et al. (+2 more)

The origin of ultra-high-energy cosmic rays (UHECRs) remains elusive, as proposed acceleration mechanisms are rarely connected directly to composition-sensitive air-shower observables. We establish such a connection for voltage-drop acceleration in near-horizon current sheets of magnetically arrested disks around supermassive black holes. Fragmentation is represented by a log-normal distribution of effective potential-sampling factors $\eta$, with $\mu_\eta=\langle\ln\eta\rangle$, $\sigma_\eta=[\mathrm{Var}(\ln\eta)]^{1/2}$, and median $\widetilde{\eta}=e^{\mu_\eta}$, while the energy gain scales with nuclear charge $Z$ and maximum potential $\mathcal{V}_{\max}$. We propagate the accelerated particles through Bethe–Heitler and adiabatic losses, proton photopion attenuation, and a mass-evolving photodisintegration cascade, and predict $\langle X_{\max}\rangle$ and $\sigma(X_{\max})$. We perform Bayesian fits to the Pierre Auger Observatory 2025 DNN measurements for two fixed injection templates and propagation distances of 10, 30, and 100 Mpc. An Auger-inspired, proton-poor injection reproduces both shower moments at 10 and 30 Mpc, with $\chi^2/\nu=0.33$ and $0.47$. At 30 Mpc, $\sigma_\eta=0.59^{+0.13}_{-0.10}$, corresponding to a multiplicative dispersion of approximately 1.8. The median sampling factor and maximum potential remain strongly degenerate, with $\widetilde{\eta}\mathcal{V}_{\max}$ more directly constrained. A metal-enriched injection containing $\sim53\%$ hydrogen gives $\chi^2$ larger by 17.9 and 25.9 at 10 and 30 Mpc. Both templates remain viable at 10 and 30 Mpc but fit poorly at 100 Mpc. Within the scenarios tested, the results favour a relatively proton-poor accelerated population and show that the shower-width evolution can constrain the stochastic structure of electric-field acceleration in nearby AGN.
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All authors E. Koutsoumpou, I. Contopoulos, A. Nathanail, A. Loules, D. Kazanas

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

3D Radiation Hydrodynamics of Blue Supergiant Envelopes

Linhao Ma, Lars Bildsten and Yan-Fei Jiang

We present the first 3D radiation hydrodynamical simulation of the outer envelope of a blue supergiant. These near Eddington limited massive stars in the Hertzsprung gap exhibit substantial photometric variability and have spectroscopic line widths indicating unexplained macroturbulent velocity fields. Our Athena++ simulations find that the convection associated with the opacity peak from iron directly extends to the photosphere, as found in previous 3D simulations of massive main sequence stars. Unlike 1D stellar models, this single convecting region imprints a velocity field that is consistent with spectroscopic models of macroturbulence. In addition, the convection in the outermost layer leads to such large density variations that the exiting radiation varies on large ($\lesssim 10 \% $) amplitudes and short ($\sim$ days) timescales. This leads to stochastic low frequency (SLF) photometric variability, with amplitudes and power spectra broadly consistent with observations.
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High Energy Astrophysical Phenomena

Dynamically Stable Magnetic Fields in Relativistic Neutron Stars: A Diverse Landscape of GRMHD Equilibria

Mai Li, Vasileios Paschalidis, Antonios Tsokaros et al. (+2 more)

Neutron stars endowed with purely poloidal or purely toroidal magnetic fields are known to be unstable, and settle into mixed poloidal-toroidal configurations. The end state of these instabilities is still poorly understood. It is unclear whether this state is unique or whether the dynamically stable configuration has memory of the initial conditions. In this work, we perform long-term, fully general relativistic magnetohydrodynamic simulations of equilibrium configurations of slowly rotating neutron stars endowed with self-consistent mixed poloidal and toroidal magnetic fields. Our initial configurations differ in their initial magnetization and magnetic field geometry. We evolve the initial equilibria for tens of Alfvén timescales until well after any instabilities have saturated. Employing an array of diagnostic tools from visualizations to the structure of the plasma four-current and a vector spherical harmonic decomposition of the settled magnetic fields, we characterize the structure of the dynamically stable, settled magnetic equilibria. We find that the settled configurations are not unique. Instead, they exhibit diverse multipolar structures, with higher-order modes contributing substantially. Our findings demonstrate that there is memory of the magnetic-field initial conditions, and that there is no unique dynamically stable magnetic-field geometry for neutron stars. Nevertheless, we find that the angle-averaged radial profile of the magnetic field toroidal to poloidal amplitude in the bulk of the settled configurations exhibits some degree of universality with typical values of order \(20\)–\(40\%\).
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All authors Mai Li, Vasileios Paschalidis, Antonios Tsokaros, Maria Mutz, Koji Uryu

High Energy Astrophysical Phenomena

Probing the Metallicity Dependence of Fast Radio Burst Progenitors with CHIME/FRB Outrigger Dwarf Host Galaxies

Y. Dong, W. Fong, K. Nimmo et al. (+24 more)

Using the fast radio bursts (FRBs) localized with the CHIME/FRB Outriggers between February and October 2025, we conduct a systematic search for dwarf host galaxies. We identify and characterize eight host galaxies for one repeating and seven apparently non-repeating FRBs, newly classifying five hosts as dwarfs with stellar masses of $\log(M_\ast/M_\odot) \approx 8.1-9.1$. We measure their gas-phase metallicities and, when combined with hosts from the literature, finding that repeating and non-repeating FRBs have statistically distinct metallicity distributions. Non-repeating FRBs inhabit relatively metal-rich environments ($12 + \rm log( O/H) \gtrsim 8.35$) across the full observed mass range, while repeating FRBs have significant preference for dwarf galaxies with $12 + \rm log( O/H) \lesssim 8.35$. Almost all repeaters associated with persistent radio sources (PRSs) are in dwarfs with $12 + \rm log( O/H) \lesssim 8.15$, suggesting a connection between repeaters in metal-poor dwarf environments and PRS formation. We also calculate the host dispersion measure ($\mathrm{DM}_{\rm host}$), Faraday rotation measure ($\mathrm{RM}_{\rm host}$), and scattering timescale ($\tau_{\rm host,1~GHz}$) for 54, 37, and 24 FRBs, respectively, including both our sample and FRBs in the literature. We find no significant ($>3\sigma$) correlation between host metallicity and $\mathrm{DM}_{\rm host}$, $\mathrm{RM}_{\rm host}$, or $\tau_{\rm host,1~GHz}$. The distinct host metallicities of apparently non-repeating and repeating FRBs may reflect metallicity-dependent massive-star evolution, with repeating FRBs and PRSs requiring more extreme magnetar birth spins than the predominantly non-repeating FRB population.
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All authors Y. Dong, W. Fong, K. Nimmo, N. Loudas, B.C. Andersen, S. Andrew, A. Cai, S. Chatterjee, A.M. Cook, A.P. Curtin, T. Eftekhari, B.M. Gaensler, J. Hessels, V.M. Kaspi, A. Khan, C. Leung, L. Mas-Ribas, K. Masui, A. Moroianu, A.B. Pearlman, J.X. Prochaska, M.W. Sammons, P. Scholz, K. Shin, S. Simha, M. Woodland, Z. Zhuang

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

AstroGenesis: A Domain-Specific Multi-Agent AI for Astrophysical Research

N. Sahakyan, M. Khachatryan, A. Mahabal et al. (+13 more)

Modern astrophysical research requires the integration of rapidly expanding scientific literature, heterogeneous observational data, and increasingly complex physical models. We introduce AstroGenesis (https://astrogenai.com), a domain-specific multi-agent AI framework that integrates literature retrieval, multiwavelength data access and analysis, theoretical modeling, and research ideation within a unified research environment.The current implementation focuses on blazar research, with specialized agents coordinated by a Supervisor Agent and Planner/Replanner architecture. These agents provide capabilities for retrieving and synthesizing literature, accessing and analyzing multiwavelength observations, performing physical modeling, and identifying research directions. A central component is the Theoretical Modeling Agent, which uses pretrained neural-network surrogate models to enable efficient broadband and multimessenger modeling through natural-language interaction. The framework provides natural-language access to science-ready multiwavelength observational data and retrieval-grounded literature through a multi-stage retrieval and ranking pipeline. The literature-retrieval system was evaluated using two benchmarks: a single-paper benchmark, in which each question targets one publication, and a multi-paper benchmark, in which questions may require evidence from several publications. At least one relevant publication was retrieved among the top five results for 76.6% of the single-paper questions and 79.2% of the multi-paper questions. Representative workflows demonstrate how literature, observational data, physical modeling, and hypothesis generation can be combined within traceable analyses. The framework is extensible to additional astrophysical domains, with the goal of streamlining research workflows and enabling efficient, connected, and reproducible scientific investigations.
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All authors N. Sahakyan, M. Khachatryan, A. Mahabal, D. Pasham, A. Khachatryan, V. Markosyan, N. Unkovskii, V. Vardanyan, D. Bégué, P. Giommi, G. Harutyunyan, D. Israyelyan, A. Tramacere, A. Casotto, Y. Wang, D. Li

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

Finite-Temperature Axion-Admixed Neutron Stars in a Quarkyonic Crossover Framework

S. K. Patra, D. Dey, Jeet Amrit Pattnaik et al. (+1 more)

We analyze the effect of axion contribution on the mass, radius and dimensionless deformability of a neutron star (NS) at finite temperature. Although the cooling of an NS is mainly due to the emission of neutrinos, additional cooling cannot be ruled out due to the emission of axions. The existence of an axion is needed for addressing the strong CP problem within the standard model. The Peccei-Quinn (PQ) symmetry breaking mechanism generates axions. They are pseudo-Nambu-Goldstone bosons. Their mass is extremely small but non-zero. This naturally resolves the fine-tuning problem of the strong CP-violating parameter ${\theta} \lesssim 10^{-10}$. We propose that a finite temperature NS feature a quark core surrounded by a baryonic medium admixed with the axions, which are pseudo scalar Goldstone bosons and are considered as strong candidates for the dark matter (DM). Unlike the original quarkyonic model, we formulate a quarkyonic-inspired smooth crossover equation of state (EoS) at finite temperatures. This smooth crossover avoids thermodynamic instabilities found in sharp first-order constructions. We add the axionic EoS to the finite temperature quarkyonic one and solve the Tolman-Oppenheimer-Volkoff equations to estimate the properties of the NS both with and without taking the axion into account at different temperatures. We find that the production of axions is maximum within a radius of $5-11$ km in the star's interior; however, the impact on the mass is significant, i.e., $\sim 60\%$ of axion mass concentrated in the central region of the star. We also find that the effect of temperature on the mass of NS is marginal unlike the radius. In general, the influence of axions is significant for the observational properties of NS. These results highlight the pivotal role of axion physics in neutron star phenomenology.
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All authors S. K. Patra, D. Dey, Jeet Amrit Pattnaik, R. N. Panda

High Energy Astrophysical Phenomena

First Pre-peak Ultraviolet Spectrum of a Tidal Disruption Event: A Fast Outflow Revealed Prior to Maximum Light in TDE2025aarm

Erica Hammerstein, Ryan Chornock, Jean Somalwar et al. (+32 more)

The tidal disruption and eventual accretion of a star by a massive black hole can lead to the launching of outflows and winds that encode important information about the tidal disruption and accretion processes. However, little is known about the existence and behavior of these outflows before optical light-curve peak, when processes such as stream-stream collisions and disk formation may be important. Here, we present the first pre-peak ultraviolet (UV) spectrum of a tidal disruption event (TDE), obtained $\sim 20$ days before optical maximum light of the nearby (redshift $z=0.0137$) TDE2025aarm, along with quasi-simultaneous infrared (IR) through X-ray observations. Our HST/STIS spectrum shows strong evidence for an early-time outflow through broad near-UV (NUV) and far-UV (FUV) absorption lines blueshifted by $\sim$10,000 km s$^{-1}$, including two new UV broad absorption features not yet identified in a TDE. We find that the FUV–IR continuum deviates significantly from a blackbody ($f_\lambda \propto \lambda^{-3.08}$), which we interpret as a signature of reprocessing through the outflow. This deviation implies that the bolometric luminosity in optical TDEs may be underestimated by a significant factor ($\sim$9 in this case) when inferred from single-temperature blackbody fits to NUV–optical photometry alone. This work further confirms that TDEs are capable of launching fast outflows at very early times and emphasizes the importance of prompt FUV spectroscopic observations of TDEs that can capture the full continuum emission and energetics.
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All authors Erica Hammerstein, Ryan Chornock, Jean Somalwar, Raffaella Margutti, Brenna Mockler, Olivia Aspegren, Itai Sfaradi, Natalie LeBaron, Jonathan Carney, Alexei V. Filippenko, A.J. Nayana, Igor Andreoni, Thomas Brink, Peter J. Brown, Adam J. Burgasser, S. Bradley Cenko, Suvi Gezari, Xiaoshan Huang, Mansi M. Kasliwal, Jon M. Miller, Sasha Mintz, Sam Rose, Peter Senchyna, Joshua D. Simon, Emma Softich, Robert Stein, Yuhan Yao, WeiKang Zheng, Richard Dekany, Matthew J. Graham, Russ R. Laher, Josiah Purdum, Ben Rusholme, Jesper Sollerman, Stefan van der Walt

High Energy Astrophysical Phenomena

Performance-portable GPU acceleration of the hybrid particle-in-cell code dHybridR

Bricker Ostler, Miha Cernetic and Damiano Caprioli

Hybrid particle-in-cell simulations are widely used to study kinetic processes in collisionless astrophysical and space plasmas, yet the high computational cost of large-scale three-dimensional runs has largely confined production studies to two dimensions or restricted domains. To address this challenge, we present a performance-portable GPU implementation of the hybrid particle-in-cell code dHybridR. The implementation combines OpenMP target offloading with specialized SYCL kernels for the most computationally expensive operations, while preserving a unified CPU-GPU codebase that supports Intel, AMD, and NVIDIA GPUs. On the exascale supercomputers Aurora and Frontier, dHybridR achieves weak scaling efficiencies of $86\%$ to $97\%$ across 49,152 accelerators, and at 256 particles per cell, its full-node GPU throughput is up to $32\times$ that of the vectorized CPU implementation at approximately $90\%$ less energy per particle-update. To our knowledge, no other hybrid particle-in-cell code has reported GPU performance at this scale, leaving dHybridR uniquely positioned to exploit exascale systems. These advances substantially lower the computational barrier to large-scale three-dimensional hybrid-kinetic simulations of collisionless plasmas.
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High Energy Astrophysical Phenomena · Cross-listed from nucl-th

Strange Quark Stars and Spontaneous Scalarization

Mina Mehraeen Nazdik and Zeinab Rezaei

Strange quark matter, as one of the most extreme dense states in astrophysics, can be explored through relativistic compact objects. Hypothetical relativistic stars composed of dense strange quark matter are characterized by the equation of state of that matter. The spontaneous scalarization of strange quark stars is influenced by the equation of state of strange quark matter. Here, we investigate how the equation of state of strange quark matter influences the spontaneous scalarization of these stars. Considering Normal, color-flavor-locked (CFL), and CFLm quark matter in the bag models and applying the scalar-tensor gravity, we calculate the structure of scalarized strange quark stars. We investigate how the effective bag constant, perturbative QCD parameter, CFL pairing gap, and strange quark mass control the mass-radius relation, the central scalar field, the scalarization thresholds, and the scalar charge. Our results demonstrate that strange quark stars in scalar-tensor gravity are viable, testable objects, and they establish a complete framework for constraining both the gravitational theory and the properties of cold quark matter with current and future multimessenger observations.
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High Energy Astrophysical Phenomena

A Tentative Line-like GeV Excess in Fermi Blazar 4FGL J1754.2+3212: Implications for Jet Physics and Beyond

Shi-Ju Kang, Yue Yin, Yong-Gang Zheng et al. (+1 more)

We report the detection of a tentative, narrow spectral feature in the GeV gamma-ray spectrum of the blazar 4FGL J1754.2$+$3212, using approximately 16.5 years of observations from the Fermi Large Area Telescope. A detailed likelihood analysis of the time-averaged spectrum reveals a localized, line-like excess at an energy of $\sim$31 GeV, with a local statistical significance of $\sim$3$\sigma$ (3 degrees of freedom), without corrections for multiple trials and the look-elsewhere effect. This feature is not well described by standard smooth continuum models such as a single log-parabola. The addition of a Gaussian component significantly improves the spectral fit. If confirmed by future observations with higher sensitivity, this feature would represent a novel phenomenon in blazar astrophysics. We explore its potential physical origins, which could involve exotic processes such as dark matter annihilation or photon-axion-like-particle oscillations, or extreme astrophysical mechanisms within a structured jet. Regardless of its ultimate nature, this finding highlights the potential of deep spectral studies of blazars to reveal non-standard physics and challenges conventional one-zone emission models. Definitive verification awaits observations by next-generation instruments, particularly the Very Large Area $\gamma$-ray Space Telescope (VLAST).
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All authors Shi-Ju Kang, Yue Yin, Yong-Gang Zheng, Qingwen Wu

High Energy Astrophysical Phenomena

Study of blazar variability through their flux distribution for CTAO long-term monitoring

A. Mikhno, J. Biteau and J.-P. Lenain on behalf of the CTAO collaboration

The CTAO Key Science Project on Active Galactic Nuclei (AGN) includes a long-term monitoring (LTM) program of blazars. The probability distribution function (PDF) of the flux extracted from unbiased blazar lightcurves can provide important insight into the physical processes taking place in relativistic jets and their connection to observed variability. The aim of this work is to define an optimal observational strategy for the CTAO LTM program using the flux distributions as a quantitative metric. To achieve this goal, we develop a simulation-based analysis framework that enables a systematic comparison of different observational strategies. Light curves were simulated as would be observed with the future CTAO for a representative set of AGN sources, following four different observational strategies within a fixed time budget. For each simulated light curve, different PDF models were fitted and their goodness of fit was evaluated. Based on the best-fit parameters, Monte Carlo simulations were generated to assess the ability of the method to recover the underlying PDF model. Since the true model used to generate the simulated light curves is known, this procedure allows us to quantify the discriminatory power of the algorithm and to identify which observational strategy maximizes model discrimination. Additionally, the algorithm has the potential of discrimination against different models for the flux distributions, such as Gaussian and lognormal. Once validated on simulations, the method is applied to archival lightcurves from existing gamma-ray instruments to extract flux PDFs and study the physical processes driving blazar variability. The analysis framework has been developed and tested on simulated CTAO-like data, showing good stability and robustness. A study to optimize the observing strategy for the CTAO LTM is presented.
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High Energy Astrophysical Phenomena

4U 1624-490 as a possible intermediate-mass X-ray binary: constraints from XRISM and NuSTAR spectroscopy

R. Iaria, A. Anitra, T. Di Salvo et al. (+7 more)

High-inclination dipping neutron-star X-ray binaries provide a direct view of accretion-flow structure because the line of sight intercepts ionised plasma above the disc. We analyse two XRISM/Resolve observations of 4U 1624$-$490 with simultaneous NuSTAR coverage, dividing the data into seven orbital-phase intervals. Phase-resolved fits include neutral partial covering, a photoionised absorber, thermal Comptonisation, and ionised disc reflection. Fe XXV and Fe XXVI absorption are detected over most of the orbit. The absorber column density, ionisation state, covering fraction, and projected velocity vary with orbital phase and the lines are systematically blueshifted, with line-of-sight velocities of about $85$-$440\ {\rm km\,s^{-1}}$. Absorption-only models leave broad Fe-K residuals, while ionised reflection improves the fits. The velocity modulation has a semi-amplitude $K=210\pm14\ {\rm km\,s^{-1}}$ and, if interpreted as orbital motion, implies a mass function $f(M)=0.83\pm0.17\,M_\odot$. Residual structure in the blue wing of Fe XXVI suggests additional kinematic complexity near the dip, but the modulation remains consistent when the two dip-adjacent measurements are excluded. If the absorber traces the neutron-star orbital motion, the inferred donor mass is $M_2=2.66\pm0.32\,M_\odot$, compatible with a Roche-lobe-filling B9 V-B9.5 V star. Under this interpretation, 4U 1624$-$490 may be an intermediate-mass rather than a classical low-mass X-ray binary. (Abridged abstract)
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All authors R. Iaria, A. Anitra, T. Di Salvo, A. Sanna, F. Barra, L. Burderi, C. Maraventano, S. Caserta, W. Leone, C. Miceli

High Energy Astrophysical Phenomena

Radio component of the multi-messenger emission from the anticipated T Coronae Borealis outburst

O. Petruk, T. Kuzyo, S. Orlando et al. (+4 more)

T Coronae Borealis is the nearest symbiotic recurrent nova and is expected to undergo a new eruption soon, offering a rare opportunity to study shock evolution and particle acceleration. We predict radio emission from the outburst and assess how radio observations can constrain the circumbinary medium (CBM) and cosmic-ray acceleration. We post-process three-dimensional (3D) hydrodynamic nova simulations with a multi-zone model for particle acceleration and radio emission, computing thermal free-free and non-thermal synchrotron radiation, including the Razin effect and free-free and synchrotron self-absorption for different CBM configurations. Early radio morphology should not be interpreted directly as the intrinsic shock geometry, because free-free absorption selectively obscures regions of the 3D emitting volume. The intrinsic radio emission is dominated by synchrotron radiation from electrons accelerated at the forward shock, with unabsorbed fluxes reaching 100-1000 Jy at GHz frequencies during the first day. Dense ejecta and shocked CBM material strongly absorb the emission, reducing the observable flux by several orders of magnitude. The source becomes largely transparent at GHz frequencies within about one month. Radio emission remains predominantly non-thermal during the first year, while thermal emission becomes important only for dense equatorial enhancements. The radio spectral index evolves from positive values caused by absorption to the optically thin synchrotron value of about 0.5 after several weeks. A short-lived polarized signal may arise during the first few days if the red giant's magnetic field contributes to the cosmic-ray-generated field. Early multi-frequency radio monitoring and polarimetry of the forthcoming T CrB eruption will probe density structure and magnetic field, testing particle acceleration in symbiotic recurrent novae.
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All authors O. Petruk, T. Kuzyo, S. Orlando, L. Chomiuk, F. Bocchino, M. Miceli, V. Beshley

High Energy Astrophysical Phenomena

Gamma-ray emission toward supergiant shells in the Large Magellanic Cloud

Qixin Yu, Olaf Reimer and Anita Reimer

The Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way, is considered to be a very good target for investigations of diffuse gamma-ray emission. Observations have shown that diffuse emissions are related to the large- and small-scale structures of the LMC. Particularly, it has been suggested that some of the small-scale bright diffuse emissions coincide with supergiant shells (SGSs) previously identified based on H$\alpha$ images. We analyzed the gamma-ray emission from the LMC using more than 15 years of observations with the Fermi Large Area Telescope in the energy range of 300$\,$MeV-300$\,$GeV. A dedicated spatial and spectral analysis was made for diffuse emission components of the LMC. Particularly, we evaluated the emissions from SGSs in detail using phenomenological approaches, and we established their corresponding spatial templates for gamma-ray emission. In the analysis of small-scale extended emissions, we detected seven H$\alpha$-selected SGSs at a significance of more than $5\sigma$. The emission structure nested inside the SGSs appears to be a non-Gaussian feature and is very likely due to a modulated cosmic-ray distribution by an ensemble of supernova remnant shocks. The spectra of the gamma-ray emission for most SGS regions favor a hadronic origin. For the large-scale diffuse emission from the LMC disk, our results suggest an interplay of hadron- and lepton-induced emission. The cosmic-ray proton energy density derived for bright SGSs is 3-10 times higher than that in the LMC disk. SGS$\,2$ was detected to be the most significant emission region among all SGSs. Based on the gamma-ray excess emission we detected in H$\alpha$-selected SGSs in the LMC, we estimate that their contribution to the total diffuse gamma-ray emission in the LMC is at a non-negligible 41% level.
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High Energy Astrophysical Phenomena · Cross-listed from nucl-th

The compressible liquid drop model with curvature included algebraically for the pasta phases in neutron stars

Dmitry Kobyakov

In the compressible liquid drop model (CLDM), the curvature energy term has been included in many works in the literature, and its effects on the presence of pasta phases have also been extensively investigated. However, the analytical part of the inclusion used to cease once a 4-order algebraic equation for the optimized size of the nucleus was written. In this paper, an optimization is suggested to the analytical part of the solution of the CLDM equations. Solving analytically the 4-order algebraic equation proves to be helpful for a straightforward identification of the denied parameter ranges in which the CLDM has no real-root solutions. Using a particular nuclear force derived from the chiral effective field theory, I investigate effects on the ground state of the nucleus curvature. Surprisingly, the inclusion has lead to a significant modification of the range of baryon density in which the bubble phases are found, disfavoring the bubble phases from the ground state. This shows a strong difference between the predictions of the CLDM with the nucleus size obtained when including the curvature term and that obtained from the $w_\mathrm{s} = 2w_{\mathrm{C+L}}$ approximation. The importance of the curvature energy in the CLDM is linked with its ability to change qualitatively the ground state. This paper presents an analytical extension to the CLDM solution, which is ready to be used with any other nuclear model in the future, in order to optimize the inclusion of the nuclear curvature energy, and is not limited to one nuclear model.
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High Energy Astrophysical Phenomena

Variable opacity and accretion-column regimes of magnetized neutron stars

Galina Lipunova and Pavel Abolmasov

Radiation-supported accretion columns of magnetized neutron stars can operate in several distinct regimes. As the accretion rate increases, magnetospheric accretion may proceed through hot-spot emission, efficiently cooling radiative shocks, and advective sinking columns. The structure of the accretion flow is determined by the global parameters such as magnetic field strength and mass accretion rate, but is also affected by opacity variations in strong magnetic fields. A decrease of opacity for photons with energy below the cyclotron energy is able to substantially augment the classical critical accretion rate separating efficiently cooling solutions from advective solutions. We propose an approach to determine the critical accretion rate above which the Basko-Sunyaev sinking solution becomes unavoidable. We also use the Basko-Sunyaev sinking solution to estimate the plasma temperature and compare the characteristic photon energy with the local cyclotron energy. For a combination of moderate magnetic fields and high accretion rates, the characteristic photon energy in advective columns typically exceeds the cyclotron energy. In this case, the opacity is not reduced by the magnetic field, and tall accretion columns are formed. Strong magnetic-field systems may retain short, efficiently cooling accretion columns over a wide range of super-Eddington accretion rates. We identify a region in accretion rate and magnetic-field strength where both efficiently cooling and advective solutions may coexist. In this region, the column may switch or oscillate between the regimes and geometries. The typical time of such oscillations are close to the replenishment time of the column, or to the thermal time near its bottom. The quasi-periodic oscillations, observed in some super-Eddington objects in the 1-100 mHz frequency range, may be related to such relaxation cycles.
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High Energy Astrophysical Phenomena

Shock-free super-Eddington accretion onto magnetized neutron star

Galina Lipunova and Pavel Abolmasov

Strong magnetic fields allow accreting neutron stars to form magnetically controlled funnel flows. At sufficiently high accretion rates, such flows become radiation-pressure-supported. In the conventional picture, the magnetospheric flow contains a nearly free-falling transonic outer region, a radiation-mediated shock, and a subsonic accretion column. We investigate the regime in which the height of the column is expected to exceed the magnetospheric radius. In this case a self-consistent transonic solution with a standing shock is no longer possible, and the entire magnetospheric flow remains subsonic. We construct a semi-analytical hydrodynamical solution for a fully subsonic, radiation-supported magnetospheric flow in the low-Mach-number limit. The magnetic field is treated in the force-free approximation, while the flow structure is determined by mass conservation, energy conservation, and the momentum equation along the field lines. We test the solution using one-dimensional time-dependent simulations with the code HACol. The shock-free solution is highly advective. Its enthalpy profile is close to the adiabatic limit, while the energy density is regulated by leakage of mass and heat through the sides of the accretion channel. Most of the observable power is expected to be released by plasma leaving the magnetospheric flow above the neutron-star surface. Such systems should be associated with super-Eddington accretion discs and low pulse fractions due to visibility selection and scattering in the disc wind. We suggest that many non-pulsating ultraluminous X-ray sources containing neutron stars may accrete in this shock-free regime.
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High Energy Astrophysical Phenomena

First X-ray detection of symbiotic nova CN Cha with eROSITA/SRG

Sara Saeedi, Jaroslav Merc, Manami Sasaki et al. (+2 more)

Aims. We report on the first X-ray detection of the symbiotic star CN Cha with eROSITA (extended Roentgen Survey with an Imaging Telescope Array) onboard Spektrum-Roentgen Gamma (Spektr-RG). Methods. We analyse the eROSITA observations from eRASS2 to eRASS5 individually, as well as the combined eRASS:5 data of all five surveys in order to investigate the X-ray spectral and timing properties of CN Cha throughout the eRASS. Results. Although CN Cha was not detected during the first eROSITA All-Sky Survey (eRASS1), it was detected for the first time in the second eRASS2. Our analysis shows that the X-ray emission of CN Cha gradually increased from eRASS2 to eRASS5. The X-ray spectrum of the source, with the main emission at <2.4 keV, allows us to classify it as a e̱ta}-type symbiotic star. However, a more detailed analysis suggests that there is very soft component, which may originate either from thermonuclear burning on the white dwarf, or from the wind collisions. Additional X-ray observations are required to better understand the origin and emission mechanisms of the source.
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All authors Sara Saeedi, Jaroslav Merc, Manami Sasaki, Lorenzo Ducci, Michael Freyberg

High Energy Astrophysical Phenomena

A Diagnostic Method for Proto-Neutron Star Magnetic Fields by Supernova Fallback Neutrinos

Akihiro Inoue, Yudai Suwa, Ryuichiro Akaho et al. (+3 more)

Young isolated neutron stars exhibit a wide range of magnetic field strengths. To understand the origin of this diversity, it is important to observationally constrain magnetic fields during the proto-neutron star (PNS) phase. We propose a novel diagnostic of PNS magnetic fields using neutrinos associated with supernova fallback. By performing one-dimensional general-relativistic magnetohydrodynamic simulations of the supernova fallback onto a magnetized PNS, we investigate how the energy spectra of the fallback neutrinos depend on the PNS magnetic field strength and the fallback accretion rate. We find that stronger magnetic fields and lower accretion rates produce softer spectra because the fallback neutrinos are emitted mainly near the magnetospheric radius, which becomes larger under these conditions. To quantify this spectral softening, we define a hardness ratio based on event rates above and below 30 MeV and introduce a hardness-event-rate diagram. Using this diagram, we identify the parameter region in which our diagnostic is applicable. Its applicability depends on the PNS magnetic field strength, fallback accretion rate, onset time of the fallback phase, and neutrino mass ordering. We also investigate the dependence on the PNS radius and find that a larger radius produces softer spectra, indicating a degeneracy between the magnetic field and PNS compactness. An independent constraint on the PNS compactness would therefore enable us to better constrain the magnetic field. These results suggest that future multimessenger observations of galactic supernovae can provide a new window into the NS diversity.
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All authors Akihiro Inoue, Yudai Suwa, Ryuichiro Akaho, Shinsuke Takasao, Kazumi Kashiyama, Hiroyuki R. Takahashi

High Energy Astrophysical Phenomena

Semi-Analytic Solutions For Warped, Super-Eddington Accretion Disks with Outflows

Gabriel Wohlfarth, Matthew Middleton, Omer Blaes et al. (+1 more)

The discovery of billion solar mass black holes at high redshift challenges any standard Eddington-limited growth model. Previous work has shown that an advective accretion disk can account for these early black holes with episodic accretion. It has also been shown that if the accretion disk of a black hole is misaligned from the spin axis of the black hole, the disk can become advective within a certain radius. While warped and misaligned accretion disks have been explored in numerical simulations, there is no fully or semi-analytic framework that self-consistently connects disk warp, mass inflow, winds, and energy transport in the super-Eddington regime. In this work, we develop a first-principles semi-analytic model of a radiation-pressure-dominated, warped accretion disk, starting from the coupled conservation equations for mass, energy, and angular momentum. We find profiles for the radial velocity and surface density that explicitly account for disk warping and mass loss and determine the radius at which the disk becomes advective. We explore changes with black hole spin, misalignment angle and viscosity, reproducing relevant results from numerical studies.
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All authors Gabriel Wohlfarth, Matthew Middleton, Omer Blaes, P. Chris Fragile

High Energy Astrophysical Phenomena · Cross-listed from physics.plasm-ph

Anisotropy and energy distribution of leptons and photons in radiative relativistic Alfvénic turbulence

Peng Liu, Xiaochun Wei and Zheng Gong

The anisotropic evolution of relativistic particles in magnetically dominated Alfvénic turbulence is a fundamental process governing nonthermal emissions in high-energy astrophysical environments. In this work, we develop a theoretical framework by deriving scaling equations that characterize the pitch-angle dynamics of leptons and photons across both weak- and strong-radiative cooling regimes. Our model reveals a universal coupling between particle energy and angular distribution, arising from the interplay of field-aligned accelerations, perpendicular drift motions, and radiation reactions. The model can uniquely identify “turning-point energies" that exhibit a nonlinear dependence on the local magnetic field and the energy-injection scale. The validity of this scaling model is confirmed through radiative particle-in-cell simulations. We show that the electron pitch-angle distribution fundamentally dictates photon emission characteristics, further revealing a pronounced statistical correlation between high-energy photon emission and turbulent vortices in the reconnection regions. By bridging the gap between small-scale kinetic interactions and broad-band spectral characteristics, this work provides a quantitative basis for interpreting the hardening of radiation spectra and complex polarization signatures in sources such as pulsar wind nebulae and gamma-ray bursts.
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High Energy Astrophysical Phenomena

Energy estimation of direct and reflected cosmic-ray events aboard balloon-borne radio detectors

Sergio Cabana-Freire, Jaime Alvarez-Muñiz and Matias Tueros

Balloon-borne radio detectors offer a highly effective approach to monitoring immense volumes of the Earth's atmosphere and ice for ultra-high-energy cosmic rays and neutrinos. Because these payloads measure radio emission from a single, highly elevated vantage point, reconstructing the air-shower energy using traditional methods is not possible. In this work, we present a comprehensive, simulation-based energy reconstruction framework tailored to the unique cosmic-ray event geometries observed by balloon payloads. For the first time, we adapt and apply this methodology to direct atmosphere-skimming cosmic-ray showers, accounting for their highly asymmetric development in rarefied air. Furthermore, we provide a major update to the energy reconstruction of reflected downward-going showers by incorporating spherical reflection and ray de-focusing mechanics directly into simulations. We rigorously validate the method across different event geometries, demonstrating a baseline energy resolution of $8-11\%$. Finally, we quantify the systematic effects of finite detector pointing resolution, unknown primary mass composition, and signal-to-noise ratio, establishing a robust framework for the analysis of cosmic-ray events in next-generation balloon missions such as PUEO and POEMMA-Balloon with Radio.
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High Energy Astrophysical Phenomena

A magnetar-powered jet-wind outflow in gamma-ray burst-associated supernovae: Application to the thermal components of GRB 060218-SN 2006aj

Yun-Peng Li and Da-Bin Lin

A rapidly rotating magnetar formed in the collapse of a massive star may release its spin-down power through both a collimated polar jet and a quasi-isotropic magnetar wind. With its long-lasting soft X-ray blackbody component and the double-peaked UV/optical light curves of SN 2006aj, GRB 060218-SN 2006aj provides a useful test case for such a two-component energy-injection scenario. We developed a magnetar-powered jet-wind model in which the two outflows interact with homologously expanding SN ejecta, driving forward shocks that produce shock-breakout emission and subsequent magnetar-powered thermal emission. We first used the jet component to fit the 0.3-10 keV blackbody flux and temperature evolution of GRB 060218 and subsequently added the wind component to model the UV/optical light curves of SN 2006aj. We find that the soft X-ray blackbody component can be reproduced mainly by shock-breakout emission from the polar ejecta driven by the jet. The double-peaked UV/optical light curves are instead produced by the interaction between the quasi-isotropic magnetar wind and a more massive ejecta component. The first optical peak is dominated by wind-driven shock-breakout emission, whereas the broader and later emission is powered mainly by the subsequent magnetar-powered thermal component. Our results suggest a two-component magnetar-engine picture for GRB 060218-SN 2006aj, in which the X-ray thermal emission and the UV/optical double-peaked evolution arise from different energy-injection channels of the same central engine.
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High Energy Astrophysical Phenomena

Multi-Band Optical Variability Characteristics of the $\gamma$-ray emitting blazar 1ES 0647+250 during its 2020 flare

K.Subbu Ulaganatha Pandian, C. S. Stalin, S. Muneer et al. (+4 more)

We present a multi-band optical variability study of the $\gamma$-ray emitting high-synchrotron-peaked blazar 1ES~0647+250 during its enhanced optical activity in 2020. The analysis utilizes 107 epochs of high-precision $B$, $V$, and $R$-band photometry obtained with the 75 cm telescope at the Vainu Bappu Observatory, combined with high-cadence $z_g$ and $z_r$ survey data from the Zwicky Transient Facility. The combined light curves reveal coherent variability across all optical filters, with the source reaching a peak $R$-band magnitude of 15.74 mag. We calculated the variability amplitude, finding the highest intrinsic variability in the B- band. Spectral analysis using color–magnitude diagrams reveals a statistically significant “Bluer-When-Brighter” trend, supported by a Pearson correlation coefficient of $r = 0.68$ ($p < 0.001$) for the (V$-$R) index. Furthermore, the discrete correlation function analysis indicates a near-zero time lag between the $V$ and $R$ bands, suggesting simultaneous emission within the observational cadence. The observed spectral hardening is consistent with synchrotron-dominated optical emission, likely associated with enhanced particle acceleration within the jet during the active phase of 1ES~0647+250.
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All authors K.Subbu Ulaganatha Pandian, C. S. Stalin, S. Muneer, S. Umayal, Aditi Agarwal, Amit Kumar Mandal, B. Natarajan