High Energy Astrophysical Phenomena
Classification of X-ray Binaries in NGC 6946 with the use of Multiwavelength Data
Senay Avdan, Hasan Avdan, Eda Sonbas et al. (+1 more)
We identified X-ray binary (XRB) candidates in the
galaxy NGC 6946 using archival Chandra data. A total of 98 XRB candidates were detected within the 0.3$-$8 keV energy range, with luminosities between 10$^{36}$ and 10$^{40}$ erg~s$^{-1}$. X-ray color-color diagrams were constructed for these sources, classifying 57 as high-mass X-ray binaries (HMXBs) and 8 as low-mass X-ray binaries (LMXBs). The X-ray temporal variability of the sources was analyzed, revealing that 40 sources exhibit variability and 26 are classified as transient sources. Possible optical counterparts for 57 sources were identified using Hubble Space
Telescope (HST) images. Based on the color-magnitude diagrams (CMDs), 40 sources were found to have masses greater than 8M$_{\odot}$ and were classified as HMXBs. Seventeen sources with masses between 5 and 8M$_{\odot}$ were classified as intermediate-mass X-ray binaries (IMXBs). The remaining sources, for which optical counterparts could not be identified, were assumed to be LMXBs. The X-ray luminosity functions (XLFs) derived for HMXBs and LMXBs, identified through both X-ray and optical classification, were best described by a single power-law model ($\alpha \sim$ 1.3$-$1.6). We supplemented traditional classification methods with a machine learning (ML) approach based on multiwavelength photometry. While traditional methods identified no AGN candidates, the ML approach detected a significant AGN population. The spatial resolution of the observing instruments and the limits of the training database hinder the reliability of the ML classifications, thus highlighting a key challenge in combining these methods for extragalactic X-ray source
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All authors Senay Avdan, Hasan Avdan, Eda Sonbas, Kalvir S. Dhuga
High Energy Astrophysical Phenomena
Relaxation of a Vlasov gas to an inhomogeneous state due to phase space mixing in an axisymmetric potential: A Newtonian analogy of the Kerr orbital motion
Ana Laura García-Perciante, Merce Guerrero, Darío Núñez et al. (+1 more)
We explore the dynamics of a Vlasov gas propagating in an external axisymmetric potential consisting of a central potential with an additional quadrupolar component which gives rise to two potential wells along the
symmetry axis. Employing independently $N$-particle simulations and statistical methods, we show that an initially homogeneous and isotropic configuration evolves to an inhomogeneous final state with overdensities located at the wells. The quadrupolar component is chosen such that the equations of motion form an integrable
Hamiltonian system, which allows one to compute the final state of the gas analytically. On the one hand, this allows one to compare the late-time behaviour of the simulations with an analytic prediction and on the other hand to understand the relaxation process through phase space mixing. Our model constitutes a Newtonian analogue of the particle motion in a Kerr spacetime, and we discuss possible applications to recently observed astrophysical phenomena.
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All authors Ana Laura García-Perciante, Merce Guerrero, Darío Núñez, Olivier Sarbach
High Energy Astrophysical Phenomena
Observed x-ray spectral indices support magnetic buoyancy sourced coronae in Seyfert active galactic nuclei
Ketevan Kotorashvili and Eric G. Blackman
Standard radiation spectra modeling of active galactic
nuclei like Seyfert
galaxies typically involves tuning the relative contributions from an optically thick disc, optically thin corona, dust
torus, and outflow by hand to match observations. The physics that quantifies this relative partitioning is not precisely understood. Here we make progress toward improving this understanding by testing the implications of a conceptual paradigm that the corona is supplied from below only by
magnetic fields whose scales are large enough to escape turbulent shredding on their buoyant rise time. We use a generalized mean field
accretion disc theory that includes both local radial angular momentum transport and angular momentum transport into a corona. We then use previous numerical simulations of magnetized turbulent
accretion disc energy spectra to connect the fraction of disc
magnetic energy that would escape to the fraction of coronal to total luminosity. Without explicitly calculating detailed particle acceleration and radiative processes, we assume that the disc radiation spectrum is thermal and the coronal radiation spectrum is a power law. We then show that the aforementioned paradigm for corona formation constrains the allowed x-ray power law index to a narrow range which agrees with observations. For the three AGNs studied here the predicted mean
photon index is $\overline{\Gamma}=1.76$, consistent with the mean observed value of $\overline{\Gamma}_{\rm obs}=1.73$, bolstering confidence in the paradigm.
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High Energy Astrophysical Phenomena
A concordance model of neutron star kicks
Cheyanne Shariat, Kareem El-Badry and Smadar Naoz
Neutron stars (NSs) receive natal kicks during
supernovae that affect their survival in binaries and motion through the
Galaxy. The global distribution of these kick velocities across the NS population remains uncertain. Here, we combine the orbital properties and birth rates of Gaia NS and Be+NS binaries with velocity constraints from young isolated
pulsars to construct a concordance model of NS natal kicks. We find that all three populations can be jointly described by a shared bimodal kick distribution. The low-velocity component follows a lognormal distribution with $\mu_{\ln v}=1.87^{+0.65}_{-0.53}$ and $\sigma_{\ln v}=0.55^{+0.85}_{-0.44}$, corresponding to a median speed of $6.5^{+5.9}_{-2.7}~{\rm km~s^{-1}}$. The high-velocity component is consistent with the distribution inferred from young
pulsars, which peaks near $200~{\rm km~s^{-1}}$. The low-velocity component contributes only $\sim10\%$ of all NS births, but produces most Gaia NS binaries and roughly half of Be+NS binaries. The same model predicts systemic recoil velocities consistent with the observed kinematics of Gaia NSs, Be+NS binaries, and NS low-mass X-ray binaries. For Gaia NSs, whose formation history remains unknown, we use this kick distribution to constrain their progenitors. We favor pre-
supernova binaries containing intermediate-mass stripped
stars with $\sim1~{\rm M_\odot}$ companions in au-scale orbits, and predict $\sim300$ such binaries in the Milky Way today, $\sim40$ in the LMC, and $\sim30$ in the SMC. Finally, we consider the implications of these kicks for NSs in hierarchical triples. We find that 2-10$\%$ of Gaia NS triples and 3-23$\%$ of Be+NS triples retain their tertiary after the
supernova. Overall, our results suggest that a shared natal kick law can account for isolated
pulsars and surviving NS binaries, where most NSs receive large kicks and low kicks are common among surviving binaries.
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High Energy Astrophysical Phenomena
A Unified Multi-Band Secular Framework for the Hierarchical Triple PSR J0435+3233 System
Achamveedu Gopakumar, Vidit Singh, Abhimanyu Susobhanan et al. (+9 more)
The millisecond
pulsar PSR J0435+3233, discovered by FAST, exhibits an unusually large apparent
spin-down rate, potentially reflecting line-of-sight acceleration from a distant companion. Recent investigations reveal a hierarchical triple consisting of an inner
pulsar-helium white dwarf binary with an orbital period of ~8 days, orbited by a G-type subgiant tertiary on an eccentric ~74 yr orbit. The current timing description admits two distinct geometrical solutions. We develop a singly averaged (SA) formulation that averages over the inner orbit while retaining the instantaneous phase of the eccentric outer orbit, thereby capturing the time-dependent quadrupolar perturbations induced by the tertiary. We show how this physical three-body description can be incorporated naturally into
pulsar-timing analyses and use it to forward-propagate the two currently viable configurations, Tv1 and Tv2, to forecast when their timing signatures become distinguishable with FAST. The SA
evolution predicts that the two configurations will become distinguishable in the coming years, with the effective projected semi-major axis, $x_{1,\mathrm{eff}}$, providing the earliest discriminating observable. We further extend the framework to predict the tertiary's spectroscopic radial-velocity and astrometric
evolution, linking FAST timing with LAMOST time-domain spectroscopy and Gaia astrometry. This multi-band framework has the potential to establish PSR J0435+3233 as an exceptionally well-constrained laboratory for three-body dynamics, compact-binary and stellar
evolution, and gravitational physics.
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All authors Achamveedu Gopakumar, Vidit Singh, Abhimanyu Susobhanan, Mauri J. Valtonen, Sayantan Bhattacharya, Di Li, Hong-Liang Yan, Jianping Yuan, Na Wang, Pei Wang, Qingdong Wu, De Zhao
High Energy Astrophysical Phenomena
Polarimetry of Black Hole Spin and Axisymmetric Emission Geometry
Vadim Bernshteyn, Daniel C. M. Palumbo, Dominic Chang et al. (+1 more)
The Event Horizon
Telescope array has measured the average polarimetric spiral morphology of black hole
accretion disks using the so-called “$\beta_2$” coefficient. This measurement is dominated by the weakly lensed, direct ($n=0$, counting
photon half-orbits) image of the disk, providing insight into
magnetic field structure around the M87* black hole. The future space-based Black Hole Explorer mission is expected to extend these capabilities, allowing for $\beta_2$ measurement of the strongly lensed $n=1$ image. In this paper, we explore the constraints $\beta_2$ measurements would impose on spacetime and
plasma geometries using a semi-analytic conical emission model. We show that
magnetic field orientation with respect to the emission cone qualitatively changes the dependence of $\beta_2$ modes on model parameters. For cone-aligned
magnetic fields, we find that the leading-order theoretical prediction from equatorial emitters of $\beta_{2,\, n=0} \approx\beta_{2,\, n=1}^*$ is the dominant trend even in off-equatorial models. Specifically, we show $\arg(\beta_{2,\, n=0}\,\beta_{2,\, n=1}^*) \approx 4\chi$, where $\chi$ is the angle determining in-cone
magnetic field orientation. We also find that $\arg(\beta_{2,\, n=0}\beta_{2,\, n=1})$ minimizes the effect of
magnetic field geometry while preserving a dependence on
spin that may be used to measure
spin direction. We find that observations of the
photon ring's $\beta_2$ complement the constraint from direct image polarization, and can greatly constrain
magnetic field structure even with an unknown external rotation measure. We also find that black hole
spin and emission cone opening angle would not be tightly constrained by $\beta_2$ alone, unless strong priors are placed on the
magnetic field geometry.
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All authors Vadim Bernshteyn, Daniel C. M. Palumbo, Dominic Chang, Tintin Nguyen
High Energy Astrophysical Phenomena
Blasts from the Past: UHECR Excesses from a Population of GRB 21009A-like Transients
Nestor Mirabal
The most prominent feature in the ultra-high-energy
cosmic ray (UHECR) sky is a significant excess of events from the vicinity of Centaurus A (Cen A). In this paper we investigate whether the excess may be caused by a past GRB 221009A-like event rather than Cen A itself. If an extraordinary gamma-ray burst (GRB) went off in one of the dwarf
galaxies within the Cen A/M83 group, intergalactic
magnetic deflections would naturally spread out and delay its
cosmic ray signal, making it visible for $\sim 2\times 10^{5}$ years. Scaling this up, we model a local population of these exceptionally bright, narrow-jet transients within the \text{Greisen–Zatsepin–Kuzmin} (GZK) horizon. For a local volumetric rate of $\dot n_0\simeq4\times10^{-8}\ {\rm Mpc^{-3}\,yr^{-1}}$ and a jet half-opening angle of $0.8^\circ$, we estimate $\simeq3-7$ UHECR excesses across the sky at any given time. Varying the jet opening angle, GZK volume, and visibility time results in a broader range of excess values. It is possible to test this hypothesis by comparing the population predictions against existing and upcoming UHECR anisotropy data obtained by the Pierre Auger Observatory and the
Telescope Array.
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High Energy Astrophysical Phenomena
The Virgo Cluster as a Case Study for Ancient Cosmic Ray Halos in Radio and X-rays
Philip F. Hopkins, Emily M. Silich, Jack Sayers et al. (+2 more)
Virgo is the nearest cool core (CC)
galaxy cluster, and is archetypal of the class of X-ray weak CCs. Despite the X-ray continuum implying a large cooling rate ($\gtrsim 10\,{\rm M_{\odot}\,yr^{-1}}$), measurements of cooler gas phases, dust, and
star formation show almost no cooling outside the M87
nucleus. It is also known that M87$^{\ast}$ is a prodigious source of
cosmic ray (CR) leptons observed in
radio and $\gamma$-rays, and recent
radio observations have shown that the M87
radio halo is low-frequency and its spectral index steepens rapidly away from the injection region+jets, consistent with an aging, low-energy CR population. We demonstrate that coupling X-ray and
radio to newer Faraday rotation limits on the
magnetic field, this implies the CRs within the CC radius ($\lesssim 40$kpc) must contribute significantly to the total pressure and soft X-ray emission, through a combination of Coulomb heating the gas and inverse-Compton (IC) scattering microwave background
photons to $\sim$keV. We show this can explain the apparent correspondence between jet/
radio power and X-ray cooling luminosity, and why the X-ray inferred cooling rate appears much larger than otherwise observed. This would also explain why the central X-ray inferred
metallicity at $\lesssim 10\,$kpc appears to be suppressed relative to the UV/optical inferred
metallicity (in a wavelength-dependent manner). Compared to more extreme
radio+X-ray sources like Perseus, we find the observations require a softer 'escaping' CR spectrum (escaping or accelerated outside $\gtrsim 1\,$kpc from M87$^{\ast}$).
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All authors Philip F. Hopkins, Emily M. Silich, Jack Sayers, Sam B. Ponnada, Isabel S. Sands
High Energy Astrophysical Phenomena
Forward Bayesian Inference for the Binary Black Hole Populations
Poulami Dutta Roy, Tanmaya Mishra, Achal Kumar et al. (+1 more)
We present PopCWB, a
Bayesian framework for the inference of the binary black hole (BBH) population based on the unmodeled search pipeline coherent WaveBurst (cWB). The standard population analysis takes an inverse approach by inferring the underlying population model parameters for a given set of detected events using the individual posterior samples. In PopCWB, we take a forward approach to population inference by varying population models to identify the most optimal one which describes the observed events. Rather than using event-level posterior samples, PopCWB uses a large set of simulated events reconstructed by cWB and their total mass, estimated with a machine-learning regression, to construct marginal probability distributions. These marginal probabilities are then used in the construction of the likelihood. We apply PopCWB to BBH events detected by cWB during the third observing run (O3) of the LIGO–Virgo–KAGRA detectors. We constrain an astrophysically motivated BBH population model that incorporates the effects of pulsational pair-instability
supernovae and dynamical mergers. The analysis predicts a maximum black hole mass of $\mathbf{45.5^{+5.3}_{-6.5} M_{\odot}}$ from stellar
evolution and an inferred local BBH merger rate of $\mathbf{23.1^{+11.3}_{-8.4} \, Gpc^{-3} yr^{-1}}$. We compare these results with the existing population models from the literature.
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All authors Poulami Dutta Roy, Tanmaya Mishra, Achal Kumar, Sergey Klimenko
High Energy Astrophysical Phenomena
Guide field effects on particle acceleration in three-dimensional relativistic magnetic reconnection
Despina Karavola, Lorenzo Sironi and Maria Petropoulou
Relativistic
magnetic reconnection is a key mechanism for particle acceleration in astrophysical
plasmas. Recent 3D particle-in-
cell simulations have shown that, for weak guide fields, particles can be accelerated at nearly the maximal rate by escaping the reconnected
plasma and gaining energy from the large-scale upstream electric field. Here, we investigate whether this fast acceleration channel persists as the guide field becomes comparable to the reconnecting field. We perform 3D simulations of relativistic pair-
plasma reconnection, complemented by corresponding 2D runs, using open boundaries along the reconnection outflows to establish a quasi-steady state. We find that increasing the guide field strength reduces the reconnection rate and causes energetic particles to become more closely aligned with the local
magnetic field. In 3D, the fast acceleration channel—absent in 2D—persists across the full range of guide fields investigated, but involves a progressively smaller fraction of particles as the guide field increases. Consequently, stronger guide fields produce steeper particle energy spectra and lower maximum Lorentz factors.
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High Energy Astrophysical Phenomena
Twins in the Shadow: Phase Transitions in Proto-Neutron Stars
Jannik Milthaler, Jan-Erik Christian, Pia Jakobus et al. (+1 more)
We study the impact of a first-order phase transition from hadronic to deconfined
quark matter on the
evolution of proto-
neutron stars. To maximize the applicability of our results, we construct an equation of state that is highly parameterizable, where we use a relativistic mean field approach to model hadronic matter and a constant speed of sound approach to model
quark matter. We approximate the cooling behavior of proto-
neutron stars as a sequence of decreasing
entropy at constant baryonic mass. Some parameterizations lead to so-called "twin-
stars", where two
neutron stars have the same (gravitational) mass but different radii. Detecting them is regarded as a smoking gun signal for phase transition. We find, however, that proto-
neutron star evolution suggests that gravitational twin
stars are only possible if the hybrid
star does not have, in addition, a baryonic twin. This rules out most twin
star pairs, especially if the phase transition occurs at large densities. In a mass-radius diagram the unpopulated hybrid branch appears to be "in the shadow" of the hadronic branch.
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All authors Jannik Milthaler, Jan-Erik Christian, Pia Jakobus, Stephan Rosswog
High Energy Astrophysical Phenomena
The 2015-2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-Dependent Viewing Geometry
Wanchaloem Khwammai, Minchul Kam, Motoki Kino et al. (+11 more)
We present high-cadence, multi-frequency monitoring of the blazar OJ 287 using KVN (22–129 GHz), ALMA (91.5–343.5 GHz), and Metsähovi (37 GHz), covering 2012–2023, together with published optical polarimetry. Within this decade-long
dataset, an exceptionally large and smooth EVPA rotation is observed only during 2015–2017. The millimeter-band EVPA rotates by more than $\sim$300 deg over $\sim$1.5 yr, with a comparably large and more rapidly evolving rotation observed at optical wavelengths, while remaining comparatively stable at other epochs. The rotation coincides with a sequence of strong
radio flares from 37 to 343 GHz whose peak amplitudes increase toward 2017 March, when the rotation ends. Modeling the ALMA 91.5 GHz light curve yields variability Doppler factors of $\delta_{\rm var} \sim 8$–$11$ during the EVPA-rotation interval, with no systematic increase across the
flare sequence. The absence of a monotonic change in $\delta_{\rm var}$ indicates that progressively enhanced relativistic beaming is unlikely to be the primary driver of the rising
flare envelope. The confinement of the large EVPA rotation to this interval, together with the nearly constant $\delta_{\rm var}$, indicates that the 2015–2017 event occurred when a single newly ejected disturbance temporarily dominated the polarized emission while propagating through structured inner-jet regions threaded by a helical
magnetic field under modest jet-direction changes. Large EVPA rotations therefore arise intermittently as the jet orientation evolves, and are realized only when emission dominance and viewing geometry align favorably.
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All authors Wanchaloem Khwammai, Minchul Kam, Motoki Kino, Sascha Trippe, Hiroshi Nagai, Seiji Kameno, Ilje Cho, Ioannis Myserlis, Jeffrey A. Hodgson, Do-Young Byun, Sang-Sung Lee, Taehyun Jung, Anne Lahteenmaki, Merja Tornikoski
High Energy Astrophysical Phenomena
Comparing the size evolution of GRB 221009A with afterglow models
S. Giarratana, O. S. Salafia, B. Marcote et al. (+10 more)
We present very long baseline interferometry (VLBI) observations of GRB221009A obtained with the global-VLBI network at 5 GHz between 231 and 750 days after the burst. The quality of our observations allowed us to measure the sub-milliarcsecond full-width at half maximum of the source surface brightness distribution with a one-sigma equivalent error of 13% and 30% at 231 and 391 days post-burst, respectively. By fitting the temporal
evolution of these new size measurements alongside those previously published in the literature at 5 and 8 GHz with a single power law $\propto t^a$, we find a slope of $a = 0.52^{+0.14}_{-0.12}$. Using the public code VegasAfterglow, we model the 5 GHz size
evolution considering the afterglow emission of the forward shock produced by a jet decelerating in either a homogeneous interstellar medium (ISM) or into a surrounding medium with a wind-like density profile. The model constrains the collimation-corrected Sedov ratio, that is, the ratio of the jet kinetic energy $E_\mathrm{jet}$ to the density of the surrounding medium ($n$ in the ISM scenario, or $A_\star$ in the wind scenario), in the range $\log[(E_\mathrm{jet}/n)\,\mathrm{erg}^{-1}\,\mathrm{cm}^{-3}] = 54.2^{+0.4}_{-0.5}$ for the ISM scenario and $\log[(E_\mathrm{jet}/A_\star)\,\mathrm{erg}^{-1}] = 51.7^{+0.4}_{-0.2}$ for the wind scenario. Among the works in the literature that fit the light curves and spectra of GRB221009A based on the standard afterglow model, and which can be reproduced with VegasAfterglow, none has best-fitting parameters that are compatible with these constraints within $2\sigma$. Moreover, we show that their predicted size
evolution is not compatible with the observed one. Our results highlight the importance of size measurements of GRB afterglows as an independent probe of their dynamics, structure, and emission mechanisms.
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All authors S. Giarratana, O. S. Salafia, B. Marcote, Z. Paragi, M. Giroletti, G. Ghirlanda, L. Nava, A. van der Horst, J. K. Leung, L. Rhodes, P. G. Edwards, M. Pérez-Torres, J. Yang
High Energy Astrophysical Phenomena
XMM-Newton Resolves Parsec-scale X-ray Jets in the PeVatron Microquasar V4641 Sgr
Naomi Tsuji, Caterina Tresoldi, Kaya Mori et al. (+34 more)
The microquasar V4641 Sagittarii (V4641 Sgr) has recently been established as one of the most powerful Galactic PeV
cosmic-ray accelerators (PeVatrons) through the detection of degree-scale gamma-ray emission to 0.8 PeV. Using XMM-Newton, we performed an X-ray survey of its gamma-ray-emitting regions during the quiescent phase. We discovered extended X-ray emission to the north and south of V4641 Sgr, aligned with the
radio-jet axis, revealing X-ray jet structures extending over several tens of parsecs for the first time. The northern and southern
knot-like features, dubbed n1 and s1, are located approximately 20 pc from the central binary and spatially coincident with the gamma-ray emission. The X-ray emission from each
knot, characterized by a
photon index of $\Gamma_X = 1.9 \pm 0.1_{\rm stat} \pm 0.5_{\rm syst}$ and an energy flux of $\sim$(2–4)$\times10^{-13}$~erg~cm$^{-2}$~s$^{-1}$ in the 2–10 keV band, is likely non-thermal in origin, indicating synchrotron radiation from accelerated electrons. The morphological coincidence between the X-ray and gamma-ray emission favors a leptonic scenario for the gamma rays, although the X-ray emission is more compact than the gamma-ray-emitting region and the hadronic scenario cannot be excluded. Through X-ray to gamma-ray modeling, we obtain a weak
magnetic field strength of 3–5~$\mu$G. We propose that the newly detected X-ray
knots represent particle-acceleration sites, such as jet termination or (re-)collimation regions powered by V4641 Sgr, or downstream structures analogous to the
knots of the outer jets in SS~433/W50. This discovery suggests that extended X-ray emission along jets would be a key signature, yet previously overlooked, in PeVatron microquasars.
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All authors Naomi Tsuji, Caterina Tresoldi, Kaya Mori, Hiromasa Suzuki, Matthew Lundy, Dmitry Khangulyan, Samar Safi-Harb, Laura Olivera-Nieto, Felix Aharonian, Shotaro Abe, Zhen Cao, Sabrina Casanova, Maria Chernyakova, Yi-Heng Chi, Stephane Corbel, E. de la Fuente, Hua Feng, Tatsuki Fujiwara, Noa Grollimund, Dezhi Huang, Yoshiyuki Inoue, Ryohei Kanazashi, Shigeo S. Kimura, Marianne Lemoine-Goumard, Jonathan Mackey, Alessio Marino, Samaresh Mondal, Kai Morita, Maxime Parra, Gabriele Ponti, Brian Reville, Megumi Shidatsu, Shunya Takekawa, Michelle Tsirou, Xiaojie Wang, Heshou Zhang, Ping Zhou
High Energy Astrophysical Phenomena
Candidate X-ray Counterparts of Ultra-High-Energy Emission of the Galactic Microquasar V4641 Sgr Revealed by Einstein Probe
Yi-Heng Chi, Shao-Qiang Xi, Ping Zhou et al. (+20 more)
Microquasars have emerged as promising PeVatron candidates capable of accelerating particles to PeV energies, with recent observations revealing ultra-high-energy (UHE) $\gamma$-ray emission from extended structures associated with these systems. Multi-wavelength studies of such extended structures are crucial for understanding the acceleration, transport, and radiation processes of UHE
cosmic rays (CRs), yet observational evidence remains limited. Here, we report the discovery of diffuse X-ray structures spatially coincident with the TeV $\gamma$-ray emission regions to the north and south of the microquasar V4641 Sgr, using the Follow-up X-ray
Telescope (FXT) onboard the Einstein Probe. We identify X-ray structures located $\sim20$-$40$ pc from V4641 Sgr, extending over several arcminutes. These structures exhibit X-ray fluxes of $\sim1$-$3\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$ and
photon indices of $\Gamma\sim2$-3 in 0.5-8 keV per individual
knot. Under the one-zone leptonic interpretation, the weak X-ray emission implies a
magnetic field of order $1-2 \mu$G. Stronger fields remain possible if only part of the TeV-PeV emission is produced by inverse Compton scattering, with the remaining emission arising from hadronic interactions. The EP-FXT observations support the presence of relativistic electrons on tens-of-parsec scales, while the origin of the UHE $\gamma$-ray emission remains open.
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All authors Yi-Heng Chi, Shao-Qiang Xi, Ping Zhou, Hua Feng, Ruo-Yu Liu, Songzhan Chen, Naomi Tsuji, Caterina Tresoldi, Gabriele Ponti, Zhen Cao, Yong Chen, Shumei Jia, Chi-Chuan Jin, Chengkui Li, Yuan Liu, Matthew Lundy, Kaya Mori, Haiwu Pan, Lian Tao, Junfeng Wang, Weimin Yuan, Juan Zhang, Haisheng Zhao
High Energy Astrophysical Phenomena
Study of the Ultra-Long GRB 220627A: Exploring a Possible Blue Supergiant Collapsar Origin
Ayush Garg, Amit Shukla, Dorottya Szécsi et al. (+1 more)
Ultra-long gamma-ray bursts (ul-GRBs) challenge standard collapsar models because their extreme durations require central engines active for $\sim10^{3}-10^{4}$ s. While the
prompt spectral
evolution of typical GRBs is difficult to resolve due to their short durations, ul-GRBs provide a unique opportunity to study spectral
evolution on extended timescales. We present a time-resolved spectroscopic study of GRB 220627A, one of the most luminous ul-GRBs known ($E_{\rm iso}\sim10^{54}$ erg, $T_{90}>1000$ s), consisting of two bright emission episodes separated by $\sim600$ s quiescent interval. Using a novel HOP
algorithm-based segmentation method, we identify optimal time intervals for time-resolved spectroscopy. A Comptonized power-law (CPL) model well describes the
prompt emission ($\sim 10$ keV-40 MeV) across all timescales. Fermi-GBM spectroscopy reveals a transition from a hard, slow-cooling synchrotron spectrum ($\alpha_{\rm I}=-0.70$) during episode-I to a softer, fast-cooling synchrotron spectrum ($\alpha_{\rm II}=-1.29$) during episode-II. This
evolution closely resembles that of the magnetar-candidate GRB 091024A, highlighting a potential progenitor degeneracy based solely on spectral
evolution. However, the extreme isotropic-equivalent energy exceeds the maximum energy available from magnetar
spin-down, strongly favouring a blue supergiant (BSG) collapsar origin. The prolonged activity and multi-episode structure are naturally explained by sustained
accretion through an extended stellar envelope. Our results demonstrate that ul-GRBs can display magnetar-like spectral
evolution while being powered by BSG collapsars. GRB 220627A highlights the need for combining time-resolved spectroscopy with energetic constraints to distinguish between progenitor scenarios and understand the physical origin of ul-GRBs.
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All authors Ayush Garg, Amit Shukla, Dorottya Szécsi, G. C. Anupama
High Energy Astrophysical Phenomena · Cross-listed from hep-ph
Kinetic Equilibrium between SIMP Dark Matter and Radiation via Internal Bremsstrahlung
Xiaoyong Chu, Ajay Kaladharan and Yassine El Mohtadi
As an alternative to standard dark matter freeze-out via two-body annihilation, strongly interacting massive particles are proposed to regulate their relic abundance through three/four-body self-annihilation. During such freeze-out, dark matter must undergo both number reduction and energy dissipation. The former requires number-reducing processes, and the latter requires an efficient energy-dissipation mechanism. In this work, we propose a novel mechanism where internal Bremsstrahlung processes establish kinetic
equilibrium between dark matter and radiation, and thus dissipate energy. More appealingly, this mechanism can be naturally realized in a class of strong dynamics models with flavor-universal
quarks and a light
axion. To demonstrate our results in a $Sp(4)_c$ dark-color model, we compute all relevant interaction terms and numerically solve the full Boltzmann equations. The same mechanism can also be applied to vector dark matter models.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.IM
Robust Bayesian non-linear pulsar timing and noise analysis using a sequential data-tempered ensemble sampler
Abhimanyu Susobhanan
We develop a sequential ensemble MCMC scheme for
Bayesian pulsar timing and noise analysis that uses data size as the tempering parameter. The method enables gradual and reliable convergence from potentially uninformed initial points by iteratively increasing the data size, thereby increasing sensitivity and progressively narrowing the target distribution until the posterior of the full
dataset is reached. The initial iterations use sub-
datasets that can be orders of magnitude smaller than the full
dataset, substantially reducing computational cost, which is further lowered by employing approximate lower-rank models justified by the reduced sensitivity of these sub-
datasets.
Bayesian inference is performed using the final MCMC chain sampled from the true target distribution after applying appropriate burn-in. We demonstrate the method using the NANOGrav 12.5-year data of PSR B1855+09, showing progressive convergence to the target posterior and that most of the computational time is spent sampling the true target distribution rather than in the preceding tempered iterations.
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High Energy Astrophysical Phenomena
The Structure in the Structure Function of Black Hole Light Curves
Daniel C. M. Palumbo, Alejandro Cárdenas-Avendaño and Lena Murchikova
Of the two black holes, M87$^\ast$ and Sgr A$^\ast$, for which event-horizon-scale imaging is currently achievable, only Sgr A$^\ast$ at our Galactic Center exhibits sub-hour variability, making light curve observations of the inner
accretion flow especially valuable for time-domain studies of the spacetime and the surrounding
plasma. Observed light curves are often sampled with variable cadence, so a structure function is often used to quantify their variability properties. In this work, we assemble a heterogeneous Sgr A$^\ast$ light curve data set from discrete observations acquired over $\sim12$ years {at 230 GHz}, with different instruments, durations, and cadences, and study its structure function in three frameworks. First, we consider analytic predictions for the features of structure functions of stationary random processes with
Fourier spectra dictated by the Matérn covariance. We then study the structure functions resulting from lensed movies of synthetic optically thin
accretion flows approximated with Matérn covariance in both space and time, and identify which features in the structure function can be attributed to the black-hole geometry. Lastly, we examine the structure functions of a library of $72$ general relativistic magnetohydrodynamic simulations. We find that, in general, magnetically arrested disk (MAD) models produce smoother structure functions with steeper slopes than standard and normal
evolution (SANE) counterparts. On time scales of $5-15$ minutes, the observed structure function slope of Sgr A$^\ast$ more closely aligns with the MAD trend, while both simulation types are generally more variable than the data on all sub-hour timescales.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR
Mesoscale Turbulence in Type Ia Supernova Deflagrations II. Evidence for Fuel Preheat Due to Flame Thermal Expansion from Lagrangian Analysis
Ryan Learn, Andrey Zhiglo and Tomasz Plewa
Thermonuclear flame fronts in Type Ia
supernovae develop hydrodynamic instabilities that generate turbulent mesoscale flow and substantial local thermodynamic variability. Recent work by \citet{Brooker+25} demonstrated systematic shortening of fuel ignition times near Rayleigh–Taylor unstable deflagration fronts, but the physical origin remained unclear. We investigate the thermodynamic
evolution of fuel near the flame front using multidimensional simulations and Lagrangian tracer-particle analysis, focusing on flame-driven expansion and ignition-time variability in partially degenerate carbon–oxygen
plasma. Tracer trajectories show that the most strongly preconditioned fuel parcels near the flame front, which form a minority of the fuel layer, undergo approximately pressure-constrained expansion with modest density reductions and temperature increases of $\approx 2\times10^{8}$~K to $\approx7\times10^{8}$~K ($\approx13$ to $63$ per cent above the unperturbed fuel temperature) between the low- and high-density models. Despite moderate thermodynamic perturbations, the strong temperature sensitivity of carbon ignition produces substantial shortening of ignition times. The results indicate a two-step mechanism: flame thermal expansion perturbs the fuel away from hydrostatic
equilibrium, while restoration of pressure balance by weak acoustic waves redistributes internal energy from degenerate to thermal components. This raises the fuel temperature and produces localized thermodynamic preconditioning. These results suggest that unsteady thermonuclear flames can shorten ignition times ahead of the flame, influencing fuel over distances larger than the laminar flame thickness. Whether this preconditioning leads to secondary ignition or contributes to deflagration-to-detonation transition remains untested here, as nuclear burning is disabled in the analyzed fuel.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.CO
Distinguishing the origin of cosmic birefringence: dark energy, dark matter, and neutrino asymmetry
Lu Yin and Eiichiro Komatsu
Cosmic birefringence, the isotropic rotation of the plane of linear polarization of
photons from sources at cosmological distances, can be generated by different parity-violating mechanisms. While the net rotation angle of a single polarized source carries little information about its physical origin, sources at various
redshifts $z$ can be used to distinguish between different origins. Parity-sensitive correlations of the $E$- and $B$-mode polarization fields of the
cosmic microwave background measured on large and small angular scales probe sources at $z\simeq 10$ and $1100$, respectively. Thus, the detailed shape of the $EB$ power spectrum is sensitive to the
redshift evolution of the birefringence source. In this paper, we calculate the $EB$ power spectrum due to a
neutrino-asymmetry current, and compare it to
axion dark energy and dark matter using a common Chern–Simons framework. We find that the $EB$ power spectrum due to a
neutrino asymmetry is suppressed at low multipoles, $\ell\lesssim 20$, similar to the dark matter case. This can be clearly distinguished from the dark energy case, which has no such suppression. Large-scale polarization measurements, including LiteBIRD, can therefore distinguish late-time dark energy birefringence from that generated at higher
redshifts by
axion dark matter or
neutrino asymmetry. We also find that the
evolution of the
neutrino density during recombination alters the shape of the $EB$ power spectrum at high multipoles. This signature can be used to distinguish between the
neutrino and
axion dark matter interpretations, using ground-based experiments such as the Simons Observatory.
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High Energy Astrophysical Phenomena
A Refined Analytic Oblate–Schwarzschild Model for Thermal X-Ray Pulse Profiles of Rotating Neutron Stars
Xi Yang, Tong Zhao and Renxin Xu
Thermal X-ray pulse profiles constrain
neutron-
star masses and radii through the effects of gravitational light bending and rotation on surface emission. The precision anticipated from the enhanced X-ray Timing and Polarimetry (eXTP) mission motivates more accurate analytic light-bending formulae that retain the efficiency needed for repeated model evaluations. We derive a refined analytic flux expression within the oblate–Schwarzschild (OS) approximation that combines bending, propagation-delay, and
lensing approximations with a spheroidal surface. We compare its profiles with numerical Schwarzschild and second-order Hartle–Thorne ray tracing on the same spheroidal surfaces. The comparisons use a selected two-spot configuration at 300 Hz with isotropic blackbody emission and three choices for surface eccentricity: a freely specified value, the Maclaurin relation for a uniformly rotating Newtonian fluid, and a fit to rotating-
neutron-
star surfaces. Across these cases, the refined OS profiles have RMS flux differences of $1.76-1.98\%$ relative to the Hartle–Thorne reference, compared with $6.50-21.39\%$ for the spherical Schwarzschild-plus-Doppler (S+D) model. All differences are normalized to the phase-averaged reference flux. The corresponding differences between Schwarzschild and Hartle–Thorne ray tracing remain below $1.1\%$. These comparisons separate errors in analytic
photon transfer from the effects of surface deformation and the rotating exterior spacetime. For the configurations examined, accounting for oblateness substantially improves agreement with ray tracing. The model retains the speed of an analytic calculation while making the physical origin of profile changes explicit.
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