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Physics Astrophysics High Energy Astrophysical Phenomena

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Friday 21 August

High Energy Astrophysical Phenomena

Relativistic blob signatures in the M87 jet at sub-parsec scales

Donaldo Mora, Alejandro Cruz-Osorio, Erika Benítez et al. (+1 more)

In this work, we present results from general-relativistic radiative-transfer calculations of black hole accretion and jet launching to reproduce the observed properties of the M\,87 jet at sub-parsec scales. For the first time, in the simulations we have included a blob component at the launching region of the jet that allowed us to reproduce the quasi-simultaneous low-energy spectral energy distribution ($10^{10} \text{ Hz} \leq \nu \leq 10^{16} \text{ Hz}$). Moreover, we obtain a better fit of the synchrotron emission in the self-absorption region ($\nu \leq 10^{11} \text{ Hz}$), showing that the inclusion of a blob component in our radiative transfer calculations results in a flatter spectrum ($\alpha_{22-86\ \text{GHz}}=0.08$). Additionally, our simulations improved the accuracy of the observed morphology of M\,87's jet, matching the data up to a distance of 0.6 mas from the core at 86 GHz. Our results reproduce the edge-brightened structure up to $\sim1.3$ mas, i.e., in a more extended region compared with previous studies, with a brighter southern edge. The obtained synthetic image of the jet reproduces additional observed knots along the southern edge.
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All authors Donaldo Mora, Alejandro Cruz-Osorio, Erika Benítez, Yosuke Mizuno

High Energy Astrophysical Phenomena

VLASS Discovery of a Luminous Galactic Radio Transient Evolving on Decade Timescales

Jessie M. Miller, Gregg Hallinan, Dillon Dong et al. (+10 more)

We present a multiwavelength analysis of the radio transient VT J1906+0849, discovered as a 70 mJy source in Epoch 1 of the Very Large Array Sky Survey (VLASS), 21 yr after an NRAO VLA Sky Survey (NVSS) non-detection. Radio observations reveal the source was first detected in 2005, peaking at $\gtrsim200$ mJy in 2014, then declining until a late 2025 rebrightening. The transient sits at a Galactic latitude of $\approx0.74^\circ$ and the properties of its optical-infrared counterpart support a Galactic origin. At $d\gtrsim15$ kpc, the extreme radio luminosity is likely powered by sustained accretion onto a compact object. However, a Swift-XRT non-detection shows it is X-ray faint relative to the Galactic X-ray binary population, and the radio emission is distinct from X-ray binaries in its temporal and spectral behavior. Broadband radio spectra suggest synchrotron self-absorption, but size constraints from equipartition and very long baseline interferometry show little to no expansion in the radio-emitting region over 5+ yr, despite significant spectral evolution. Near-infrared spectroscopy reveals a single broad emission line with a stable centroid but variable width and luminosity. We attribute this feature to blueshifted Br$\gamma$ tracing a persistent asymmetric $\approx2000$ km s$^{-1}$ outflow. These properties are unlike any previously identified Galactic radio source. One possible interpretation is that VT J1906+0849 is a young analog of the microquasar SS 433, with a dense disk wind confining a continuously powered synchrotron outflow. This jet-wind interaction explains the compact, slowly expanding radio source and may contribute to the absence of bright X-ray emission.
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All authors Jessie M. Miller, Gregg Hallinan, Dillon Dong, Adolfo S. Carvalho, S. T. Myers, Jean Somalwar, Casey Law, B. M. Gaensler, Vikram Ravi, Laura Chomiuk, Assaf Horesh, Delina Levine, Yuyang Chen

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

The Random Magnetic Field of the Milky Way

Michael Unger and Glennys R. Farrar

We constrain the large-scale isotropic random component of the Galactic magnetic field using the Planck reconstruction of the 408 MHz synchrotron sky. Our analysis simultaneously fits the random-field structure, the synchrotron contributions of the coherent field and local foregrounds, and isotropic and dipolar offsets. A new element of the analysis is the use of excess polarized emission at 30 GHz to model local foregrounds, allowing us to retain 97% of the sky without absorbing these structures into Galaxy-wide features of the magnetic field. Across variations in the coherent-field model, cosmic-ray electron distribution, sky mask, and field profile, we consistently find that the dominant random-field component is a vertically compact disk with a local rms strength of about 4 $\mu$G and a 1/e scale height of approximately 1 kpc, substantially thinner than in most previous models. An annular enhancement in the inner Galaxy improves the fit to the data, whereas we find no evidence for either a large-scale spiral pattern or a thick random-field disk. The fits also yield an intensity monopole of 4-7 K, whose possible origin we discuss. We quantify the implications of the inferred random field for the angular smearing of ultrahigh-energy cosmic rays. Compared with previous models, the sky-median smearing angle is smaller by up to a factor of 1.7, and by up to 2.4 in individual directions.
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High Energy Astrophysical Phenomena

Generation of TeV Photons by PeV Neutrinos in Dense Astrophysical Environments

Jun-Chen Wang, Hanlin Song, Hao Li et al. (+2 more)

Recent observations by IceCube and KM3Net of PeV-scale ultra-high-energy (UHE) neutrinos, together with detections of TeV-PeV photons from various sources such as the Crab Nebula, the Galactic Center, and gamma-ray burst by ground-based observatories including Tibet AS$\gamma$, MAGIC, Carpet-3, and LHAASO, point to the existence of extreme astrophysical environments capable of accelerating particles to ultra-high energies. These findings motivate investigations of possible connections between UHE neutrinos and photons in such environments. Theoretically, dense regions surrounding compact objects can efficiently produce UHE neutrinos. In this work, we calculate the production of UHE photons from neutrino-nucleon interactions, and note that if these interactions occur in the outer, optically thin regions of dense environments, the resulting photons could potentially be observed. In our model, an incident neutrino scatters off a nucleon, generating secondary partons that hadronize into pions and subsequently decay into UHE photons. We calculate the resulting photon energy spectra and find that for incident (anti)neutrinos with energies above 1 PeV, the probability of producing photons with energies exceeding 1 TeV is greater than 13%. As a concrete application, we show that this mechanism can quantitatively account for the preburst TeV photons observed in GRB 221009A, providing a natural explanation for both their energies and lead times. These findings establish a plausible mechanism linking UHE neutrino events to gamma-ray observations, providing new insights into hadronic processes in extreme astrophysical environments and supporting multi-messenger astronomy studies.
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All authors Jun-Chen Wang, Hanlin Song, Hao Li, Jie Zhu, Bo-Qiang Ma

High Energy Astrophysical Phenomena

Constraining neutron-star properties with ensembles of thermonuclear bursts: application to SRGA J144459.2-604207

Duncan K. Galloway, Serena Jones, Luke Waterson et al. (+3 more)

Deducing the properties of the host neutron stars from thermonuclear (type-I) X-ray bursts remains a challenge, due to incomplete data, multidimensional parameter space, and dearth of suitable models and analysis tools. Here we describe further development of the BEANSP package, with a new "ensemble" analysis mode utilising the consistent and regular "clocked" bursting exhibited by some sources. This mode requires only one model evaluation per epoch, and so is much more efficient than the previous "train" mode. We apply the code to the best-known source exhibiting "clocked" bursting, GS 1826-24, utilising a grid of KEPLER models pre-calculated for this purpose, and find good agreement with a previous study. We performed experiments on simulated data, and recovered input parameters related to the burst ignition with reasonable accuracy, but less so for the system distance, emission anisotropy and neutron star mass and radius. Finally, we assembled a set of 14 daily burst epochs covering the 2024 outburst of the accretion-powered millisecond pulsar SRGA J144459.2-604207, and attempted to constrain the system properties of this object by comparing to SETTLE model predictions. We find reasonably good agreement between the observations and model predictions for a mildly sub-solar fuel composition, with H-fraction $X\approx0.54$ and CNO metallicity $Z_{\rm CNO}\approx0.01$. However, we note that the inferred H-fraction is in excess of the limit of 0.4 established separately, and the adopted model may not provide sufficiently accurate predictions for this burst ignition regime. The inferred distance depends on assumptions about the system inclination and corresponding anisotropy of the persistent emission, and is likely in the range 6-11 kpc. Future applications with more physically realistic models are a promising avenue for this and other sources with H-rich bursts.
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All authors Duncan K. Galloway, Serena Jones, Luke Waterson, Tao Fu, Adelle J. Goodwin, Zhaosheng Li

High Energy Astrophysical Phenomena

CtaAgnVar : a Pipeline for Variability Studies with the Cherenkov Telescope Array Observatory

G. Grolleron, J.-P. Lenain, J. Biteau et al. (+2 more)

Blazars are active galactic nuclei whose relativistic jet points toward the observer, accelerating particles that radiate across the electromagnetic spectrum. Characterizing their flux and spectral variability constrains the physics of the central black hole and its jet, making blazars a key science case for the Cherenkov Telescope Array Observatory (CTAO). The CTAO, with two sites and a sensitivity five to ten times better than current Imaging Atmospheric Cherenkov Telescopes, should reconstruct AGN variability at very high energies with unprecedented precision. To quantify these capabilities for blazars and transients, we developed CtaAgnVar, a pipeline based on Gammapy, the open-source Python package for gamma-ray analysis selected as core library of the CTAO Science Analysis Tools. It covers the full chain from a time-dependent spectral energy distribution to the simulation of a realistic observation sequence, accounting for source visibility, and to the reconstruction of spectra and light curves. It provides variability estimators, including a new statistical estimator quantifying the detection of hysteresis patterns in hardness-ratio diagrams. Released as open source, it lets the instrument enter only through the response functions and array location, so its use is not restricted to the CTAO. We simulated over 15 years of CTAO observations of PG 1553+113, whose high-energy emission shows a periodicity in Fermi-LAT data. Assuming the very-high-energy emission follows the high-energy one, the CTAO would recover this periodicity above 5 sigma with 15 years of 30 min observations on a weekly cadence, the period determination being then limited by the light-curve sampling rather than by photon statistics.
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All authors G. Grolleron, J.-P. Lenain, J. Biteau, C. Boisson, M. Cerruti

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

From Timbuktu to SKA: Who owns the Astronomy knowledge Africa produces?

Rhea Koch and Amare Abebe

Africa has deep and diverse traditions of astronomical knowledge, ranging from archaeological astronomy, pharaonic stellar timekeeping, and manuscript astronomy in Timbuktu and the Sahel, to ecological seasonal astronomy in southern Africa and calendrical computation in the Ethiopian tradition. These multiple epistemic traditions form part of a long intellectual history that precedes and intersects with contemporary astronomy on the continent. In the context of this long-standing intellectual heritage, an important question arises regarding the contemporary circulation of African astronomical knowledge within the global scientific system. This study therefore examines where African astronomical knowledge is produced, validated, and circulated in modern scholarly communication. Using bibliometric data from the Web of Science Core Collection, our analysis investigates publication and citation patterns in the research area Astronomy and Astrophysics between 2000 and 2025, with particular attention to the publication venues of African scholars. The findings highlight a critical paradox: while African researchers are active contributors to global astronomical discovery, the intellectual capital generated through this work is largely stored, validated, and circulated through publication systems located outside the continent. This pattern reflects broader asymmetries within global scholarly communication, where dominant publishing infrastructures shape visibility, citation impact, and authority. By applying a decolonial lens to metrics such as citation impact and ownership, the paper calls for a critical reassessment of the academic practices that sustain epistemic coloniality. It concludes that achieving scientific equity requires a strategic shift in publication choices to build and fortify a sovereign African knowledge archive.
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High Energy Astrophysical Phenomena

EWOCS VII : Unveiling the faint diffuse X-ray emission in Westerlund 1

Albacete-Colombo J.F., Andersen M., De Becker M. et al. (+16 more)

Westerlund 1 (Wd 1) is the closest supermassive star cluster to the Sun, with over 100,000 stars of all spectral types down to brown dwarfs. This population strongly heats the surrounding interstellar medium (ISM), making Wd 1 a key site to study stellar feedback on intracluster gas. We present the most detailed X-ray study to date of its diffuse emission, aiming to separate and quantify the point-source contribution to the true diffuse emission. We analysed 36 Chandra ACIS-I observations within an 8x8 arcmin window. After removing 4922 point sources with ACIS Extract using energy-dependent PSF models and adaptive 99% enclosed-energy masks, we subtracted PSF-wing contamination and corrected for background, then applied adaptive smoothing in soft, medium, and hard bands. Spectral fitting suggests a shocked thermal plasma, with diffuse X-rays correlating spatially with massive stars. The core (Region #2) fits an APEC+PSHOCK model, with temperatures of 1.81+/-0.24 and 4.11+/-1.98 keV and a 6.7 keV Fe K-alpha line. The outer Region #1 shows charge-exchange emission, fitting a CXE+NEI model with softer temperatures of 0.27+/-0.04 and 1.09+/-0.05 keV. Column densities range from 1.84-2.24x10^22 cm^-2, and the total (0.5-8.0 keV) diffuse luminosity is ~9.6(+/-0.3)x10^33 erg/s. Near the core, hard emission likely arises from thermalised Wolf-Rayet winds and wind-wind collisions, softened by adiabatic expansion and turbulent mixing with cooler ISM. The soft component extends farther out, possibly from CXE in lower-extinction regions. No non-thermal contribution is found. A cluster-wind model predicts a luminosity ~2.2 above that observed, consistent with a low-density, partially evacuated intracluster medium. The resulting wind-to-X-ray efficiency, eta ~4.3x10^-6, is about two orders of magnitude below Cygnus OB2, yielding comparatively subluminous diffuse X-ray emission.
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All authors Albacete-Colombo J.F., Andersen M., De Becker M., Mackey J., Larkin C.J.K., Guarcello M.G., Anastasopoulou K., Sciortino S., Greco E., Miceli M., Sapienza V., Flaccomio E., Filócomo A., Stevens I.A., Bayo A., Drake J.J., Gennaro M., Gunderson S.J., Fraschetti F

High Energy Astrophysical Phenomena

Nucleosynthesis in the Accretion-induced Collapse of magnetised, Rotating White Dwarfs

Laurenz Thümmler, Takami Kuroda and Masaru Shibata

Accretion-induced collapse (AIC) of a rotating oxygen-neon-magnesium white dwarf (WD) is an alternative channel of neutron-star formation, in which a mass-accreting star near the Chandrasekhar mass collapses instead of being disrupted thermonuclearly. If the progenitor WD is sufficiently magnetised, it can drive magnetorotational outflows analogous to those of magnetorotational supernovae and may serve as a site for rapid neutron-capture ($r$-process) nucleosynthesis. We post-process tracer particles from five three-dimensional general-relativistic neutrino-(magneto)hydrodynamic AIC simulations, spanning four initial rotation rates, with the WinNet nuclear reaction network. Ejecta mass, the fraction of matter reaching nuclear statistical equilibrium, and the mass ejected beyond the iron group all increase monotonically with rotation rate, with magnetic fields amplifying these trends. The slowest-rotating model synthesises essentially only iron-group material, while the most rapidly rotating and magnetised model reaches the second $r$-process abundance peak at mass number $A \approx 130$, with a trace, non-robust signal up to $A \approx 244$. The corresponding $^{56}$Ni mass rises from $0.007$ to $0.014 M_\odot$, lower than representative yields for core-collapse supernovae and hypernovae by factors of five to more than forty, implying a correspondingly faint radioactively powered component. Contrary to the picture established for magnetorotationally driven supernovae, the heaviest ejecta do not track the polar outflow: they instead reside in equatorial-to-mid-latitude, moderate-entropy lobes, while the polar column is itself a local minimum in some heavy elements. We conclude that rapidly rotating, magnetised AIC events could contribute to the Galactic yields of trans-iron and weak $r$-process nuclei, although none of our models produces a robust third $r$-process peak.
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High Energy Astrophysical Phenomena · Cross-listed from nucl-th

Neutron star matter with hyperons: Bayesian comparison of nucleonic and SU(6)/SU(3) hyperonic models

Athira S., Vishal Parmar, Monika Sinha et al. (+1 more)

We investigate neutron star matter with hyperons within a density-dependent relativistic mean-field framework using Bayesian inference, considering three composition scenarios: purely nucleonic matter, hyperonic matter under SU(6) flavor symmetry, and hyperonic matter under SU(3) symmetry with free vector-sector parameters. The analysis incorporates constraints from empirical nuclear matter properties, theoretical inputs at low densities, and multimessenger observations of neutron stars. We find that the SU(6) scheme, grounded in the quark model and isospin counting rule, leads to a significantly softer equation of state. In contrast, the additional flexibility of the SU(3) framework enhances vector repulsion and yields a comparatively stiffer equation of state consistent with observational bounds across the explored parameter space; in particular, the posterior distributions favor values of the vector coupling ratio $\alpha_v$ lower than the SU(6) limit $\alpha_v = 1$. These differences are reflected in neutron star observables, including mass–radius relations, tidal deformabilities, direct Urca thresholds, and oscillation properties, all of which remain compatible with current constraints within the SU(3) scenario. We further examine structural signatures through the curvature of the mass–radius relation and find that, although hyperon-rich configurations can induce noticeable variations, such features depend sensitively on the stiffness of the equation of state and are therefore not universally robust indicators. Bayesian model comparison further shows that present constraints do not meaningfully discriminate between the purely nucleonic and SU(3) hyperonic scenarios, while providing positive, but not decisive, evidence against the more restrictive SU(6) framework.
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All authors Athira S., Vishal Parmar, Monika Sinha, Ignazio Bombaci

High Energy Astrophysical Phenomena

Hyperonic compact stars with vector portal dark matter

Prafulla K. Panda, Deepak Kumar, Hiranmaya Mishra et al. (+1 more)

The appearance of hyperons in the core of neutron stars generally softens the equation of state (EOS), posing a longstanding challenge to the existence of observed two-solar-mass compact stars. We investigate whether repulsive interactions mediated by a dark-sector vector portal can provide an additional source of high-density pressure and thereby modify the structure of hyperonic compact stars. The baryonic sector is described within the modified quark-meson coupling (MQMC) model, in which the octet baryons are treated as confined relativistic constituent-quark systems interacting self-consistently through the $\sigma$, $\omega$, and $\rho$ fields within a mean field approximation. The dark sector consists of a fermionic dark matter coupled to baryonic matter through a neutral vector mediator $Z^\prime$, generating an additional repulsive contribution to the dense-matter EOS. We investigate the resulting equation of state, mass–radius relation, tidal deformability, and moment of inertia. The resulting mass–radius relations satisfy the observational bounds from massive pulsars, including PSR J0740 + 6620, with maximum neutron star masses reaching approximately $2 M_{\odot}$. The resulting changes in tidal and rotational observables provide additional avenues for testing the dark-sector interaction through multimessenger observations.
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All authors Prafulla K. Panda, Deepak Kumar, Hiranmaya Mishra, Sudhanwa Patra

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

EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics

Paolo Soffitta, Sebastien Guillot, Ian Hutchinson et al. (+137 more)

The Enhanced X-ray Polarimetry Observatory (EXPO) is a mission concept proposed to ESA as an M8 candidate, with a prospective launch in 2041. Building on the scientific success of IXPE, EXPO is designed to overcome its two main limitations, the narrow 2-8 keV energy band and the very slow repointing time, and to enable new scientific capabilities. A wide energy band and fast repointing are essential for investigating the hard X-ray emission of magnetars and black-hole binaries, particle acceleration in supernova remnants and pulsar-wind nebulae, radiative transfer in highly magnetized plasmas, X-ray reflection in accretion flows and active galactic nuclei, and the prompt and afterglow emission of gamma-ray bursts and magnetar flares. EXPO comprises five focusing X-ray telescopes and gas photoelectric polarimeters based on the Timepix ASIC family with InGrid amplification, enabling three-dimensional track imaging and operation in the 2-35 keV band through optimized low- and medium-energy detector configurations. The mirror modules use proven electroformed nickel technology with Au-C coatings and an XMM-like focal length of 7.5 m. The polarimeters are complemented by a coded-mask Wide Field Instrument (WFI), derived from SVOM/ECLAIRs for continuous monitoring of a 2 sr field of view; a Spectral Imaging Camera (SIC), based on stacked CMOS and CdTe detectors for broadband imaging spectroscopy and accurate spectro-polarimetric decomposition; and an Instrument Control Unit (ICU) for payload management, onboard WFI image reconstruction, transient identification, and autonomous spacecraft repointing requests. These capabilities extend X-ray polarimetry into the hard X-ray domain and open a new observational window on fast transients, time-domain astrophysics, and multi-messenger astronomy.
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All authors Paolo Soffitta, Sebastien Guillot, Ian Hutchinson, Fabio Muleri, Mark Pearce, Andrea Santangelo, Daniele Spiga, Ivan Agudo, Gino Bruno Amata, Jaroslaw Bakala, Elisabetta Baracchini, Stefano Basso, Jorg Bayer, Benedikt Bergmann, Jaroslaw Borek, Enrico Bozzo, Soren Krinstian Brandt, Carl Budtz-Jorgensen, Vadim Burwitz, Stefano Cesare, Jerome Chenevez, Enrico Costa, Elisa Costantini, Vincenzo Cotroneo, Walter Cugno, Riccardo Ferrazzoli, Nicolas De Angelis, Desiree Della Monica Ferreira, Klaus Desch, Giorgio Dho, Giuseppe Di Persio, Sergio Fabiani, Davide Fiorina, Szymon Gburek, Markus Gruber, Allan Hornstrup, Jennifer Jones, Jochen Kaminski, Mozsi Kiss, Irfan Kuvvetli, Carlo Lefevre, Hannah Lerman, Honghui Liu, Pasqualino Loffredo, Niels Lund, Hemanth Manikantan, Melissa McHugh, Paul O'Brien, Stephane Paltani, Giovanni Pareschi, Stefano Piacentini, Vladislavs Plesanovs, John Rankin, Ajay Ratheesh, Selina Ringsborg Howalt Owe, Alda Rubini, Miguel Andres Sanchez Carrasco, Emanuele Scalise, Konrad R. Skup, Gianpiero Tagliaferri, Chris Tenzer, Berend Winter, Silvia Zane, Lorenzo Amati, Alessio Anitra, Maria Cristina Baglio, Thibault Barnouin, Stefano Bianchi, Fabrizio Bocchino, Markus Boettcher, Niccolo Bucciantini, Sara Capecchiacci, Fiamma Capitanio, Martina Cardillo, Eugene Churazov, Stefano Covino, Filippo D'Ammando, Laura Di Gesu, Alessandro Di Marco, Tiziana Di Salvo, Michal Dovciak, Lorenzo Ducci, Hua Feng, Giorgio Galanti, Giancarlo Ghirlanda, Vittoria Elvezia Gianolli, Andrea Gnarini, Emanuele Greco, Jeremy Heyl, Adam Ingram, Svetlana Jorstad, Philip Kaaret, Ildar Khabibullin, Fabian Kislat, Henric Krawczynski, Josefin Larsson, Ioannis Liodakis, Frederic Marin, Andrea Marinucci, Lorenzo Marra, Alan Marscher, Herman Marshall, Giorgio Matt, Marco Miceli, Riccardo Middei, Romana Mikusincova, Alexander Mushtukov, Kirpal Nandra, Lara Nava, Giuseppe Maria Oppedisano, Salvatore Orlando, Simone Pagliarella, Alessandro Papitto, Pierre-Olivier Petrucci, Oleh Petruk, Jakub Podgorny, Juri Poutanen, Arne Rau, Agata Roszanska, Felix Ryde, Vincenzo Sapienza, Patrick Slane, James Steiner, Valery Suleimanov, Jiri Svoboda, Daniele Tagliacozzo, Antonella Tarana, Fabrizio Tavecchio, Roberto Taverna, Francesco Tombesi, Sergey Tsygankov, Youli Tuo, Roberto Turolla, Francesco Ursini, Alexandra Veledina, Jacco Vink, Martin C. Weisskopf, Fei Xie, Haocheng Zhang, Menglei Zhou

High Energy Astrophysical Phenomena

Excitation and Recombination Data for Nd I-V with Applications to Kilonovae

N. Ferguson, L. P. Mulholland, M. McCann et al. (+2 more)

Neodymium (Nd), a key r-process element, significantly influences the opacity and emergent spectra of kilonovae resulting from neutron star mergers. We present new atomic data for the first five ionization stages of Neodymium, with emphasis on recombination rate coefficients and electron-impact excitation relevant to non-local thermodynamic equilibrium (non-LTE) modelling. Using the relativistic atomic structure code autostructure, we compute energy levels, radiative and dielectronic recombination rates using the isolated-resonance approximation, and excitation data for these ions. Excitation datasets are constructed using both direct (distorted-wave) and resonant excitation approaches, allowing a systematic assessment of the impact of resonance contributions on effective collision strengths. For the case of Nd ii, we compare collisional strengths with those produced by the Dirac Atomic R-matrix Codes (darc). These comparisons demonstrate the importance of resonance-mediated excitation, especially for low-charge ions where near-threshold resonances significantly enhance rates. In terms of recombination for higher ionization stages, dielectronic recombination (DR) is found to dominate over a wide temperature range, while radiative recombination (RR) is comparable with DR at low temperatures for near-neutral species. Our results highlight the pivotal role of non-LTE atomic physics in interpreting kilonovae observations and constraining nucleosynthesis yields. This work strengthens the atomic foundation necessary for linking observed electromagnetic signals to the physics of compact object mergers and heavy element formation.
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All authors N. Ferguson, L. P. Mulholland, M. McCann, C. P. Ballance, M. G. O'Mullane

High Energy Astrophysical Phenomena

Pulse profiles of accreting neutron stars - A review of analysis methods, observations, and theoretical models

Katja Pottschmidt, Peter Kretschmar, Ekaterina Sokolova-Lapa et al. (+25 more)

X-ray pulsars are highly magnetized ($B\sim 10^{12}$ G) neutron stars accreting from a donor star. Their characteristic X-ray emission arises from accreted material decelerated from relativistic velocities near the magnetic poles of the neutron star. As our line of sight onto the magnetic poles changes with the rotation of the neutron star, the X-rays are periodically modulated, resulting in X-ray pulsations. The shape of the pulse profiles depends on the physics of the interaction between the bright X-rays from the magnetic poles with the infalling matter, the location of the magnetic poles on the neutron star with respect to its spin axis, and on the properties of the space-time around the neutron star. In this review we give a pedagogical introduction to the accretion mechanisms operating in the various types of accreting neutron star systems and the observational techniques used to characterize the pulse profiles. We summarize how the pulse profiles depend on X-ray luminosity and energy and discuss the attempts to connect theoretically these observables with the physical accretion mechanisms. We conclude with an outline of future observational needs and further developments for theoretical models of magnetic accretion.
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All authors Katja Pottschmidt, Peter Kretschmar, Ekaterina Sokolova-Lapa, Elena Ambrosi, Ralf Ballhausen, Peter A. Becker, Katrin Berger, McKinley C. Brumback, Joel B. Coley, Robin H. D. Corbet, Antonino D'Aì, Megan E. DeCesar, Carlo Ferrigno, Felix Fürst, Nazma Islam, Ingo Kreykenbohm, Vicente Madurga-Favieres, Pragati Pradhan, Jakob Stierhof, Philipp Thalhammer, Brent F. West, Michael T. Wolff, Aafia Zainab, Nicolas Zalot, Christian Malacaria, Richard E. Rothschild, Jörn Wilms, Kent S. Wood

High Energy Astrophysical Phenomena

GRB 220101A: a most energetic $10^{54}$ erg long GRB triggered by two supernovae 3.5 seconds apart

R. Ruffini, Y. Aimuratov, L. M. Becerra et al. (+10 more)

GRB 220101A is a long GRB, with a total energy exceeding $10^{54}$ erg with a redshift $z = 4.61$ and one of the largest ever high-quality multi-wavelength observational coverage, from a large number of space-based and ground-based telescopes. We interpret this source in a doubly Binary driven peta nova (BdP-N) model. The progenitor is composed of a massive CO core of $\sim 10\,M_\odot$, highly magnetized with $B \sim 10^{6}$ G, associated to a neutron star (NS) and a white dwarf (WD) with orbital periods from minutes to hours. The large GRB luminosity is explained by a sequence of 7 episodes: episode 1 is triggered by a new kind of pair supernova (HB) which originates from the collapse of the strongly magnetized CO core. Accretion of the HB supernova ejecta (the ejecta) onto the white dwarf companion triggers after 3.5 sec the episode 2: the second supernova emitting neutrinos and creating a new neutron star ($\nu$NS). The ejecta, interacting with the magnetosphere of the binary NS companion originate the episode 3: the Ultra relativistic Prompt Emission (UPE) emission by far the most energetic episode of this GRB, with the formation of a powerful jet normal to the plane of the GRB. Following the UPE energy loss, the accretion of the ejecta on the NS companion leads to the episode 4: the formation of a black hole (BH) of $2.3 \ M_\odot$ leading to the observed GeV afterglow emission. Further accretion of the ejecta spin up the $\nu$NS to a period of $1.3$ ms which gives origin to the episode 5: the birth of a pulsar. The interaction of this milli-second pulsar with the remnants lead to the Episode 6: the synchrotron emission observed in the X-ray, optical and radio, The episode 7 is a 56.7 ms pulsar, as observed $10^{10}$ s after the first burst in the crab nebula.
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All authors R. Ruffini, Y. Aimuratov, L. M. Becerra, Chris L. Fryer, Liang Li, G. J. Mathews, M. T. Mirtorabi, R. Moradi, F. Rastegarnia, J. A. Rueda, C. Sigismondi, S. S. Xue, Yu Wang

High Energy Astrophysical Phenomena

A Dark Matter Masquerade: Degeneracies in Black Hole and Accretion Inference from X-ray Reflection Measurements and Prospects for Compact Dark Matter Halo Constraints

Leif Lui, Zijian Zhang and Alejandro Torres-Orjuela

X-ray reflection spectroscopy is an established probe of black hole (BH) spin and accretion geometry, and many studies have examined its systematic uncertainties. In this work, we explore a possible environmental effect associated with compact dark matter (DM) halos. We adopt numerical BH-DM spacetimes and perform general-relativistic ray-tracing calculations to generate broadened Fe\, K$\alpha$ line profiles. For the configurations studied here, compact DM halos shift the lines toward lower observed energies relative to their Kerr counterparts. We fit the simulated profiles with standard Kerr models and find that the inferred spin and inclination can differ from their input values; in some cases, the spin is overestimated. These results suggest that compact DM halos may introduce an additional uncertainty in reflection-based measurements. Conversely, reflection models that include the environment could ultimately help constrain key halo parameters, such as its mass and characteristic scale, complementing gravitational-wave probes.
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High Energy Astrophysical Phenomena

Effects of Ambient Medium Profiles on the Evolution of Relativistic Magnetized Jets

Xufan Hu and Yosuke Mizuno

Relativistic radio-galaxy jets are conventionally divided into two morphological classes: FR~I and FR~II. Among them, FR~I jets have relatively low radio luminosities and edge-darkened morphologies, indicating they undergo substantial deceleration and disruption on sub-kiloparsec to kiloparsec scales, often developing a turbulent and plume-like morphology. Here, we investigate the evolution of relativistic magnetized jets in stratified ambient media using 2D and 3D relativistic magnetohydrodynamic simulations with several ambient profiles. When the jet and ambient pressures are initially balanced, a Bondi-like atmosphere confines the jet to a broad parabolic shape ($R_j\propto z^{0.649\pm0.003}$) and produces a series of recollimation shocks. Over-pressured jets instead produce a conical shape and allow the development of the current-driven kink instability in the downstream. Based on the simulations, we derive a kink-stability criterion that depends primarily on the jet power and ambient pressure. We also calculate multi-frequency emission maps using a special relativistic radiative transfer code. Each recollimation shock appears as a bright knot, resembling the knot complex downstream of HST-1 of the M87 jet, whereas a jet with a single recollimation shock develops a bright feature close to its base and thus exhibits an FR~I-like morphology.
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Thursday 20 August

High Energy Astrophysical Phenomena

AT 2016blu: Accretion-Powered Outbursts in a Luminous Blue Variable and Compact Object Binary

Mojgan Aghakhanloo, Núria Torres-Albà, Nathan Smith et al. (+33 more)

We present the first X-ray detection of the supernova (SN) impostor AT 2016blu in NGC 4559. AT 2016blu exhibited 27 detected quasiperiodic (~113 day) outbursts between 2012 and June 2026, presumably triggered by periastron passages in an eccentric binary system in which the primary is a $\gtrsim 33~M_{\odot}$ luminous blue variable (LBV). AT 2016blu was serendipitously observed by Chandra in 2001-2002, prior to its first documented outburst in 2012, but the stacked archival data show no X-ray detection. We monitored the source's visual-wavelength variability around its predicted outburst time and triggered a Chandra Target of Opportunity program in March 2026, obtaining observations with detections over five closely spaced epochs. These data indicate an X-ray luminosity of $\log L_{\rm X} \approx 38.64 \pm 0.11$~erg~s$^{-1}$ and an accretion rate of $\dot{M} \gtrsim 8\times10^{-8}~M_{\odot}~\mathrm{yr^{-1}}$, consistent with the presence of a compact companion. We therefore conclude that AT 2016blu is the first known case of an LBV SN impostor whose outbursts are driven by intermittent accretion onto a compact object. Given that the system consists of a massive star and a compact companion, AT 2016blu is a high-mass X-ray binary, similar to SN 2010da, although the donor star in SN 2010da is a red supergiant.
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All authors Mojgan Aghakhanloo, Núria Torres-Albà, Nathan Smith, Raphael Baer-Way, Shazrene Mohamed, Alexei V. Filippenko, WeiKang Zheng, Thomas G. Brink, D. Andrew Howell, Brian Hsu, Moira Andrews, Joseph R. Farah, Curtis McCully, Kathryn Wynn, Réka Könyves-Tóth, Conor L. Ransome, Enrique Rathmann, Tamás Tordai, Sjoerd Dufoer, Charles Cynamon, Franz-Josef Hambsch, Thomas Rutherford, Simone Leonini, Franky Dubois, Ivo Peretto, Donald F. Collins, Lorenzo Betti, Lara Fossi, Irene Parenti, Vincenzo Pettina, Giulio Giordano Chiti, Maria Hofheinz, André Steenkamp, Sauro Gaudenzi, Nikola Antonov, Giuseppe Pappaa

High Energy Astrophysical Phenomena

Investigating the Multiwavelength Emission of Binary SMBH Candidate SDSS J095036.75+512838.1

Cassandra Daniele, Adi Foord, Jessie Runnoe et al. (+2 more)

We present results from a multiwavelength study of the binary AGN candidate SDSS J095036.75+512838.1, which was identified as a candidate binary SMBH system due to its significant and variable broad H$\beta$ line shifts. We use available infrared-to-X-ray data to investigate the system's accretion properties. Analyzing recent Chandra observations, we characterize the X-ray emission and test the binary SMBH hypothesis. We find that the X-ray spectrum is well-modeled as a single AGN, with a 2–10 keV luminosity of $1.3 \times10^{43}$ erg s$^{-1}$. By assembling a comprehensive multiwavelength spectral energy distribution, we further examine the possibility of circumbinary accretion. The infrared-optical emission matches that expected from a single AGN, though we observe an excess in emission between $\sim 9500-19000 \r{A}$ and a deficit near $\sim 2600 \r{A}$. Archival optical spectra also suggest a deficit near $\sim 8500 \r{A}$. We investigate whether these SED anomalies can be explained by long-term variability or interstellar reddening. We find no evidence of dimming, while reddening can account for the deficit in UV emission. However, the origin of the near-infrared excess remains unclear. Ultimately, the SED of SDSS J095036 is well described by emission from a single, reddened AGN. Future high-resolution infrared and X-ray spectral observations will be crucial for disentangling the origin of the infrared excess and testing the binary SMBH scenario in SDSS J095036.
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All authors Cassandra Daniele, Adi Foord, Jessie Runnoe, Michael Eracleous, Niana N. Mohammed

High Energy Astrophysical Phenomena · Cross-listed from nucl-th

Trace Anomaly and Effective Topological Sources in Neutron Stars

Federico Nola

This work investigates whether the trace anomaly can diagnose the stellar response to a topological scalar field in tensor multi-scalar gravity. Eleven cold tabulated equations of state (EoSs) were examined, with six selected after conservative GR stability and speed of sound checks over the tabulated density range. Their fiducial topological configurations were compared with the corresponding GR models under complementary matching prescriptions. After controlling for stellar mass and EoS dependence, the GR trace source strength $S_T$ remained strongly correlated with the topological mass response, with a partial Spearman coefficient $\rho = 0.975$ within the sampled fiducial sequences. Near $1.4\,M_\odot$, the topological configurations were systematically more compact, with reductions in the Jordan frame radius of $6.7$–$8.7\%$ at fixed baryonic mass and $0.87$–$1.29\,\mathrm{km}$ at fixed gravitational mass. These shifts are comparable to current uncertainties from the Neutron Star Interior Composition Explorer (NICER) and their observational impact depends on the source. Despite the global deformation, the interior effective source remained dominated by matter, with a median topological contribution of about $0.8\%$. The GR matter trace therefore emerges as a useful diagnostic of the fiducial topological stellar response.
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High Energy Astrophysical Phenomena

Probabilistic kilonova prediction from gravitational wave inferred binary neutron star parameters

Xiao-Fei Dong, Ik Siong Heng, Gavin P. Lamb et al. (+10 more)

Kilonovae provide a key electromagnetic window into binary neutron star mergers, revealing the properties of the merging system while probing r-process nucleosynthesis of the Universe. However, only a few kilonova candidates have been detected to date, with AT2017gfo being the only one associated with a gravitational wave event, GW170817. In this work, we present Genova, a probabilistic framework for predicting kilonova spectra and light curves directly from gravitational wave posterior samples of binary neutron star mergers. This method uses a conditional normalising flow to learn the distribution of rest-frame spectra conditioned on the source-frame component masses, tidal deformabilities, viewing angle and time since merger. Other kilonova model parameters, such as ejecta opacities, are marginalised over during training, so that their effects are propagated into the predicted spectra as predictive uncertainty. In the self-consistency test, the flow model reproduces the median light curves with residuals typically below $\sim 0.1$ mag, and the ratio of the predicted central 68% interval widths remains predominantly between $0.8$ and $1.4$ over $\sim 0.4$–$8.0$ days. Comparisons with a physically distinct kilonova model show that the probabilistic prediction can remain informative beyond the model used for training. We apply Genova to GW170817/AT2017gfo using multi-band observations, including newly re-reduced $Y$-, $J$-, $K_s$-band photometry from the Visible and Infrared Survey Telescope for Astronomy, which we present in this work. The resulting predictive intervals broadly encompass the observations while capturing both gravitational wave posterior uncertainty and the variation induced by marginalised kilonova model parameters.
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All authors Xiao-Fei Dong, Ik Siong Heng, Gavin P. Lamb, Chris Messenger, Nikhil Sarin, Benjamin Rayson, Nial R. Tanvir, Jessica Irwin, Thomas Wallace, Skye Rosetti, Bo Milvang-Jensen, Andrew J. Levan, Jens Hjorth

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

Universal Relations for Neutron Stars from Asymptotic Analysis

Syo Kamata, Josuke Minamiguchi and Shuhei Minato

Dimensionless observables of neutron stars, such as the moment of inertia, the tidal deformability, the spin-induced quadrupole moment, and the compactness, satisfy the I–Love–Q and Love–$C$ universal relations to percent-level accuracy over a wide range of equations of state. We investigate analytically the origin of this insensitivity in the stellar structure equations. We derive asymptotic expansions of these relations directly from the differential equations and boundary conditions for slowly rotating, tidally deformed stars described by a general piecewise-polytropic equation of state. Although the differential equations allow several forms of the asymptotic expansion, we find that only one class is consistent with the observed universality, and we use this class to analyze the universal relations. Because the observables are determined at the stellar surface, information about the deep-interior equation of state can enter them only through the parameters that survive in the asymptotic expansion at the surface. We find that the universal relations depend only on three parameters: two integration constants and one polytrope index of the outermost segment. By determining how these parameters deform the relations, we show that, throughout the region of parameter space occupied by realistic equations of state, the resulting deviations remain at the percent level, consistent with the observed accuracy of the universal relations. Within the same formalism, we classify violations of universality according to the additional degrees of freedom or input data responsible for them. Since the construction relies only on the underlying differential equations and boundary conditions, the same procedure can be applied to other systems once the corresponding equations and boundary conditions are specified.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR

SN 2021pfs: A Type Ia Supernova Likely Affected by Progenitor Metallicity, as Revealed by Comparison with Its Twin Counterpart

Abdusamatjan Iskandar, Xiaofeng Wang, Ali Esamdin et al. (+16 more)

We present extensive photometric and spectroscopic observations of the normal type Ia supernovae (SNe Ia) 2021pfs, which occurred in the Seyfert 2 galaxy NGC 5427 at a redshift 0.009. SN 2021pfs reached an absolute B-band peak magnitude of $M_{\rm max}(B)=$-19.28 $\pm$ 0.40 mag. The mag and a post-peak decline rate of $\Delta m_{15}(B)=$1.13 $\pm$ 0.06 mag. The observed properties of this nearby SN Ia closely resemble those of SN 2011fe, including the main optical spectroscopic features and photometric evolution. Despite their similar decline rates, SN 2021pfs rose more rapidly in the $U$ band but more slowly in the $r$ and $i$ bands compared to SN 2011fe in very early phases. This photometric difference, particularly at short wavelengths, can introduce a systematic uncertainty of up to $\sim$12% in distance estimates. Analysis of the host galaxy's local and global environment shows an environment consistent with producing a higher-metallicity progenitor for SN 2021pfs than that of SN 2011fe.This higher progenitor metallicity may explain the observed photometric discrepancy and the resulting distance between SN 2021pfs and SN 2011fe, though a larger sample of such "twin" SNe Ia is needed to confirm this trend and assess its impact on cosmological measurements.
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All authors Abdusamatjan Iskandar, Xiaofeng Wang, Ali Esamdin, Wenxiong Li, Xiangyun Zeng, Ruifeng Huang, Hu Maokai, Guoliang Lu, D. Andrew Howell, Curtis McCully, Samuel Wyatt, Daichi Hiramatsu, Estefania Padilla Gonzalez, Craig Pellegrino, Megan Newsome, Lluis Galbany, David J. Sand, Nathan Smith, Huei Sears

High Energy Astrophysical Phenomena

J0011+3443: a GPS compact symmetric object, gravitational lens, or dual AGN?

Efthalia Traianou, Tingting Liu, Roman Gold et al. (+1 more)

We present new multi-frequency VLBA observations of J0011+3443 (TXS 0008+344, z = 0.89) at 2.3, 4.9, 8.5, and, for the first time, 23.6 GHz. The source consists of two compact components A and B at a projected separation of 314 +/- 2 pc, plus a third feature C detected at 23.6 GHz at 0.6 mas from A. Archival low-resolution radio measurements confirm an integrated gigahertz-peaked spectrum, with an observed-frame peak frequency of 0.73 +/- 0.08 GHz and a peak flux density of 879 +/- 125 mJy. Comparison with nearly frequency-matched low-resolution measurements shows that the VLBA recovers 0.85 +/- 0.11 of the 4.85 GHz flux density and 0.50 +/- 0.06 of the 8.46 GHz flux density. The lower recovered fraction at 8.5 GHz suggests that low-surface-brightness emission is resolved out or falls below the VLBA surface-brightness sensitivity. We therefore interpret the VLBA component spectra as spectra of the compact recovered emission only. The 23.6 GHz morphology, the absence of a detected flat-spectrum core, the similar compact spectra of A and B, and the steep integrated GHz spectrum favor an interpretation of J0011+3443 as a GPS-class compact symmetric object, possibly in a short-lived or relic phase, although a dual-AGN origin cannot be excluded without multi-epoch astrometry.
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All authors Efthalia Traianou, Tingting Liu, Roman Gold, Richard Mushotzky

High Energy Astrophysical Phenomena

Quasi-periodic Eruptions from Recurrent Satellite Black Hole Transits through Magnetized Galactic Nucleus Accretion Disks

Na Wang, Jing-Tong Xing, Tong Liu et al. (+2 more)

Quasi-periodic eruptions (QPEs) are recurrent soft X-ray flares from galactic nuclei, but their origin remains uncertain. The delayed ultraviolet (UV) counterpart detected in ZTF19acnskyy provides a new constraint on viable models. We present a two-channel model in which a satellite black hole (sBH) repeatedly crosses a nuclear accretion disk threaded by a large-scale magnetic field. Gravitational focusing and dynamical drag generate hot, optically thick ejecta whose expansion and photon diffusion power the soft X-ray QPE. For fiducial Bondi-scale parameters, the model yields a characteristic X-ray duration of $\sim10^3\ \mathrm{s}$ and luminosity of $\sim10^{42}\ \mathrm{erg\,s^{-1}}$. For ZTF19acnskyy, the model reproduces the observed day-scale X-ray duration and energetics. Simultaneously, the sBH motion compresses and bends the background magnetic field, triggering in-disk reconnection. The reconnection channel provides the energy budget and photon-diffusion delay required for the variable UV component. Unfavorable magnetic fields or diffusion times longer than the QPE recurrence period can weaken or smear out the UV signal, potentially explaining the lack of clear UV counterparts in other QPE sources.
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All authors Na Wang, Jing-Tong Xing, Tong Liu, Ya-Ping Li, Shuang-Liang Li

High Energy Astrophysical Phenomena

Multi-scale Memory and Regime Shift in the Hyperactive Repeating FRB 20240114A

Wen-Long Zhang, Sheng-Lun Xie, Jun-Jie Wei et al. (+5 more)

We present a statistical analysis of FRB~20240114A, a hyperactive repeating fast radio burst, based on 11,553 bursts detected by FAST over 214 days. Our main findings are fourfold. (1) On the most active day (MJD~60381, 3,197 bursts in 4.38 hr), event-rate coherence analysis reveals persistent correlated activity extending up to 3600~s, the longest reported for any repeating FRB, showing memory persists even in intense bursting epochs. (2) The waiting-time distribution on this day is well described by three exponentials, whereas the full 214-day sample develops a threshold power-law tail, indicating burst statistics depend on the observational baseline, with long-range correlations emerging only over longer timescales, a hallmark of self-organized criticality. (3) Rescaled range (R/S) analysis of waiting times reveals a broken power law, with Hurst exponents $H_1=0.63\pm0.02$ (short-lag weak memory) and $H_2=1.04\pm0.02$ (long-lag non-stationary drift). The break corresponds to $\sim$1 hour, consistent with the 3600~s coherence limit. R/S analysis of energies similarly exhibits a break ($H_1=0.60\pm0.01$, $H_2=1.10\pm0.05$) at a different lag, reinforcing that non-stationarity affects both temporal and energetic properties. (4) Energy distributions exhibit waiting-time-dependent slopes that are consistent with the full and daily samples, and the high-energy cutoff remains constant across waiting-time groups, suggesting that the maximum energy scale is an intrinsic source property. Together, these results establish a multi-scale memory framework: the source behaves stochastically on short timescales but exhibits systemic non-stationarity over months, providing benchmarks for burst models and highlighting the need for long-term, high-cadence monitoring to capture temporal complexity.
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All authors Wen-Long Zhang, Sheng-Lun Xie, Jun-Jie Wei, Di Xiao, Long-Xuan Zhang, Shuang-Xi Yi, Fa-Yin Wang, Xue-Feng Wu

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

The Kick Velocities of Neutron Stars in Binary Systems

Paul Disberg, Ilya Mandel and Ryosuke Hirai

Neutron stars (NSs) receive natal kicks on their formation in supernovae (SNe). We consider constraints placed on the natal kick magnitudes by NSs in different classes of binary systems. We compare observed systems to predictions from the COMPAS rapid population synthesis code, where we apply kick models with varied natal kick prescriptions. Specifically, we compare binary orbits (i.e., periods and eccentricities) and systemic kick estimates of (1) Gaia observations of NS-harboring binaries (Gaia NSs), (2) NS low-mass X-ray binaries (LMXBs), (3) NS-white dwarf binaries (NSWDs), (4) NS high-mass X-ray binaries (HMXBs) and in particular Be X-ray binaries (BeXBs), and (5) double NSs (DNSs). In this comparison, we find that we can reproduce most of the observed properties of the Gaia NSs, LMXBs, and NSWDs with natal kicks calibrated to the velocities of young isolated pulsars, although we need a small amount of "rocket" kicks to explain the Gaia NS eccentricities. The HMXBs and DNSs, in contrast, show evidence of significantly reduced NS natal kicks. In particular, we find that an apparent correlation between eccentricity and systemic kick for DNSs can be explained by Blaauw kicks, if the natal kicks are $\lesssim 10$ km s$^{-1}$. Although our model does not align well with low-metallicity Gaia NSs, high-eccentricity BeXBs, and DNS mass estimates, we provide alternative hypothetical explanations for these systems. We conclude that a model in which NSs that are formed in binaries with high-mass companions receive significantly reduced natal kicks can provide a relatively consistent explanation for the observed NSs in binary systems.
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High Energy Astrophysical Phenomena · Cross-listed from physics.plasm-ph

Revisiting the Growth Rate of the Relativistic Tearing Instability: The Role of the Non-ideal MHD Structure

Kaoru Sugimoto, Kunihito Ioka and Masaru Shibata

Magnetic reconnection in magnetically dominated pair plasmas is a key process in high-energy astrophysical systems. We revisit the relativistic tearing instability in a Harris current sheet and derive an improved analytical expression for its linear growth rate and the most unstable wavenumber. The key modification is the treatment of the vector potential perturbation in the non-ideal magnetohydrodynamic (MHD) region. Instead of the conventional constant-A approximation, we use an extrapolated-A approximation, in which the ideal-MHD solution is linearly extrapolated into the non-ideal region. Comparison with two-dimensional particle-in-cell simulations shows that the revised theory improves the prediction of the most unstable wavenumber. The improvement is most pronounced at low particle drift velocities, where the particle gyroradius is smaller than the current-sheet thickness and the fastest-growing mode shifts to longer wavelength. The resulting analytical expressions provide an updated benchmark for magnetically dominated reconnection and its applications to high-energy astrophysical plasmas, including gamma-ray bursts and fast radio bursts.
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High Energy Astrophysical Phenomena

First Observational Evidence for QPO-like Coevolution between Characteristic Damping Timescales and X-ray Time Lags among AGNs

Haoyang Zhang and Li Zhang

Quasi-periodic oscillations (QPOs) and stochastic variability provide complementary probes of the inner accretion flow around supermassive black holes in active galactic nuclei (AGNs). Previous multi-epoch studies of narrow-line Seyfert 1 galaxy RE~J1034+396 revealed a coevolution between the QPO frequency and the X-ray time lag, but whether the stochastic variability participates in the same structural evolution has remained unclear. We analyze the multi-epoch XMM-Newton observations of RE~J1034+396 and model the soft (0.3–1~keV) and hard (1–4~keV) light curves separately using a damped random walk process. We obtain reliable characteristic damping timescales (CDTs) for 17 observations, with the soft-band CDT consistently longer than its hard-band counterpart. When combined with the X-ray time lag, the hard-band CDT traces a counterclockwise closed loop that closely resembles the previously reported QPO-frequency–time-lag loop, whereas the soft-band CDT exhibits more complex trajectory. Under plausible dynamical, thermal, and viscous interpretations, the hard-band CDT is associated with characteristic scales in the inner hot accretion flow/corona. The observed loops may represent different projections of a common cyclic evolution of the inner accretion flow/corona, as the three timing observables may respond to its dynamical, stochastic, and radiative properties on different timescales. These results provide the first observational support among AGNs for the physical picture in which changes in the spatial extent of the hot inner flow/corona simultaneously affect QPO and stochastic variability.
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High Energy Astrophysical Phenomena · Cross-listed from physics.plasm-ph

Characterizing the temporal evolution of Biermann-battery-driven magnetic reconnection in laser-ablated plasmas

T. Morita, Y. Muramoto, S. Isayama et al. (+30 more)

This paper presents an experimental investigation of magnetic reconnection between two laser-produced expanding plasmas, focusing on the quantitative evaluation of the reconnection rate and energy conversion under varying initial conditions. By changing the separation distance between the drive laser focal spots (1 mm and 2 mm), we systematically controlled the inflow parameters. The reconnection region was probed using a two-directional laser Thomson scattering (LTS) system, which simultaneously measured the local plasma parameters parallel to the outflows and along the current sheet. Based on our established method incorporating macroscopic energy and mass conservation laws to derive the upstream magnetic field directly from LTS spectra, we characterized the temporal evolution of the current sheet and the reconnection rate. The larger spot separation allows the plasma bubbles to expand for a substantially longer time before the formation of a reconnection current sheet, resulting in different upstream conditions. Nevertheless, both configurations yielded comparable upstream magnetic fields and reconnection rates. These results suggest that the reconnection rate is relatively insensitive to the global inflow conditions and is primarily controlled by the local physics of the reconnection layer once a current sheet is formed.
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All authors T. Morita, Y. Muramoto, S. Isayama, M. Edamoto, M. Hanano, R. Ishikawa, Y. Kanesada, K. Koba, H. Kondo, S. Kurimaru, K. Maeda, Y. Maenosono, S. Matsukiyo, A. Morita, Y. Nagamatsu, G. Nakayama, T. Ogawa, K. Oshida, Y. Pan, K. Sakai, T. Sano, Y. Sato, N. Shimoda, J. Shiota, Y. Sudo, Y. Suzuki, T. Takezaki, S. J. Tanaka, K. Tomita, S. Ueno, S. Yakura, R. Yamazaki, Y. Sakawa