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Physics Astrophysics High Energy Astrophysical Phenomena

astro-ph.HE Watch High Energy Astrophysical Phenomena

Sunday 27 September

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

Stochastic gravitational-wave background from self-interacting superradiant clouds

Yin-Da Guo, Richard Brito and Chen Yuan

Gravitational-wave (GW) observations offer a powerful probe of new fundamental fields. One well-motivated source is black hole (BH)–boson cloud systems, in which an ultralight scalar field forms a cloud around a rotating BH via superradiance and emits long-lived nearly monochromatic GWs. In this work, we compute the stochastic GW background (SGWB) from such systems, extending previous work by including scalar self-interactions, and discuss its detectability with next-generation ground-based GW detectors. We find that self-interactions can suppress the SGWB and thereby relax existing LIGO-Virgo-KAGRA constraints inferred from null searches. Namely, the LIGO detectors at design sensitivity are insensitive to a SGWB produced by scalar fields with a decay constant $f_\mathrm{s} \lesssim 3\times 10^{17}$ GeV, independently of the scalar field mass. Looking ahead, we show that a moderately self-interacting cloud can still produce a detectable SGWB with next-generation detectors. Under conservative assumptions, for a decay constant $f_\mathrm{s} = 10^{17}$ GeV, the Einstein Telescope (ET) will be sensitive to scalar masses in the range $\sim[10^{-13.0},10^{-11.8}]$ eV, while Cosmic Explorer (CE) will be sensitive to scalar masses in the range $\sim[10^{-13.2},10^{-11.7}]$ eV. We also find that the minimum decay constants that still yield a detectable SGWB for ET and CE are $f_\mathrm{s}\sim 6\times10^{16}\,$GeV and $\sim 3\times10^{16}\,$GeV, respectively.
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High Energy Astrophysical Phenomena

Evidence for an extended Galactic proton component beyond the cosmic-ray knee

Jieshuang Wang

The cosmic-ray (CR) knee at a few petaelectronvolts (PeV) is commonly associated with the acceleration limit of Galactic protons. Recent detection of multi-PeV photons from the microquasar Cygnus X-3 suggests proton acceleration to tens of PeV. However, the contribution of such type of sources to CRs remains unclear. Here we analyse proton and helium spectral data spanning from 0.1 to 1000~PeV and find that the knee strongly disfavours a single terminal cutoff. Full-spectrum analyses incorporating measurements down to tens of gigaelectronvolts (GeV) require a proton contribution extending beyond $100\,\mathrm{PeV}$. This contribution either extends continuously from GeV to 100~PeV energies or belongs to a distinct component whose spectrum has a spectral break at the knee. Within source-population interpretations, its connection to lower-energy Galactic cosmic rays suggests a Galactic origin. The component from GeV energies is conventionally associated with supernova remnants, but extending it beyond $100\,\mathrm{PeV}$ challenges acceleration theory and $\gamma$-ray observations of ordinary remnants . A spectral break, by contrast, can arise naturally from rigidity-dependent transport. This interpretation makes the knee a probe of Galactic CR transport, and connects the extended proton spectrum to extreme Galactic accelerators, such as super-Eddington X-ray binaries, that can also produce PeV $\gamma$ rays and neutrinos.
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High Energy Astrophysical Phenomena

Testing Spin Prior Assumptions of Binary Black Hole Parameter Inference and Their Astrophysical Implications

Chen-He Wu, Wei-Hua Guo, Shu-Jin Hou et al. (+2 more)

With the growing number of binary black hole (BBH) candidates in the Gravitational-Wave Transient Catalog 5.0 (GWTC-5.0), an increasing number of BBH systems with distinct properties have been reported. The individual spins are relatively less constrained in the parameter estimations for most events. While the LIGO-Virgo-KAGRA Collaboration (LVK) adopts a uninformative spin prior by default, some binary evolution scenarios have strong predictions on component spins. In this work, we perform Bayesian parameter inference for a subset of BBH events spanning a broad range of properties in the chirp mass, mass ratio, and effective inspiral spin ($\mathcal{M}_{\rm c}$-$q$-$\chi_{\rm eff}$) parameter space. We consider three different spin priors: the LVK spin prior and two astrophysically motivated priors, assuming a non-spinning primary BH ($\chi_1 = 0$) or a non-spinning secondary BH ($\chi_2 = 0$). We find that the non-spinning primary BH prior is disfavored by the data for several systems with high effective inspiral spins, including GW190517_055101, GW190412, GW241113_163507, and GW231028_153006. We suggest that standard common-envelope evolution represents a promising formation channel that can be tested with gravitational-wave observations, and propose a novel method for estimating the merger rate of systems formed through alternative evolutionary channels. We further suggest that the binary evolution involving mass-ratio reversal may provide a possible formation pathway for these systems.
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All authors Chen-He Wu, Wei-Hua Guo, Shu-Jin Hou, Yuan-Zhu Wang, Ying Qin

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

Probing Solar Chameleons with XENONnT Ionization-Only Data

Guan-Wen Yuan

Screened scalar fields provide a possible connection between dark energy and laboratory-scale new physics while remaining compatible with existing fifth-force constraints. Solar production offers a direct probe of these models, and ionization-only measurements extend the sensitivity of liquid-xenon detectors into the sub-keV regime. We present the first search for solar chameleons using low-energy S2-only data, based on the 7.83 tonne-year XENONnT ionization-only data set. The predicted electronic-recoil spectrum is folded through the run-dependent detector response and tested directly in corrected-S2 (cS2) space. A joint binned likelihood combines the seven cS2 bins of science runs SR0, SR1, and SR2. We sample the five-dimensional parameter space $\{\beta_e,\beta_\gamma,M_e,\Lambda,n\}$ and derive posterior constraints on the conformal and disformal electron couplings. The data extend the recoil-energy reach to approximately $0.04~{\rm keV}_{ee}$, where the few-electron detector response and instrumental backgrounds must be modeled explicitly. In the disformal-dominated regime, the result is expressed in terms of $\beta_{\rm eff}\equiv\beta_\gamma({\rm eV}/M_e)^4$. We obtain the upper limit $\log_{10}\beta_{\rm eff}<-3.64$. Although the present analysis reaches a lower energy threshold than our previous XENONnT electronic-recoil analysis, the resulting constraint is weaker, primarily because the disformal absorption rate decreases rapidly toward low recoil energies, where the background rate also becomes larger. This analysis establishes low-energy ionization data as a new experimental channel for testing solar chameleons and screened dark-energy models.
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Saturday 26 September

High Energy Astrophysical Phenomena

Pulse profiles of tall accretion columns in X-ray pulsars

Bagration Megrelishvili and Pavel Abolmasov

Accreting X-ray pulsars (XRPs) are strongly magnetized neutron stars (NSs) distinguished by a periodic variability pattern related to the rotation of their magnetospheres (pulsations). Many of these objects show complex asymmetric pulse profiles that are difficult to explain in the framework of existing models. We calculate the pulsations of X-ray radiation from a magnetized NS in the regime of supercritical accretion, when the radiation shock wave is high above the NS surface, and the observed radiation comes from two optically thick accretion columns located downstream of the shocks. We show that the accretion column beam pattern calculated from the first principles has a potential of reproducing the variety of XRP pulse shapes. The surfaces of the columns are assumed to radiate locally as blackbodies with the temperature distribution following the solution of Basko and Sunyaev (1976). The adopted geometry of the field is an inclined dipole. It is taken into account that the two accretion flows above the shocks attenuate and scatter the radiation of the accretion columns. Using the ray-tracing technique, we obtain a vast variety of pulse shapes controlled by the mass accretion rate, magnetic angle, the direction towards the observer, and the parameters that determine the particular set of magnetic field lines being loaded with accreting matter. We propose a universal dimensionless parameter describing the asymmetry of a pulse quantitatively. The model naturally produces asymmetric pulse profiles observed in many XRPs. Its applicability, however, is limited to the brightest objects like pulsating ultraluminous Xray sources and strong flares of Galactic XRPs.
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High Energy Astrophysical Phenomena

Global Spline Fit: A unified data-driven view of the cosmic-ray spectrum and mass composition from GeV to the highest energies

Anatoli Fedynitch, Kozo Fujisue, Hans Dembinski et al. (+1 more)

The energy spectrum and mass composition of cosmic rays are measured by two complementary classes of instruments: space- and balloon-borne detectors that resolve individual elements up to sub-PeV energies, and ground-based air-shower observatories that extend the reach to beyond $10^{11}$ GeV but resolve only broad mass groups. The Global Spline Fit (GSF) is a data-driven model that combines both into a single, self-consistent description of the flux of all elements from hydrogen to nickel. The flux of four leading mass groups is parametrized by cubic basis splines, imposing smoothness but no astrophysical expectation on the spectral shape, while the energy-scale offsets of the participating experiments are cross-calibrated within their quoted systematic uncertainties as part of the fit. The model is defined at the local interstellar spectrum, with the effect of solar modulation at the lowest energies accounted for in the fit. The fit uses the most precise recent data, among them the elemental spectra from AMS-02, CALET, and DAMPE, the knee-region measurements of LHAASO, and the fluorescence-based composition of the Pierre Auger Observatory, and describes about one thousand data points with $\chi^2/\mathrm{ndf}\approx 1.3$, or 0.83 after de-weighting localized disagreements between data sets, demonstrating that the global body of cosmic-ray data is consistent once energy scales are aligned. The model delivers the flux, the mass composition, and their full covariance, along with a compact reduced representation for uncertainty propagation. We further provide the cosmic-ray nucleon flux, which differs from widely used parametrizations by 20-50% over four decades in energy. This exceeds the remaining model uncertainty several times over and has direct consequences for atmospheric neutrino and muon flux predictions.
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All authors Anatoli Fedynitch, Kozo Fujisue, Hans Dembinski, Ralph Engel

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

AGN hosting jets. II: Deriving fundamental information from semi-analytical models and the LoTSS sample

Pau Beltrán-Palau, Manel Perucho, José-María Martí et al. (+1 more)

Galactic activity has been shown to play a fundamental role in galaxy and cluster evolution throughout the history of the Universe. In the last decades, a number of works have tackled the determination of radio luminosity functions and their evolution, with the aim to understand and quantify this role. In this work we propose a different approach, based on the application of a semi-analytical model to produce mock populations of radio loud active galaxies throughout several orders of magnitude in power, and the comparison with observational samples. The aim of this work is to test the ability of our semi-analytical model to reproduce luminosity and size distributions of large numbers of sources from the radio-loud active galactic nuclei sample from the LoTSS survey. We apply our semi-analytical model for the evolution of radio galaxies and generate hundreds of thousands of sources applying random selection of parameters within probability distributions for each of them. Our results show that the model is able to reproduce the main features of the observed sample, and identifies jet mass-load included in our model and activity periods as critical parameters that should be addressed in more detail in future work. These initial results allow us to establish a maximum activity period ranging from 10 to 200 Myr within an active time distribution probability p(t_max) \propto 1/t_{max}. We also reproduce the radio luminosity function for the z<0.6 population, and the jet power spectrum from 10^{34.5} W to 10^{38} W. Our results show that mass-load is a fundamental parameter in the determination of radio source luminosities with distance, mainly in the low-power regime. Our model predicts that at least 3-4 x 10^{5} radio galaxies must exist in the LoTSS field in order to obtain the 17045 observed ones. This population is vastly dominated by low power sources.
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All authors Pau Beltrán-Palau, Manel Perucho, José-María Martí, Xavier López-López

High Energy Astrophysical Phenomena

15-Year X-ray Study Links Variable Mass Accretion to Changing Look Behavior in AGN NGC 7582

Santanu Mondal, V. Jithesh, Neeraj Kumari et al. (+1 more)

Changing Look Active Galactic Nuclei (CLAGN) are a subclass of AGN that exhibit spectral state transitions in both optical/UV and X-ray bands on timescales of days to years. In this work, we studied fifteen years of X-ray data of the CLAGN NGC 7582 taken from Suzaku, XMM-Newton, and NuSTAR to understand the origin of the CL behavior. Our study reveals that the obscuration (Hydrogen column density, $N_H$) along the line of sight is associated with the failed wind scenario. The variation in mass accretion is the key that can solely explain the origin of both the failed wind and CL behavior. The mass accretion behavior in NGC 7582 resembles the hard spectral state of black hole binaries. Furthermore, the observed CL timescales in NGC 7582 can be explained either by the dynamical timescale at the BLR or by viscous and thermal timescales at the boundary layer of the corona for a black hole mass of $5.3^{+0.5}_{-0.3} \times 10^7 M_\odot$. Our study thus highlights a direct physical link between the corona and the BLR, which may be potential drivers of the CL behavior.
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All authors Santanu Mondal, V. Jithesh, Neeraj Kumari, Tek P. Adhikari

High Energy Astrophysical Phenomena

Constraints on Supermassive Black Hole Binaries from the Lack of Resolvable Sources in the NANOGrav 15-yr Dataset

Emiko C. Gardiner, Kayhan Gültekin, Philippe Turgeon et al. (+116 more)

Pulsar timing arrays (PTAs) have found evidence of a gravitational wave background (GWB) consistent with a population of supermassive black hole binaries (SMBHBs), but have not yet resolved a continuous wave (CW) from an individual binary. We present the first SMBHB population constraints incorporating PTA upper limits on CWs, based on the NANOGrav 15-year (NG15) data. Using semi-analytic models to generate simulated populations, we calculate each sample's GWB likelihood by free spectral comparison and CW nondetection (nCW) likelihoods by (1) mapping each realization's highest CW signal-to-noise ratio (S/N) to a detection probability and (2) counting the fraction of realizations without a binary close enough that NG15 would have detected it. CW nondetection favors populations in which many lower-mass sources, rather than a few ultramassive ones, dominate the GWB. This information is complementary to that provided by the GWB amplitude, breaking the degeneracy between the masses and number of binaries required to reproduce the observed spectrum. Relative to GWB-only constraints, the joint posteriors favor higher galaxy merger rates, lower $M_\mathrm{BH}$-$M_\mathrm{bulge}$ scatter, shorter binary lifetimes, and steeper small-separation hardening. The most detectable sources in the S/N-weighted joint posterior have chirp masses $\sim 10^{9.6} M_\odot$, luminosity distances $\sim$1300 Mpc, and frequencies $\sim$6 nHz, while the nCW-only constraints favor lower frequencies, coinciding with the NG15 4 nHz candidate. These constraints will tighten if CW nondetection continues.
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All authors Emiko C. Gardiner, Kayhan Gültekin, Philippe Turgeon, Bence Bécsy, Laura Blecha, Nikita Agarwal, Gabriella Agazie, Akash Anumarlapudi, Anne M. Archibald, Zaven Arzoumanian, Anjana Ashok, Jeremy G. Baier, Paul T. Baker, Adam Brazier, Paul R. Brook, Sarah Burke-Spolaor, Rand Burnette, Robin Case, J. Andrew Casey-Clyde, Maria Charisi, Shami Chatterjee, Tyler Cohen, James M. Cordes, Neil J. Cornish, Fronefield Crawford, H. Thankful Cromartie, Kathryn Crowter, Megan E. DeCesar, Paul B. Demorest, Heling Deng, Lankeswar Dey, Timothy Dolch, Graham M. Doskoch, Elizabeth C. Ferrara, William Fiore, Emmanuel Fonseca, Gabriel E. Freedman, Nate Garver-Daniels, Peter A. Gentile, Kyle A. Gersbach, Joseph Glaser, Deborah C. Good, Aiden Gundersen, C. J. Harris, Jeffrey S. Hazboun, Ross J. Jennings, Aaron D. Johnson, Megan L. Jones, David L. Kaplan, Anala Kavumkandathil Sreekumar, Luke Zoltan Kelley, Matthew Kerr, Joey S. Key, Nima Laal, Michael T. Lam, William G. Lamb, Bjorn Larsen, T. Joseph W. Lazio, Natalia Lewandowska, Tingting Liu, Duncan R. Lorimer, Jing Luo, Ryan S. Lynch, Chung-Pei Ma, Dustin R. Madison, Ashley Martsen, Cayenne Matt, Alexander McEwen, James W. McKee, Maura A. McLaughlin, Natasha McMann, Bradley W. Meyers, Patrick M. Meyers, Matthew T. Miles, Chiara M. F. Mingarelli, Andrea Mitridate, Cherry Ng, David J. Nice, Shania A. Nichols, Stella Koch Ocker, Daniel J. Oliver, Ken D. Olum, Timothy T. Pennucci, Benetge B. P. Perera, Polina Petrov, Nihan S. Pol, Henri A. Radovan, Scott M. Ransom, Paul S. Ray, Joseph D. Romano, Jessie C. Runnoe, Alexander Saffer, Shashwat C. Sardesai, Ann Schmiedekamp, Carl Schmiedekamp, Kai Schmitz, Levi Schult, Brent J. Shapiro-Albert, Xavier Siemens, Joseph Simon, Sophia V. Sosa Fiscella, Ingrid H. Stairs, Daniel R. Stinebring, Kevin Stovall, Abhimanyu Susobhanan, Joseph K. Swiggum, Jacob Taylor, Stephen R. Taylor, Mercedes S. Thompson, Jacob E. Turner, Michele Vallisneri, Rutger van Haasteren, Joris P. W. Verbiest, Sarah J. Vigeland, Haley M. Wahl, Kevin P. Wilson, Caitlin A. Witt, David Wright, Olivia Young

High Energy Astrophysical Phenomena

Spin wandering of an accreting neutron star near torque balance: implications for gravitational wave searches

Andrew Melatos

Torque balance between gravitational radiation reaction and magnetocentrifugal accretion is one reason why accreting neutron stars rotate slower than centrifugal break-up. Random processes in the accretion disk, such as Rayleigh-Taylor instabilities and flicker noise, drive the spin frequency $f_\ast$ and gravitational wave frequency $f_{\rm gw}\propto f_\ast$ to wander stochastically around their torque balance values. Here it is shown theoretically, in terms of an idealized Ornstein-Uhlenbeck model, that the standard deviation of $f_{\rm gw}$ is given by $\sigma_{f_{\rm gw}}(h_0) \propto f_{\rm gw} h_0 D \beta T^{1/2}$, or equivalently $\sigma_{f_{\rm gw}}(F_{\rm X}) \propto f_{\rm gw}^{1/2} F_{\rm X}^{1/2} D \beta T^{1/2}$, where $h_0$ is the characteristic wave strain, $F_{\rm X}$ is the X-ray flux, $D$ is the source distance, and $\beta$ and $T$ are the fractional amplitude and autocorrelation time-scale of the stochastic torque. One can use $\sigma_{f_{\rm gw}}(h_0)$ and $\sigma_{f_{\rm gw}}(F_{\rm X})$ to check the physical consistency of a detection candidate in a gravitational wave search by comparing with $\sigma_{f_{\rm gw}}$ observed. Spin wandering impacts search design, because it sets the maximum time interval $\max(T_{\rm coh})$, during which the signal may be treated as coherent. The Ornstein-Uhlenbeck calculation predicts $10^{-1} \lesssim \max(T_{\rm coh}) / (1\, {\rm day}) \lesssim 10^3$, with $3\times 10^{-2} \lesssim \beta T^{1/2} / (1 \, {\rm s^{1/2}} ) \lesssim 4$, for Rayleigh-Taylor instabilities and flicker noise. The predicted $\max(T_{\rm coh})$ is shorter typically than the year-long observation runs of audio-band interferometers operating today, emphasizing the utility of semi-coherent search algorithms, which divide the data into multiple coherent segments of duration $\max(T_{\rm coh})$.
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High Energy Astrophysical Phenomena

Polarization Signatures of Sgr A* with Tilted Hybrid Magnetic Fields

Yushu Feng, Hao Yin, Songbai Chen et al. (+1 more)

The magnetic field geometry in the near-horizon region is very important in shaping polarimetric signatures of black hole images. With a hybrid magnetic field consisting of an axisymmetric split-monopole field and a tilted uniform external field, we investigate the effects of external magnetic field inclination on the polarimetric signatures of Sgr A* by employing a stationary semi-analytical radiatively inefficient accretion flow (RIAF) model and general relativistic polarized radiative transfer simulations. Our results show that the external magnetic field tilt significantly modifies the electric vector position angle (EVPA) pattern and brightness distribution. The induced non-axisymmetry from the tilted magnetic filed component renders polarization morphology dependent on the observer's azimuthal angle, which is a novel behavior absent in purely axisymmetric magnetic field configurations. In combination with Event Horizon Telescope (EHT) data, we analyse the possibility of such hybrid magnetic fields around Sgr A*.
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All authors Yushu Feng, Hao Yin, Songbai Chen, Jiliang Jing

Friday 25 September

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

A Delayed Rejection Reversible Jump Markov Chain Monte Carlo Method for Multi-Resolution Maps of the Stochastic Gravitational Wave Background with Pulsar Timing Array Data

Taha T. Moursy and Nihan S. Pol

Pulsar timing array anisotropy analyses often use a naive counting argument to set resolutions of inferred maps of the stochastic gravitational wave background (GWB). We present a data-driven method in the form of a delayed rejection reversible jump sampler tailored to multi-resolution pixel decompositions of the angular power density of the GWB. We also consider a rapid, frequentist alternative based on information criteria statistics. We verify our methods with a series of injection-and-recovery simulations, finding that the standard counting argument would lead to drastic overfitting of the data and that our data-driven methods reduce the number of parameters in the models by one to three orders of magnitude yet can achieve higher resolution than the standard counting argument when justified by the data. We make our sampler and frequentist method implementation available on GitHub.
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High Energy Astrophysical Phenomena

Not all spins of black-hole binaries formed in clusters are isotropic and not all spins of black-hole binaries formed in isolation are aligned

Sofia Dossena, Davide Gerosa and Tristan Bruel

Spin directions are promising observables for distinguishing between the formation channels of merging stellar-mass binary black holes with gravitational-wave observations. In this work, we challenge the standard expectation that binaries formed via dynamical interactions in stellar clusters have isotropically distributed spins and that binaries formed in isolation have spins aligned with the orbital angular momentum. For dynamically formed binaries, we account for observational findings suggesting that the spins of stars in clusters may exhibit a preferred degree of alignment and construct a simple geometrical description of the resulting black-hole spin directions. This results in an analytical joint distribution for the polar angles of first-generation black-hole binaries, whose key feature is a correlation between the two spin-orbit tilts. We show that, under reasonable assumptions and independently of the initial spin configuration, spins are fully randomized after a single black-hole merger, implying that binaries formed through hierarchical mergers have isotropically distributed spin orientations. For isolated binaries, we introduce a simple model for spin evolution as a function of the natal kick imparted during the first supernova explosion, while accounting for tidal interactions and mass-ratio reversal. The resulting distribution is also fully analytic. We investigate the impact of relativistic spin precession, as well as that of the finite escape speed of clusters, on our geometrically derived distributions. Our results provide a simple analytical framework for modeling the spin directions of merging black holes in gravitational-wave population fits.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Milky Way White Dwarfs as Inelastic Dark Matter Detectors

Javier F. Acevedo

We show that white dwarfs in the Galactic Center are a sensitive probe of inelastic dark matter (IDM) annihilation. IDM particles traversing a white dwarf may become gravitationally captured without fully settling in its interior if their energy drops below the kinematic threshold for inelastic scattering. In this regime, a fraction of the incoming particles remains bound on long-lived orbits that extend beyond the white dwarf's volume, enabling annihilation to partially proceed outside of the object. Some of the annihilation products subsequently escape, rather than being absorbed by the stellar material, producing a potentially detectable signal. We estimate the collective gamma-ray flux from the external annihilation sourced by these systems, as a function of the IDM parameters, and compare it to H.E.S.S. observations within the central degree of the galaxy. From this comparison, we derive tentative limits on the IDM scattering cross-section at the TeV mass scale, under largely conservative assumptions for the IDM density in this region. As a concrete application, we map these results into specific parameter constraints for dark photon-mediated and higgsino models with mass splittings at the MeV scale.
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High Energy Astrophysical Phenomena

Correlated Signatures of Plasma Lensing in Fast Radio Bursts

Paz Beniamini and Pawan Kumar

Plasma lensing has been proposed to explain narrow spectra, multiple burst copies, frequency drifts, interference fringes, and polarization changes in fast radio bursts (FRBs). Because many of these features are not unique to lensing, a convincing identification requires several correlated observables to be reproduced by one lens model. We derive such relations for a one-dimensional Gaussian plasma lens near a fold caustic. We distinguish the phase separation of the merging images from their observable group-delay difference and identify three regimes: resolved burst copies, overlapping images with negligible mutual coherence, and coherent spectral interference. We connect the magnification, image delay, spectral-envelope width, fringe spacing, differential dispersion, time-frequency caustic curvature, and the brightness and ordering of the third image. We derive separate finite-source constraints from large fold magnification and from fringe visibility, and relate the wave-optics magnification scale to the plasma column of the lens. High-contrast fringes simultaneously constrain the source size and the mutual coherence of the two image fields, which limits the intrinsic coherence of the FRB radiation once the observed spectrum and instrumental bandpass are accounted for. We also derive the conditions for differential Faraday rotation in magnetized lenses. Finally, we show that clean lensing by a volume-filling turbulent screen requires fine tuning between rare strong fluctuations and caustic confusion, suggesting that sparse sheets, filaments, interfaces, or shocked clumps are more promising lensing structures.
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High Energy Astrophysical Phenomena

Discovery of minute-scale polarization angle rotation in a supermassive black hole jet

Aristeidis Polychronakis, Ioannis Liodakis, Dmitry Blinov et al. (+7 more)

Relativistic jets from supermassive black holes oriented towards our line of sight called blazars have puzzled the astronomical community for decades. Their fast jets and preferential alignment creates a fog of relativistic effects that obscures their true properties. Optical polarimetry - tracing the uniformity, direction, and evolution of the magnetic field in the jets, has revealed a unique phenomenon to blazars; coherent changes of the magnetic field manifesting as rotations of the polarization angle ($\Psi$). The origin of those rotations has been debated for decades with several proposed models to explain them. We detected an extremely fast - minute time-scale - rotation in the polarization angle of $\sim136^\circ$ over the span of about 80 minutes, making it the fastest known rotation by almost an order of magnitude. Our optical polarization observations combined with radio very long baseline interferometry allow us to reject different models of particle acceleration and strongly point to relativistic effects and the interaction of standing and moving shock waves as the origin of the extreme polarization variations in jets.
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All authors Aristeidis Polychronakis, Ioannis Liodakis, Dmitry Blinov, Anastasia Glykopoulou, Sebastian Kiehlmann, Karan Pal, Georgios F. Paraschos, Lena Debbrecht, Swati Ravi, Herman L. Marshall

High Energy Astrophysical Phenomena

A Multidimensional General-Relativistic Boltzmann Solver for Neutrino Transport: Implementation, Discretization and Optimization

Arthur Offermans, Harry Ho-Yin Ng, Patrick Chi-Kit Cheong et al. (+2 more)

We present the implementation of a multidimensional general-relativistic Boltzmann solver for neutrino transport using the finite volume method. We extend the general-relativistic magnetohydrodynamics solver Gmunu to discretize the full $6$D phase space. We discretize the momentum space in spherical coordinates in the comoving frame, and the position space in the lab frame in Cartesian, cylindrical or spherical coordinates. As in the M1 scheme, we expand the interaction kernels up to first order in a Legendre series. We present a discretization scheme that ensures consistency between the number-conservative and non-conservative formulations in empty flat spacetime, multiple dimensions and coordinate systems, conserves energy to $\sim 1\%$ with $20$ energy bins in 1D tests. We discuss optimizations of the implicit solver for stiff source terms to minimize its computational cost. In particular, we introduce a method that leverages the Legendre expansion of the kernels to reduce the dimensionality of the problem. In a 1D core-collapse supernova snapshot, our method yields a speed-up factor of $300$ compared to a full-matrix LU method for $14$ angular bins when including energy- and species-coupling interactions. Finally, we validate our implementation on standard test cases and report quadratic convergence in space, energy and propagation angles. We compare our solver to the M1 scheme in simplified 1D configurations and find differences of about $10\%$ in the free-streaming luminosities in relaxation test cases, which we attribute primarily to the M1 closure relation. The average energies, on the other hand, agree between the two methods. We compare both methods on a core-collapse supernova test case and find an overall good agreement in the fluid variables' profiles at the time of core bounce for $20$ energy bins.
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All authors Arthur Offermans, Harry Ho-Yin Ng, Patrick Chi-Kit Cheong, Anthony Mezzacappa, Tjonnie Guang Feng Li

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

The 91T/99aa-like Type Ia Supernova 2019vrq, Part I: Photometry, Spectroscopy, and Spectropolarimetry of a Nearly Standard Candle

Yi Yang, Peter Hoeflich, J. Craig Wheeler et al. (+16 more)

Among the various major subtypes of Type Ia supernovae (SNe), the luminous 1991T-like (Filippenko et al. 1992; Phillips et al. 1992) and 1999aa-like (Krisciunas et al. 2000; Garavini et al. 2004) events stand out as particularly important for constraining the explosion mechanism. Their white dwarf (WD) progenitors are thought to have masses close to or exceeding the Chandrasekhar limit. We present spectrophotometric and polarimetric time-series observations of the overluminous Type Ia SN2019vrq. Between -9 and +12 days relative to the light-curve peak, its continuum polarization is consistent with zero, as seen in most normal-bright SNeIa and despite a possible rise up to 0.2% toward longer wavelengths as seen in subluminous SNeIa. Polarization across major spectral features is marginal ($\lesssim$0.3%) with the exception of the high-velocity CaII near-infrared triplet, which reaches $\sim$0.5%. This low polarization indicates a substantially spherically symmetric electron-scattering photosphere and minimal large-scale ejecta asymmetry. Early-time spectra reveal hot, doubly ionized ejecta dominated by SiIII and blended iron-group features. Intermediate-mass-element lines, such as CaII and SiII, are more clearly seen than in 91T-like events, though these lines remain weaker than in normal SNeIa. The intrinsically higher peak luminosity of overluminous 91T/99aa-like SNeIa (Yang et al. 2022; Phillips et al. 2024), their comparatively short formation timescale for a massive WD progenitor - a channel that becomes more common toward high redshift, and the moderate viewing-angle dependence of their spectrophotometric properties as inferred from their persistently low polarization, together suggest that this subclass may serve as a promising standard candle at high redshift.
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All authors Yi Yang, Peter Hoeflich, J. Craig Wheeler, Dietrich Baade, Lifan Wang, Aleksandar Cikota, D. Andrew Howell, Curtis McCully, Alexei V. Filippenko, Ferdinando Patat, Chris Ashall, Mattia Bulla, Avishay Gal-Yam, Melissa. L. Graham, Daichi Hiramatsu, Divya Mishra, Estefania Padilla Gonzalez, Craig M. Pellegrino, Steve Schulze

High Energy Astrophysical Phenomena

Neutron Economy and Freeze-out Dynamics in the Late-Time Cold r-Process

Mengke Li and Bradley Meyer

Standard r-process models often treat the freeze-out phase as a passive, uniform smoothing of the global abundance pattern. We demonstrate, however, that the final pattern is instead actively sculpted by a dynamic “neutron economy” that operates well after the free neutron source is exhausted. By analyzing cold neutron star merger outflows, we identify a distinct neutron exporter-importer relationship: the second-peak region ($A \sim 130$) transitions early into a $\beta$-decay dominated regime, acting as a net exporter, while the third-peak ($A \sim 195$) and Rare Earth ($A \sim 164$) regions remain capture-dominated neutron importers. Targeted simulations reveal that the final peak structures emerge from a competition between two opposing forces: a global “push” driven by neutron capture shifting mass toward heavier nuclei, and a local “pull” from beta delayed neutron emission returning material back toward lighter masses. We identify $^{136}\text{Pd}$, $^{135}\text{Ag}$, and $^{130}\text{Ru}$ as the primary exporters driving this economy, while third-peak fine structure is governed by $N=126$ isotones (Sm to Tb). Physically, this transport reflects a fundamental drive toward shell closure, where nuclei that overshoot the $N=82$ closed shell shed mass to migrate back toward this stable configuration, while nuclei in the third peak capture these redistributed neutrons to anchor themselves at the $N=126$ shell. Crucially, our models overproduce the odd-even staggering in the third peak compared to solar data. This discrepancy suggests current models likely overestimate the $\beta$-delayed neutron emission probabilities ($P_n$) for these $N=126$ nuclei, highlighting them as high-priority targets for theoretical refinement and future FRIB experiments.
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High Energy Astrophysical Phenomena

4FHL: The Fourth Catalog of Hard Fermi-LAT Sources

The Fermi-LAT Collaboration

We present a catalog of sources detected above 50 GeV by the Fermi Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope using 16 years of observations. This is the Fourth Catalog of Hard Fermi-LAT Sources (4FHL), containing 673 objects detected in the 50 GeV-2 TeV energy range. The sensitivity and localization are improved by factors of $\sim$2 relative to the previous LAT catalog at the same energies (i.e. the 2FHL catalog). The increased photon statistics, nearly doubling the number of detections relative to 2FHL, enables more detailed population studies, including the identification of fainter sources detected above 50 GeV. The majority of the sources (84.7%) are associated with extragalactic counterparts, primarily BL Lacertae objects, including 36 objects at redshift $z>1$. Galactic sources account for 10% of the sample, mainly pulsar wind nebulae and supernova remnants, while $\sim$5% remain unassociated or of uncertain classification. Twenty sources are reported with no counterpart in previous LAT catalogs. The 4FHL catalog provides the deepest all-sky survey above 50 GeV to date. By focusing on this high-energy regime, it provides a fundamental link between space-based gamma-ray observations and the TeV range probed by current and future ground-based observatories. It serves as a useful reference for guiding future observations, including new potential targets for follow-up with Cherenkov telescopes.
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High Energy Astrophysical Phenomena

A statistical hardness classification of ultraluminous X-ray sources using long-term Swift/XRT monitoring

Sinan Allak, Wei Yu, Menglei Zhou et al. (+2 more)

We characterize long-term hardness behavior in 29 ultraluminous X-ray sources (ULXs) using over 3500 archival Swift/XRT observations. We classify observations as soft-band dominated, hard-band dominated, or uncertain from the hardness ratio (HR; 1.5–10/0.3–1.5 keV count rates) and its confidence bounds. We derive $f_{\rm soft}$ and $f_{\rm hard}$ and define the Crossing Index (CI) as the fraction of consecutive confidently classified observations on opposite sides of $HR=1$. We analyze these descriptors with rule-based classification and unsupervised hierarchical clustering. The rule-based analysis identifies eight soft-band-dominated, five hard-band-dominated, and 16 mixed-band systems; clustering yields a similar three-group structure. Known or candidate accretor types tend to occupy band-dominated groups, whereas unknown accretors are more common in the mixed-band group. All four sources with reported long-term X-ray modulation discussed in an orbital or super-orbital context are classified as mixed-band. The three-cluster solution is preferred in 25 of 29 leave-one-out trials, while bootstrap tests find the band-dominated groups more stable than the mixed-band group. As the classification uses observed count rates, the groups represent Swift/XRT signatures rather than intrinsic spectral states. Response-folding tests show that modeled HR depends on absorption column density and photon index. Bayesian reanalysis of a representative low-count source reduces CI from 56.3% to 38.5% while preserving its mixed-band classification. Random downsampling produces no systematic shift in CI; halving the observations changes median CI by at most $\sim10\%$ across four sources. Larger samples with independent absorption constraints can test links among these observational classes, accretor type, long-term periodic modulation, and other X-ray properties.
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All authors Sinan Allak, Wei Yu, Menglei Zhou, Honghui Liu, Andrea Santangelo

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

Probing the cosmic web with fast radio bursts: II. Scintillation

Sharon Lapiner, Nir Mandelker, Paz Beniamini et al. (+1 more)

We consider the formation of multiphase gas in cosmic web objects (CWOs) through cooling and fragmentation within the shock-heated medium of cosmic sheets and filaments, and the circumgalactic medium (CGM) of haloes. This leads to dense, cool ($\sim10^{4}\,\mathrm{K}$), small-scale cloudlets in pressure equilibrium with their surroundings. Such ionized overdensities can affect the propagation of radio waves emitted by fast radio bursts (FRBs), allowing us to probe the small-scale structure in CWOs by examining their impact on the scintillation pattern of the observed FRB flux. While we find that the probability of encountering a high-temperature filament or sheet with cold cloudlets is rather small and usually confined to high redshifts, the expected high rates of FRBs may still permit detection through scintillation along rare sightlines. We suggest scintillation from the Milky Way (MW) interstellar medium as a promising probe: CWO plasma screens can suppress it, placing constraints on their cold-gas properties. Filaments and sheets can suppress MW scintillation from sources at $z_\mathrm{S}\gtrsim2$ and $3$, respectively, while cold and turbulent cloudlets in the CGM can be probed by sources at redshifts as low as $z_\mathrm{S}\sim0.5$, constraining remarkably low volume-filling fractions, $f_\mathrm{v}$ (e.g. $f_\mathrm{v}\gtrsim10^{-5}$ for $z_\mathrm{S}\gtrsim2$). To provide testable predictions, we estimate the fraction of FRBs expected to encounter CWOs that may suppress MW scintillation below a given frequency, and express this as a function of the average extragalactic dispersion measure. Future FRB samples could map these constraints onto bounds on the existence and properties of cloudlets in CWOs, such as their density, size, $f_\mathrm{v}$, and turbulence.
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All authors Sharon Lapiner, Nir Mandelker, Paz Beniamini, S. Peng Oh

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

Set Transformer inference of the neutron star equation of state from stellar observations

Márcio Ferreira, Valéria Carvalho, Michał Bejger et al. (+1 more)

We develop a permutation-invariant Set Transformer to reconstruct the equation of state (EoS) of dense matter from variable-size, unordered sets of neutron star (NS) observations. The model takes stellar masses together with radii, tidal deformabilities, or both, and predicts either the pressure $P(n)$ or the sound speed $c_s^2(n)$ on a fixed density grid, along with density-dependent uncertainties. Nothing in the architecture prescribes which star informs which density: self-attention couples all observations nonlinearly, and each density point reads the full set through its own learnable query, so the star-to-density mapping is learned from the data. Trained on independent piecewise-polytropic and Gaussian-process EoS ensembles, the model provides well-calibrated predictions whose uncertainty increases in density regions that stable stars cannot probe. Reconstruction errors decrease with the number of observations, while tidal deformability generally improves accuracy at a fixed observation count, even when it carries its own measurement noise. Sensitivity analysis reveals a density-local mapping: in the pressure models, predictions at density $n$ depend most strongly on stars whose central densities are near $n$. We also show that the sensitivity of the model to the inferred stellar compactness provides information on the minimum central density. These results demonstrate that set-based neural inference, in which the star-to-density mapping is learned rather than assumed, can extract physically interpretable EoS information with calibrated uncertainties.
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All authors Márcio Ferreira, Valéria Carvalho, Michał Bejger, Constança Providência

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

Bayesian inference of non-Maxwellian distribution functions from collective Thomson scattering spectra

Kentaro Sakai, ByungJun Kang, Akito Nakano et al. (+1 more)

We investigate Bayesian inference of non-Maxwellian electron distribution functions from collective Thomson scattering (CTS) spectra. We represent distribution functions using multiple candidate models of different complexity and perform inference on synthetic spectra generated from known non-Maxwellian distribution functions. Our analysis demonstrates that a sufficiently flexible model can approximate the overall shape of the ground-truth distribution function. The most plausible model is identified based on model evidence, a statistical measure representing the probability to obtain the observed data given the model. When the candidates include the ground-truth model, the model evidence favors the ground-truth model. Without the ground-truth model as a candidate, the model evidence favors the candidate model with the fewest parameters that adequately approximates the data. The posterior probability density function of the most plausible model reveals the characteristic features and associated uncertainties of the underlying distribution function. The origin of the additional spectral peaks is attributed to a combination of modifications to the dispersion relation and reduced Landau damping. This enables the objective and data-driven identification of distribution functions directly from observed CTS spectra.
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All authors Kentaro Sakai, ByungJun Kang, Akito Nakano, Takeo Hoshi

High Energy Astrophysical Phenomena

Convolutional non-parametric Gamma-Ray Signal and Background Separation

Scarlet Betterman, Emmanuel Moulin and Martin White

In very-high-energy gamma-ray astronomy, signals must be separated from the residual background which arises from misidentified cosmic-ray protons. Building on previous work, we introduce an ensemble of variational autoencoders that aims to perform automatic signal-background separation with minimal assumptions that include separability of the spatial and energy distributions in both the signal and background, with no prior specification of the number of components or the point-like/diffuse nature of the signal itself. In addition, we do not assume knowledge of which region of the coordinate space is background-dominated or where the signal is supposed to be located in the field of view. We test the model on an analytic point-source mixture scenario, a realistic simulation of dark matter annihilation in the Galactic centre, and real observations of the Crab nebula and MSH 15-52 from the public H.E.S.S data release. The model proves capable of completely reconstructing the signal and background at a pixel by pixel level in all scenarios, whilst also denoising the inputs. Stable performance and reasonable error estimates are obtained even for low signal-to-background ratios.
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High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR

Near-infrared spectra of chemically peculiar stars and applications to kilonova spectra

Nanae Domoto, Masaomi Tanaka, Wako Aoki et al. (+11 more)

Kilonova spectra provide information on rapid neutron-capture nucleosynthesis in neutron star mergers. However, it remains challenging to identify individual elements in kilonova spectra, primarily due to incomplete atomic data for heavy elements at near-infrared wavelengths. Recent work demonstrated that spectra of chemically peculiar stars offer excellent laboratory for studying kilonova spectra. In this paper, to expand our understanding of strong transitions of heavy elements at near-infrared wavelengths, we present high-resolution near-infrared spectra of 14 chemically peculiar stars covering a range of effective temperatures, surface gravities, and abundance patterns, obtained with Subaru/IRD. The Sr II, Ce III, and Gd III lines, previously suggested to be among the strongest transitions, are detected in the spectra of most of our targets, confirming their intrinsic importance. In addition, Eu II and Nd III lines are detected in several stars. By comparing observed equivalent widths and abundance ratios, we show that under a highly heavy-element-enriched abundance pattern, Eu II and Nd III lines may contribute to kilonova spectral features comparably to Sr II. Importantly, we find no unidentified lines systematically stronger than these transitions despite the larger stellar sample. These results further support the primary importance of the Sr II and Ce III transitions and provide new insights into the potential contribution of Eu II and Nd III.
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All authors Nanae Domoto, Masaomi Tanaka, Wako Aoki, Salma Rahmouni, Tomoyuki Kudo, Hiroki Harakawa, Teruyuki Hirano, Takayuki Kotani, Takashi Kurokawa, Masayuki Kuzuhara, Jun Nishikawa, Takuma Serizawa, Motohide Tamura, Akitoshi Ueda

High Energy Astrophysical Phenomena

Long-term X-ray variability of the NLS1 Ark 564

Neha P R, Nikita S Arun, Savithri H. Ezhikode et al. (+4 more)

We present a systematic spectral study of the X-ray emission from the Narrow- Line Seyfert 1 (NLS1) galaxy Ark 564 using Swift/XRT observations spanning sixteen years. The source exhibits significant variability in both the soft and hard X-ray bands, along with a strong positive correlation between them, indicating a closely coupled emission mechanism within the accretion flow-corona system. A simple absorbed power-law model is statistically insufficient to describe the spectra, confirming the presence of an additional soft component. To investigate the origin of the soft excess, we performed multi-model spectral fitting. Incor- porating a warm Comptonization component yields a statistically improved fit, supporting the scenario in which the soft excess arises from Comptonization in a warm, optically thick corona. We also applied a relativistic reflection model to representative observations; however, given the moderate spectral resolution and limited high-energy coverage of Swift/XRT, the current data do not allow us to robustly determine whether reflection provides a superior or physically preferred description. Our results indicate that warm Comptonization offers a consistent explanation for the observed soft excess in Ark 564 within the limitations of the available data, while deeper broadband observations are required to defini- tively distinguish between competing models and comprehensively characterise the long-term X-ray spectral variability of the source
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All authors Neha P R, Nikita S Arun, Savithri H. Ezhikode, Ranjeev Misra, Bari Maqbool, Jeena K, Joe Jacob

High Energy Astrophysical Phenomena

Deciphering the VHE gamma-rays spectra of Extreme High-Frequency peaked BL Lac Objects through a Photohadronic Model

Sarira Sahu, D. I. Páez-Sánchez, A. U. Puga Oliveros et al. (+7 more)

Extreme high-frequency peaked BL Lac objects (EHBLs) have their synchrotron peak above $\sim 1$ keV and their inverse Compton peak extends up to several TeVs. Due to low luminosity of EHBLs, these objects are often undetected, and only a small number of them are thus known. As a result, they are poorly understood. Using a photohadronic model and including extragalactic background light correction we analyzed 22 very high-energy (VHE) gamma-ray spectra of 15 EHBL sources observed in different periods. We find that the photohadronic model used can adequately describe the observed VHE spectral energy distribution and furthermore, we classify them according to the value of the photon spectral index, $\delta$. The redshift of the EHBL HESS J1943+213 is also tightly constrained using the same photohadronic model. Within the sample, a plurality of flaring epochs fall in the high-emission state which is followed by the low-emission state and the very-high-emission state respectively. These fractions of flaring epochs characterize only the present sample, not the EHBL population at large. Due to the anomalous behavior of the VHE gamma-ray spectra of the EHBL H 2356-309, we suggest it to be a new subclass of EHBL having hard and very soft intrinsic spectra during different flaring epochs.
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All authors Sarira Sahu, D. I. Páez-Sánchez, A. U. Puga Oliveros, M. E. Iglesias Martínez, Jose Guerra Carmenate, R.de J. Pacheco-Aké, G. Sánchez-Colón, Subhash Rajpoot, P. Fernández de Córdoba, Gaetano Lambiase

Thursday 24 September

High Energy Astrophysical Phenomena

Radiative properties and optical appearance of a thin accretion disk around a charged-PFDM black hole

Taiyang Zhang, Zongyuan Qin, Qian Feng et al. (+1 more)

Accretion onto magnetically charged black holes in a perfect fluid dark matter (PFDM) background opens a new window for testing strong-field gravity. This paper investigates the radiative properties and optical appearance of a thin accretion disk surrounding a charged-PFDM black hole. We numerically compute the radiative energy flux, temperature distribution, and radiative efficiency, and employ a ray-tracing method to construct the direct images, secondary images, redshift distribution, and observed flux. By comparing with the Schwarzschild and pure PFDM cases, we find that the thin-disk efficiency of the charged-PFDM black hole lies between the two, with the magnetic charge partially counteracting the dark-matter-induced efficiency enhancement; for M87*, the efficiency is estimated to be 7%–8%. The optical appearance is predominantly governed by the PFDM parameter, while the magnetic charge plays only a marginal role. Larger inclination angles give rise to stronger Doppler asymmetry, producing the characteristic “hat-like” shape. These results provide falsifiable predictions for future high-resolution observations, such as those by ngEHT, to distinguish dark-matter environments from magnetic-charge effects.
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All authors Taiyang Zhang, Zongyuan Qin, Qian Feng, Zheng-Wen Long

High Energy Astrophysical Phenomena

Tracing the Host-Galaxy Environments of FRBs with a New Sample of Localized Sources Detected with the DSA-110

Frank M. Verdi, Vikram Ravi, Myles B. Sherman et al. (+7 more)

We characterize the source environments of a sample of fast radio bursts (FRBs) detected with the Deep Synoptic Array (DSA-110). We present new analyses of a sample of 24 FRBs which have been localized to host galaxies with spectroscopic redshifts. Four of these bursts have been confidently associated with galaxies near or beyond redshift 1, and several are published here for the first time. Combining these new bursts with previously analyzed DSA-110 sources, we use a sample of 43 DSA-110 FRBs with known redshifts to measure host-galaxy contributions to dispersion measure (DM) and Faraday rotation measure (RM). We find characteristically large and significantly correlated host-galaxy contributions to DM and RM, suggesting that most FRBs reside in overdense, magnetized regions within their host ISM. Additionally, we identify a subset of FRBs with detectable scattering, and show that their scattering timescales and inferred host DM values are broadly consistent with an empirical relation established for Galactic pulsars. Combined with two-screen modeling of some bursts, our results indicate that density fluctuations within the host-galaxy ISM are the dominant source of extragalactic scattering. Comparisons of FRB host-galaxy properties with pulsars and simulations of young and old progenitor populations support a scenario in which FRBs preferentially form and remain associated with HII regions throughout their emission lifetime, favoring magnetars formed in core-collapse supernovae as the sources of FRBs.
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All authors Frank M. Verdi, Vikram Ravi, Myles B. Sherman, Jakob T. Faber, Stella Koch Ocker, Kritti Sharma, Casey J. Law, Gregg Hallinan, Kaitlyn Shin, Liam Connor