High Energy Astrophysical Phenomena
Flux Dependence of the 1\,keV Soft X-ray Feature in SAX J1808.4-3658 with NICER
Katherine Bruce, Anne M. Lohfink, Jaiverdhan Chauhan et al. (+3 more)
We present a systematic X-ray spectral analysis of the 2019 and 2022 outbursts of the accreting millisecond
pulsar SAX J1808.4-3658 with extit{NICER} with a particular emphasis on the nature of the longstanding $\sim$1\,keV spectral feature. We report here the first clear evidence that the 0.9-1.0 keV feature varies systematically with source brightness, exhibiting strong correlations with both continuum and blackbody flux. This establishes that the feature is directly coupled to the
accretion process. The scaling and energy distribution of the feature differ between outbursts, corresponding to a $\sim$2\,dex change in reprocessor density (log\,$n_e \approx 19.1$ in 2019 vs. $\sim$17.3 in 2022), which naturally explains the stronger, broader feature in 2019 and the weaker, more localized $\sim$1\,keV feature in 2022, in agreement with high-density reflection models. These results provide the strongest constraints to date on the origin of the $\sim$1\,keV feature in SAX J1808.4-3658.
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All authors Katherine Bruce, Anne M. Lohfink, Jaiverdhan Chauhan, Suchismito Chattopadhyay, Sachiko Tsuruta, Andrew C. Liebmann
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Observational Signatures of Dyonic Black Holes in Non-Linear Electrodynamics
Sobhan Kazempour, Sichun Sun, Zhiqing He et al. (+1 more)
We study the observational features of thin-disk
accretion and emission spectra for static, spherically symmetric dyonic black holes in non-linear electrodynamics (NLED). In contrast to classical Maxwell theory, where electromagnetic duality assumes equal charges, the coupling parameters in NLED introduce a new gravitational potential and modify the geometry in the strong-field regime. We also study the kinematic characteristics of particles on circular orbits, such as their angular velocity, specific energy, and specific angular momentum. It is found that modifications of NLED lead to a shrinkage of the ISCO radius, resulting in a shift of the radiation spectrum thermal maximum towards smaller radii and, consequently, an increase in the local effective temperature in comparison with the standard Schwarzschild and Reissner-Nordström solutions. Additionally, we compute null-geodesic orbits to determine the optical edge of both the
photon sphere and the black hole shadow. In order to assess the observational validity of the aforementioned alternatives, we carry out a $\chi^2$ goodness-of-fit test through the angular diameter constraints at the horizon scale collected by the Event Horizon
Telescope (EHT) concerning Sagittarius A* ($\text{Sgr A}^*$). We prove that both these dyonic NLED and purely
magnetic models have good statistical agreement with experimental data.
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All authors Sobhan Kazempour, Sichun Sun, Zhiqing He, Chengye Yu
High Energy Astrophysical Phenomena
Shortening the Baseline: Characterising Type Ia Supernovae Using $\Delta m_{10}(X)$
Joel M. Velasco, Paolo A. Mazzali, Christopher Ashall et al. (+8 more)
An explorative study of Type Ia
Supernovae (SNe Ia) light curves is presented. $\Delta m_{t}(X)$ decline-rate metrics are used to evaluate alternative characterisations of light curve diversity and the Luminosity-Width Relation (LWR). Using a sample of 362 SNe Ia from the Carnegie
Supernova Project (CSP-I and CSP-II), Gaussian Process regression is used to fit $B$- and $g$-band light curves and measure decline-rates across multiple post-maximum timescales. The LWR is calibrated using a clean subsample of SNe Ia with host-independent distance moduli and Fitzpatrick (1999) reddening corrections. Decline-rates $\Delta m_{10}(B)$ and $\Delta m_{10}(g)$ are found to capture the LWR effectively, while mitigating the severe degeneracy that affects longer timescales for fast-declining ($\Delta m_{15}(B) \geq 1.6$ mag) events. Continuous, piecewise linear models are presented that seamlessly anchor the transition between normal and fast-declining regimes. The decline-rate $\Delta m_{10}(X)$ closely approximates the results of the more complex colour-stretch parameters, requiring less post-maximum temporal coverage than $\Delta m_{15}(B)$ or $s_{BV}$, and offers an even greater sensitivity for slow decliners. $\Delta m_{10}(B)$ provides a precise and practical tool for large-scale photometric surveys and real-time observer classification.photometric surveys and real-time observer classification.
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All authors Joel M. Velasco, Paolo A. Mazzali, Christopher Ashall, Christopher R. Burns, Mark M. Phillips, Lluis Galbany, Nidia Morrell, Maximilian D. Stritzinger, Peter Hoeflich, Eric Y. Hsiao, Syed A. Uddin
High Energy Astrophysical Phenomena
Descalarization bursts from Galactic supramassive neutron stars: a kHz continuous-wave target
José Carlos Olvera M., Daniela D. Doneva, Pablo Cerdá-Durán et al. (+2 more)
In massive scalar-tensor
gravity, a scalarized
neutron star must shed its scalar hair as it collapses into a black hole. Using 3D simulations of a realistic rotating progenitor we show that dispersion stretches the emitted burst into a quasi-monochromatic signal in the kHz frequency range lasting $>10^4$ yr and amenable to continuous-wave methods. We find that detecting these signals in directed searches of
supernova remnants without a central
pulsar requires sensitivity depths of 233–762 $\rm{Hz^{-1/2}}$ with the A+-ET-CE detector network. We estimate up to $\sim3$ of these signals in transit at any observational epoch.
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All authors José Carlos Olvera M., Daniela D. Doneva, Pablo Cerdá-Durán, José A. Font, Stoytcho S. Yazadjiev
High Energy Astrophysical Phenomena
The Origin of the Fermi Halo-like Emission: A New Model of the Fermi Bubbles
Ilias Cholis, Leo Qiyuan Hu and Yi-Ming Zhong
We study the gamma-ray sky at high latitudes for energies between 0.3 and 900 GeV. We search for a contribution in the high-latitude sky and in the region of the Galactic Stellar
Halo that would be beyond known astrophysical emissions, i.e., the galactic diffuse emission, the isotropic mostly extragalactic gamma-ray background, the Fermi Bubbles, Loop I, the contribution from the Galactic Center Excess, and the known point and extended gamma-ray sources detected by the Fermi-LAT Collaboration. We perform a sequence of template fits to the Fermi gamma-ray data, testing different regions of interest of the galactic sky, utilizing a large set of alternative galactic diffuse emission models, and implementing alternative fitting schemes to assess the impact of increased modeling complexity of the gamma-ray sky. We find that the
halo-like emission recently reported by Totani (2025) is real, but it is likely due to emission from the Fermi Bubbles rather than dark matter annihilation in the Galactic
halo. We provide new templates for the Fermi Bubbles, including an updated model for the Flat Bubbles and a detailed model of their substructures, the Cocoons and the Inner Bubbles. In combination, these components account for the previously reported
halo-like emission. We also estimate the associated fluxes and spectral properties of the Flat Bubbles, the Inner Bubbles, and the Cocoons. This updated model for the Fermi Bubbles provides new input toward understanding their origin.
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High Energy Astrophysical Phenomena
Accreting Neutron Stars in Low-Mass X-ray Binaries
M. C. Baglio, S. E. Motta, A. P. Nitindala et al. (+1 more)
Neutron star low-mass X-ray binaries (NS LMXBs) are systems in which a
neutron star accretes matter from a Roche-lobe-filling companion via an
accretion disc, releasing gravitational energy with an efficiency of order 10-20% of the rest-mass energy. Unlike black holes,
neutron stars possess a solid surface and a
magnetic field, making NS LMXBs uniquely rich laboratories for high-energy astrophysics and dense-matter physics. In this chapter we review the current understanding of
accretion onto
neutron stars in LMXBs, tracing matter from the outer disc radii to the stellar surface across five decades of observational discovery. We survey the principal classes of accreting
neutron star systems - canonical atoll and Z-sources, ultracompact X-ray binaries, accreting millisecond
pulsars, and transitional millisecond
pulsars - and discuss how their spectral states, timing properties, and multiwavelength behaviour encode the physics of the inner
accretion flow. We examine thermonuclear Type-I X-ray bursts as probes of nuclear burning and
neutron star structure, and review the boundary and spreading layers where accreted matter decelerates onto the stellar surface. We discuss relativistic jets and ionised disc winds as regulators of the
accretion-ejection cycle. Finally, we present X-ray polarimetric results from the Imaging X-ray Polarimetry Explorer (IXPE), which for the first time provide new, though model-dependent, constraints on the geometry of the emission regions - disc, corona, spreading layer, and jets - that spectroscopy alone cannot resolve, alongside complementary optical and infrared polarimetric observations.
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All authors M. C. Baglio, S. E. Motta, A. P. Nitindala, J. Poutanen
High Energy Astrophysical Phenomena
Discovery of a Nearby Ultra-fast Magnetic White Dwarf Propeller: Common Pathways to Magnetic Cataclysmic Variables
Antonio C. Rodriguez, Kareem El-Badry, R. Michael Rich et al. (+11 more)
The fastest spinning white dwarfs (WDs), which approach break-up speeds, are found in close binary systems. In the two known
magnetic WD “propellers”, AE~Aqr and LAMOST J0240+1952, the
magnetic WD
spins so rapidly that it ejects the material transferred by its Roche-lobe-filling companion rather than accreting it. We report the discovery of the third such system, 1eRASS J042633.5$-$250226 (1eRASS J0426/Valeria), at only 265 pc, identified by combining SRG/eROSITA and Gaia data via the “X-ray Main Sequence”. TESS photometry shows AE~Aqr-like flaring and orbital modulation with $P_{\mathrm{orb}} \approx 6.62$ hr, and ZTF shows month-long high/low state changes. Phase-resolved Palomar/DBSP spectroscopy confirms the orbital period and yields a donor radial velocity of $K_2 = 202\pm7$ km s$^{-1}$. High-speed Palomar/CHIMERA photometry detects optical pulsations on three nights with the strongest detection at $P_\textrm{spin} = 26.8$ sec, though further observations are needed to confirm this as the WD
spin. Spectral fitting of the donor in the low state reveals a late K-type
star with $T_{\mathrm{donor}} = 4100\pm150$ K. Zeeman splitting in the optical spectrum yields a candidate field strength of $B_{\mathrm{WD}} = 26 \pm 3$ MG, meaning the system will evolve into a polar. Like other WD propellers, the system is X-ray faint ($L_X \approx 7\times 10^{30}$ erg s$^{-1}$), but is not detected in the
radio, and a
spin period is not detected in X-rays by SRG/eROSITA. The proximity of this system and evolutionary model constraints on the timeframe of the propeller phase yield a birth rate density of $\sim8\times10^{-16} \textrm{pc}^{-3} \textrm{yr}^{-1}$, which agrees with that of
magnetic CVs, suggesting that a substantial fraction of
magnetic CVs pass through a propeller phase.
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All authors Antonio C. Rodriguez, Kareem El-Badry, R. Michael Rich, Kevin N. Ortiz Ceballos, Ingrid Pelisoli, Casey J. Law, Kaya Mori, Boris Gaensicke, Edo Berger, Liam Connor, Ilkham Galiullin, Ilaria Caiazzo, Matthew Graham, Samuel Whitebook
High Energy Astrophysical Phenomena
Neutrino flavor conversion alters the lepton-emission self-sustained asymmetry in core-collapse supernovae
Noah Roux, Irene Tamborra and Maryna Mesiura
We solve the
neutrino equations of motion in a half annulus, accounting for two spatial dimensions. At any location in the annulus, the
neutrino momentum is characterized by a polar-angle distribution, two effective azimuthal angles, and an energy distribution. We explore how
neutrinos change their flavor while decoupling from matter in the presence of the
neutrino-driven hydrodynamical instability LESA (i.e., the Lepton-number Emission Self-sustained Asymmetry) during the stalled-shock phase of a core-collapse
supernova. In our setup, LESA is responsible for an excess of $\nu_e$'s ($\bar\nu_e$'s) in the Northern (Southern) region of the annulus. Crossings in the $\nu_e$-$\bar\nu_e$ angular distributions appear in the ELN-depleted Southern region of the annulus, while crossings in the energy distributions of $\nu_e$-$\nu_x$ and $\bar\nu_e$-$\bar\nu_x$ are present throughout the annulus. We find that collisional instabilities drive flavor conversion just before
neutrino decoupling, fast instabilities are prominent in the Southern region of the annulus, and slow instabilities dominate otherwise. Flavor conversion is most efficient in the Southern region and is responsible for a $5$-$10\%$ enhancement of the LESA dipole, independent of the
neutrino-mass ordering. Our findings suggest that direction-dependent flavor conversion can have implications for the development of global
neutrino asymmetries and hydrodynamical instabilities in core-collapse
supernovae.
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High Energy Astrophysical Phenomena
Wind-Fed Magnetic Flux and the Emergence of X-ray Coronae in Active Galactic Nuclei
Rostom Mbarek
We propose a scenario for the onset of radiatively efficient Active Galactic
Nuclei (AGN) in which circumnuclear stellar and
supernova-driven winds supply mass and partially ordered
magnetic flux to a compact disk reservoir around a supermassive black hole. A coherent poloidal component surviving advection to small radii strengthens inward, lowering the gas
plasma beta until mean Maxwell stresses overtake MRI
turbulence, compress the disk vertically, and open a magnetically dominated wedge above it. Within a quasi-force-free description of this wedge, the field self-organizes toward a critical twist at which the net radial transport changes sign, defining a
magnetic saturation radius $r_c\sim10\,r_g$. The
magnetic squeezing of the disk fixes the pressure balance at the interface between the disk and wedge, confining the accumulated flux at $r_c$: flux advected toward this boundary is preferentially shed, fixing $B_r\propto r^{-2}$ in the interior. From the field
topology alone we then derive a normalized saturation flux $\phi \sim 50$, consistent with the MAD plateau of GRMHD simulations. The resulting compact, time-dependent saturation region is a natural candidate for the AGN X-ray corona, being collisionless, magnetized, and, at high compactness, plausibly pair-rich, with hadronic pair injection providing a conditional additional channel. Its size, variability, and spectral hardness are controlled by $r_c$, the compactness, and the magnetization. We connect these scalings to observed coronal sizes, variability, and the power partition between the corona and jet. We further propose that Little Red and Blue Dots could represent a disk-reservoir phase preceding luminous AGN activity.
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High Energy Astrophysical Phenomena
Exemplary Merging Clusters II: XMM-Newton and Chandra X-ray Observations of MACS J1752.0+4440 and ZwCl 1856.8+6616
K. HyeongHan, K. Finner, W. Lee et al. (+10 more)
Using deep XMM-Newton and Chandra observations, we compare the double-
radio-relic
galaxy cluster mergers MACS J1752.0+4440 and ZwCl 1856.8+6616. Both are 1:1 mass-ratio mergers with similar relic separations but differing nearly an order of magnitude in total mass. In MACS J1752.0+4440, the surface-brightness discontinuity at the northeastern relic yields Mach $M_{\rho}=1.4\pm0.2$, while the temperature jump provides a weaker constraint, corresponding to $M_{kT}\lesssim2.3$ at the $2\sigma$ level, lower than the
radio-derived $M_{\rm radio}=3.1\pm0.1$. No discontinuity is detected at the southwestern relic. Thermodynamic maps and axial profiles reveal surviving relatively low-
entropy cores embedded in Mpc-scale stripped-gas structures. The nearly uniform X-ray tail brightness and a southwestern plateau coincident with a third
lensing mass peak favor a non-zero impact parameter and complex assembly history. In ZwCl 1856.8+6616, we do not detect a clear surface brightness discontinuity at either relic. The northern X-ray peak leads its brightest cluster
galaxy (BCG) and
lensing peak, while the hotter southern X-ray peak is laterally offset from its BCG and
lensing peak. The asymmetric intracluster emission favors a phase near or after first apocenter, when displaced gas can develop a slingshot-like morphology. We present two TNG-Cluster analogs in which a minor merger followed by an off-axis major collision produces extended X-ray tails resembling MACS J1752.0+4440 and later leads to gas structures resembling ZwCl 1856.8+6616. X-ray morphology independently confirms the merger phases inferred from relic-based chronometry, placing MACS J1752.0+4440 before and ZwCl 1856.8+6616 after first apocenter, and demonstrating that similar relic and
lensing configurations can correspond to distinct dynamical states.
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All authors K. HyeongHan, K. Finner, W. Lee, E. Silich, W. Forman, C. Jones, A. Tümer, S. Randall, J. ZuHone, M. James Jee, D. Wittman, R. J. van Weeren, M. Troxel
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Impact of site-dependent noise configurations on Einstein Telescope science at low frequency
Francesco Crescimbeni, Matteo Di Giovanni, Davide Rozza et al. (+9 more)
We investigate how site-dependent Newtonian noise configurations affect the scientific performance of the Einstein
Telescope (ET), a third-generation gravitational-wave observatory. We compare the baseline triangular and 2L detector geometries, with arm lengths of 10km and 15km, respectively, under three site-dependent noise scenarios constructed from measurements in the Euregio Meuse-Rhine region, Lausitz, and Sardinia. The differences among the adopted sensitivities are concentrated mainly below 10Hz and thus have the greatest impact on science cases that rely on the early inspiral of light objects, or on massive systems whose characteristic frequencies lie in the low-frequency band. We quantify these effects for four ET science cases: (1) detection and parameter estimation of intermediate-mass black-hole binaries; (2) post-Newtonian tests of General Relativity based on the early inspiral; (3) black-hole ringdown spectroscopy with massive remnants; and (4) early-warning and sky-localization capabilities for binary
neutron-
star mergers. Using
Bayesian parameter estimation, we find that improved low-frequency sensitivity can significantly enhance ET's scientific performance across all four cases. Within the configurations considered here, a 2L network in which at least one of the two L-shaped interferometers attains a noise level comparable to that assumed for the Sardinian site generally provides better performance than the triangular configurations. Conversely, strong degradation of the sensitivity below 10Hz can substantially reduce, and in some regimes offset, the scientific gains expected from increasing the arm length from 10km to 15km. These results highlight the importance of site selection and low-frequency noise mitigation, alongside network geometry, for fully realizing ET's scientific potential in the largely unexplored frequency band below 10Hz.
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All authors Francesco Crescimbeni, Matteo Di Giovanni, Davide Rozza, Andrea Contu, Alessandro Cardini, Domenico D'Urso, Carlo Giunchi, Luca Naticchioni, Marco Olivieri, Mike Lindner, Andreas Rietbrock, Paolo Pani
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
A weakly modelled view of the joint compact-binary mass plane: population structure and spectral-siren cosmology
Viola De Renzis, Alessandro Agapito and Michele Mancarella
We introduce a flexible mixture model for the joint source-frame component-mass distribution of compact binaries, inferred simultaneously with cosmology, together with a framework that identifies structures in the reconstructed density, quantifying their posterior stability, local prominence, and cosmological relevance. Applied to GWTC-5.0, the workflow clarifies several familiar and emergent structures. The $\sim35~M_\odot$ feature is classified as a clear near-equal-mass peak despite an ambiguous interpretation in the primary-mass projection. A weaker unequal-mass concentration near $(21,12)~M_\odot$ is recovered frequently but with lower prominence. A tentative joint transition near $60~M_\odot$ appears at mildly unequal masses, with a more robust manifestation in the primary-mass projection. For cosmology, the two-dimensional mass model constraint on $H_0$ is $33\%$ narrower than a model that flexibly reconstructs only the primary mass, and only $\sim~10\%$ wider than analyses based on strongly parameterized one-dimensional spectral structure. At the feature level, we show that the $\sim10~M_\odot$ and $\sim35~M_\odot$ concentrations depend on $H_0$ mainly through a common scale which captures the entire $\sim~(1+z)$ cosmological rescaling of both masses; the intermediate unequal-mass feature instead retains residual dependence in the orthogonal mass-ratio direction. More generally, we argue that population prominence, formal inclusion in an astrophysical feature catalogue, and cosmological relevance need not coincide. These results suggest that a genuinely two-dimensional view may be required to fully exploit spectral-siren information. Our workflow provides a reproducible way to detect population structure and identify which aspects of its geometry are associated with spectral-siren constraints. The feature finder is directly applicable to other population reconstructions.
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High Energy Astrophysical Phenomena
Widespread steep X-ray spectra of luminous $z\simeq5.85-7.6$ quasars and their implications for future X-ray surveys
A. Tortosa, L. Zappacosta, E. Piconcelli et al. (+51 more)
Luminous
quasars (QSOs) at the Epoch of Reionization (EoR, $z\gtrsim6$) host billion-solar-mass black holes assembled within the first Gyr of
cosmic time. Their X-ray emission probes the innermost accreting regions and constrains the physical state of the corona during early supermassive black hole (SMBH) growth. Previous results from the HYPerluminous
quasars at the Epoch of ReionizatION (HYPERION) sample of $z \gtrsim 6$ QSOs, powered by the most rapidly accreted SMBHs, suggested a link between X-ray spectral slope and long-term growth efficiency. We test whether the steep X-ray spectra of luminous z>6 QSOs are a general property of early SMBH growth or a peculiarity of the most efficiently grown SMBHs, using a sample extended mainly to the lowest growth efficiencies. We analysed the largest and most X-ray sensitive sample of 28 luminous QSOs at $z \simeq 5.85-7.56$ (26 used in the correlation analysis), combining HYPERION targets, luminous non-HYPERION sources, and QSOs powered by moderately accreting SMBHs newly observed in our XMM-Newton Large Programme. We re-derived black hole masses and growth indicators for the entire sample homogeneously, incorporating the latest JWST-based spectroscopic measurements to ensure the most consistent and high-quality characterization across all targets. We found no statistically significant correlation between coronal spectral parameters and BH growth indicators: luminous $z \gtrsim 6$ QSOs systematically show steep X-ray continua, with high
photon indices and/or low high-energy cutoffs, consistent with cold coronae, possibly linked to rapid
accretion. Finally, if steep X-ray slopes are common among early AGN, they may significantly reduce count rates, at fixed intrinsic luminosity, in future surveys of the EoR, affecting the detectability of steep-spectrum X-ray sources and thus the census of the early AGN population.
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All authors A. Tortosa, L. Zappacosta, E. Piconcelli, I. Saccheo, G. Lanzuisi, E. Bertola, F. Vito, M. Bischetti, M. Brazzini, M. Castellano, S. Carniani, A. Comastri, S. Gallerani, R. Gilli, F. La Franca, P. Madau, R. Maiolino, G. Miniutti, B. Musiimenta, C. Piscitelli, F. Salvestrini, F. Tombesi, R. Tripodi, C. Vignali, M. Volonteri, A. Bongiorno, L. Borrelli, M. Brusa, I. V. Chilingaria, F. Civano, S. Cristiani, V. D'Odorico, D. De Cicco, C. Done, M. Elvis, X. Fan, C. Feruglio, F. Fiore, A. Grazian, M. Guainazzi, F. Haardt, A. Luminari, N. Menci, R. Middei, F. Nicastro, M. Paolillo, M. Perna, S. Puccetti, R. Schneider, V. Testa, R. Valiante, L. Vallini, E. Vanzella, G. Vietri
High Energy Astrophysical Phenomena · Cross-listed from astro-ph.IM
BOLT: Imaging Lightning and Terrestrial Gamma-ray Flashes at the Pierre Auger Observatory
Eric-Teunis de Boone for the Pierre Auger Collaboration
Lightning-related phenomena are known to interact with and influence all detector systems of the Pierre Auger Observatory. Notably, the Surface Detector has recorded unique signals associated with Terrestrial Gamma-ray Flashes (TGFs), which are rare phenomena linked to the initial processes of lightning. Studying TGFs in coincidence with their parent lightning discharges helps to constrain the mechanisms responsible for their production, providing key insights into how these high-energy emissions are generated and evolve within thunderstorms. To image the parent lightning discharges, we are developing the Broadband Observatory of Lightning and TGFs (BOLT), an interferometric lightning mapping array that enhances the unique capabilities of the Observatory by providing high-resolution 3D (spatial and temporal) imaging of lightning propagation. BOLT consists of VHF
radio detectors that have been previously developed for the Auger Engineering
Radio Array, sensitive in the 30 MHz to 80 MHz range. The array is co-located with the Surface Detector to enable direct correlation between lightning activity and candidate TGF signals at the Pierre Auger Observatory. The BOLT array is in commissioning, with successful station firmware modifications to acquire and stream multi-second waveforms.
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High Energy Astrophysical Phenomena
The NANOGrav 15 yr Dataset: Modified Functional Forms for Profile-Evolution Time Delays in Pulsar Timing Models
Sofia V. Sosa Fiscella, Michael T. Lam, Olivia West et al. (+49 more)
Radio pulsars exhibit frequency-dependent variations in the shape of their pulse profiles that introduce systematic delays in the observed pulse times of arrival (TOA) across an observing band. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) models these delays using a frequency-dependent (FD) log-
polynomial function whose coefficients are fit to TOA measurements at different
radio frequencies. However, FD parameters are prone to absorbing power from other chromatic effects, such as interstellar dispersion, leading to fitted FD values that overestimate profile
evolution and vary significantly across
datasets. To separate these effects, we introduce two new mathematical formulations of profile
evolution: a monomial expansion (IFD) and a Legendre
polynomial expansion (LIFD). We test these models on observations of 11
pulsars in the NANOGrav 15-year
dataset and on simulated data, and also compare their performance to that of frequency-resolved pulse templates. We find that Legendre-based models generally predict timing delays that more closely replicate the true profile
evolution, reducing the residual offset relative to FD by as much as 85% for the simulated data. Additionally, we find that the choice of profile
evolution model can bias the fitted dispersion model, with FD-based models overestimating the injected DM by 0.013 pc cm^-3 compared to 0.0002 pc cm^-3 for LIFD. These results support the use of Legendre bases to mitigate covariances between chromatic effects, directly benefiting targeted timing studies of these effects and the development of advanced noise-modeling techniques for nanohertz gravitational-wave detection.
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All authors Sofia V. Sosa Fiscella, Michael T. Lam, Olivia West, Ruggero Valdata, Gabriella Agazie, Akash Anumarlapudi, Anne M. Archibald, Zaven Arzoumanian, Paul T. Baker, Paul R. Brook, H. Thankful Cromartie, Kathryn Crowter, Megan E. DeCesar, Paul B. Demorest, Timothy Dolch, Elizabeth C. Ferrara, William Fiore, Emmanuel Fonseca, Gabriel E. Freedman, Nate Garver-Daniels, Peter A. Gentile, Joseph Glaser, Deborah C. Good, Jeffrey S. Hazboun, Ross J. Jennings, Megan L. Jones, David L. Kaplan, Matthew Kerr, Duncan R. Lorimer, Jing Luo, Ryan S. Lynch, Alexander McEwen, Maura A. McLaughlin, Natasha McMann, Bradley W. Meyers, Cherry Ng, David J. Nice, Timothy T. Pennucci, Benetge B. P. Perera, Nihan S. Pol, Henri A. Radovan, Scott M. Ransom, Paul S. Ray, Ann Schmiedekamp, Carl Schmiedekamp, Brent J. Shapiro-Albert, Taylor Starkman, Ingrid H. Stairs, Kevin Stovall, Abhimanyu Susobhanan, Joseph K. Swiggum, Haley M. Wahl
High Energy Astrophysical Phenomena
Deep Learning for Cherenkov Astronomy: Performance of GammaLearn on LST-1
G. Grolleron, C. Plard, M. Dell'aiera et al. (+5 more)
The Cherenkov
Telescope Array Observatory (CTAO) marks the next generation of Imaging Atmospheric Cherenkov
Telescopes (IACTs), offering a sensitivity improvement of up to a factor of 10 over current instruments. Its first prototype, the Large-Sized
Telescope (LST-1), is already in operation at the Roque de los Muchachos Observatory in La Palma, Spain. Deep learning methods have shown significant promise in reconstructing key properties of incident particles, such as energy, arrival direction, and type, using simulated data. Unlike traditional approaches that rely on simplified image shape parameters, deep learning can exploit the full temporal and charge information of the recorded events, providing enhanced performance, particularly at low energies (~20 GeV) accessible by LST-1. This capability is especially valuable for observing distant extragalactic sources like Active Galactic
Nuclei, which are key to probing fundamental physics and cosmology. In this work, by producing sensitivity curves, we evaluate the performance of GammaLearn, a deep learning framework tailored for IACT data analysis, by comparing it to the standard analysis used with LST-1 and applying it to real observational data from LST-1.
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All authors G. Grolleron, C. Plard, M. Dell'aiera, T. François, V. Pollet, J. Talpaert, S. Caroff, T. Vuillaume on behalf of the CTAO-LST Project
High Energy Astrophysical Phenomena · Cross-listed from astro-ph.GA
Changing-look Active Galactic Nuclei from the Dark Energy Spectroscopic Instrument. VII. Testing Disk-Corona Diagnostics with eROSITA
Wei-Jian Guo, Shuo Zhai, Zhu-Heng Yao et al. (+3 more)
Changing-look active galactic
nuclei (CL-AGN) have been proposed to follow an X-ray-binary-like, V-shaped relation between the optical/ultraviolet-to-X-ray spectral slope, $\alpha_{\rm OX}$, and the Eddington ratio, $\lambda_{\rm Edd}\equiv L_{\rm bol}/L_{\rm Edd}$. We test this interpretation using CL-AGN selected from the Dark Energy Spectroscopic Instrument (DESI) and control samples matched to the eROSITA all-sky survey (eRASS). We find that the apparent positive correlation between $\alpha_{\rm OX}$ and the optical-continuum-based Eddington ratio, $\lambda_{\rm Edd,opt}$, is not unique to CL-AGN, but also appears in matched non-changing-look samples. In broad-H$\alpha$ AGN, our main statistical sample, this structure is largely driven by the known dependence of $\alpha_{\rm OX}$ on ultraviolet luminosity and by the covariance between the rest-frame 2500 Angstrom luminosity, $L_{2500}$, the black-hole mass, $M_{\rm BH}$, and $\lambda_{\rm Edd,opt}$. When an X-ray-based Eddington ratio is used instead, the relation reverses sign because the X-ray luminosity, $L_X$, enters the two axes with opposite signs. An apparent $\alpha_{\rm OX}$-Eddington-ratio V-shape can therefore be induced by the luminosity
estimator and is not a standalone
accretion-state diagnostic. As a complementary test, we examine the eRASS effective
photon index, $\Gamma_{\rm eff}$, inferred from catalogue band fluxes. In broad-line AGN, it shows a weak positive association with the X-ray-independent optical Eddington ratio. However, the weaker hard-band trend depends on the null model and does not isolate coronal softening. These results motivate further X-ray spectroscopy, combined with independently estimated Eddington ratios, to test for intrinsic changes in the disk-corona system.
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All authors Wei-Jian Guo, Shuo Zhai, Zhu-Heng Yao, Huaqing Cheng, Jingwei Hu, Jun-Jie Jin
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Analytic parity-violating greybody factors of a dynamical Chern-Simons black hole
Abhishek Rout
We study the low-frequency scattering of gravitational and pseudoscalar waves off a non-rotating black hole in dynamical Chern-Simons (dCS)
gravity, where the axial (odd-parity) gravitational sector is irreducibly coupled to the dynamical pseudoscalar while the polar sector remains general relativistic. Working to first order in the dimensionless coupling zeta = (ell_CS/M)^4 and using matched asymptotic expansions, we obtain an exact closed form for the graviton-to-pseudoscalar conversion probability, d_2 = 3(pi^2-9)^2/8, and a high-precision reconstruction, in a proven transcendental basis, of the graviton greybody (absorption) asymmetry, kappa_2 = (21/80) pi^4 + (27/8) pi^2 - 89833/1536, for the quadrupole (ell=2) channel. Both reduce to combinations of pi^4, pi^2, and rational numbers, with no ln2 or odd-zeta terms in the reconstructed basis; the transmission-phase sector, by contrast, contains an analytically derived zeta(3)-9/8 static integral, while its overall matching prefactor is identified numerically. The leading reflection-phase asymmetry vanishes, so that no polarization rotation is imprinted on the reflected wave at leading low-frequency order, while a subleading O(omegazeta) asymmetry remains in the transmitted phase. The normalized flux asymmetries are numerically consistent with a 1/(Momega)^2 falloff in the
geometric-optics limit, where the leading remaining flux effect is conversion loss. The matched-asymptotic results are checked against direct two-channel integration, while the one-dimensional integral determining kappa_2 is evaluated to high precision within an analytically controlled transcendental basis. The results provide analytic targets linking dCS parity violation in absorption and mode conversion to the axial/polar quasinormal-mode splitting.
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High Energy Astrophysical Phenomena
A relativistic inflow candidate in a quasar at cosmic noon
Yerong Xu, Xiurui Zhao, Elena Bertola et al. (+14 more)
Cosmic noon ($z\sim2$) marks the peak epoch of
cosmic star formation and black hole
accretion activity, where strong
accretion and outflows are expected. We report two candidate absorption features in the XMM-Newton spectrum of the
quasar WISEA~J094835.95+432302.6 at $z=1.893$. Two redshifted features occur at rest-frame energies of $\sim4.65$ and $\sim5.75$keV with confidence levels of 99.3% and 94.1%, respectively, estimated from Monte Carlo (MC) simulations considering the look-elsewhere effect. These features can be explained by the Fe XXV/XXVI K-shell transitions from one ultra-fast inflow (UFI) with an apparent velocity of $0.37^{+0.02}_{-0.01}c$. If confirmed by follow-up XMM-Newton observations, these detections would extend reported UFI candidates from the local Universe to
cosmic noon, offering a rare laboratory for black hole
accretion at the peak of their growth.
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All authors Yerong Xu, Xiurui Zhao, Elena Bertola, Valentina Braito, Francesca Civano, Luca Comisso, Luigi Gallo, Vittoria E. Gianolli, Elias Kammoun, Giorgio Lanzuisi, Stefano Marchesi, Mar Mezcua, Ciro Pinto, James N. Reeves, Daniele Rogantini, Núria Torres-Albà, Lizhong Zhang
High Energy Astrophysical Phenomena · Cross-listed from astro-ph.GA
A Gravitationally Lensed Low-Luminosity AGN From The Cosmic Noon
Marko Mićić, Olivia Holmes, Xinyu Dai et al. (+3 more)
Strong gravitational
lensing provides a unique tool of studying intrinsically faint high-
redshift active galactic
nuclei (AGN). In particular, low-luminosity AGN (LLAGN) can probe black-hole growth in low-mass
galaxies that current X-ray surveys cannot otherwise access. We investigate the nature and physical properties of a faint X-ray source triply imaged by the
galaxy cluster SDSS J2243$-$0935, and determine whether the source hosts an AGN. We combine archival Hubble Space
Telescope imaging and Chandra X-ray observations with the published strong-
lensing models of SDSS J2243$-$0935. We constrain the source
redshift geometrically, derive magnification maps, and infer the physical properties of the host
galaxy. The
geometric lensing analysis gives a source
redshift of $z=2.14^{+0.12}_{-0.17}$, consistent with the photometric
redshift estimate, and a total magnification of $\mu=78\pm7$. The source is triply lensed and detected only in the HST F160W band, with an intrinsic magnitude of $m_{\rm AB}=28.2\pm0.2$. The rest-frame optical emission implies a stellar mass of $8.6\lesssim\log(M_\star/M_\odot)\lesssim9.4$. The unobscured
star formation rate is constrained to $\mathrm{SFR}_{\rm UV}<0.009\,M_\odot\,\mathrm{yr}^{-1}$, although substantial dust obscuration remains possible. Chandra detects X-ray point-source counterparts to all three HST-detected lensed images. The combined X-ray spectrum contains $100\pm11$ net counts and is consistent with a moderately absorbed power law, with $\Gamma \simeq 1.48$ and $N_{\rm H}=(5.5\pm4.4)\times10^{22}\,\mathrm{cm}^{-2}$. After correcting for gravitational magnification, we obtain an intrinsic rest-frame $2$–$10$ keV luminosity of $L_{\rm X}=1.2\pm0.3\times10^{42}\,\mathrm{erg\,s^{-1}}$. The X-ray emission is likely dominated by an AGN hosted by a low-mass
galaxy at $z\simeq2.14$, making this system a strongly lensed LLAGN at
cosmic noon (abridged).
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All authors Marko Mićić, Olivia Holmes, Xinyu Dai, Kevin Hainline, Haikun Xue, Zijun Gao
High Energy Astrophysical Phenomena
Quasi-periodic eruption spectral-timing: EMRIs crossing warped accretion disks
Joheen Chakraborty, Andrew Mummery, Eliot Quataert et al. (+3 more)
Quasi-periodic eruptions (QPEs) are repeating X-ray transients from galactic
nuclei potentially caused by a stellar-mass orbiter repeatedly colliding with the
accretion disk of a supermassive black hole (SMBH) in an extreme mass-ratio inspiral (EMRI). In this picture, the eruption arrival times encode the orbit and its relativistic precessions, while the quiescent spectral energy distribution (SED) probes the same disk and SMBH, providing two independent constraints on the same orbit-disk system. We present a joint spectral-timing framework that takes advantage of this by simultaneously fitting an analytic QPE timing model built on Kerr geodesic frequencies and a warped disk geometry, coupled to a self-irradiated disk SED model sharing the same SMBH mass,
spin, and disk parameters. We apply it to the QPE sources GSN 069 and eRO-QPE2, showing that the QPE timings and quiescent spectra can be reproduced simultaneously in each, providing circumstantial support for the EMRI model and warped disks in these systems. However, given the sparse timing data in both sources, neither admits a unique solution. In eRO-QPE2, we identify six physical modes which reproduce the timings and SED comparably well, motivating denser observing campaigns to identify one unambiguously. Our results demonstrate both the promise of QPE spectral-timing as a precision probe of SMBHs, their
accretion disks, and orbiting companions, and the aliasing subtleties of sparsely sampled QPE timings.
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All authors Joheen Chakraborty, Andrew Mummery, Eliot Quataert, Riccardo Arcodia, Itai Linial, Erin Kara
High Energy Astrophysical Phenomena · Cross-listed from astro-ph.SR
Wave-Growth-Limited Particle Confinement in a Near-Parallel Interplanetary Shock Observed by Parker Solar Probe
A. Kouloumvakos, I. C. Jebaraj, M. A. Malkov et al. (+11 more)
Collisionless shocks confine the particles they accelerate with the upstream wave field they themselves drive. Streaming particles resonantly amplify Alfvén waves, and the waves then confine the same particles by pitch-angle scattering. The rigidity a shock precursor can hold is therefore capped by how fast those waves grow, and that coupling has not previously been constrained in-situ at any interplanetary shock. Here we report Parker Solar Probe measurements of a fast, near-parallel coronal mass ejection-driven shock at 0.24~AU on 2023 March 13 that constrain this wave-particle coupling through the energetic-particle population it accelerates. We measure the spectral folding energy, the boundary between the confined and the upstream-escaping
protons, and find that it falls from $\sim$6 to $\sim$1.5~MeV as $E_\mathrm{fold}\propto\Delta t^{\,\alpha}$ with $\alpha\simeq0.34$, where $\Delta t$ is the time remaining before shock arrival. One precursor convective time before arrival this boundary is at $E_\mathrm{c}\simeq 2.46$~MeV, a characteristic confinement energy of a precursor of scale $L_\mathrm{EP}$. Read through the quasi-linear marginal-stability condition, the same
evolution implies a resonant wave-growth timescale $\tau_\mathrm{growth}(E)\propto E^{1/\alpha}\simeq E^{2.92}$ that exceeds the precursor convective time above $E_\mathrm{c}$. The post-shock pitch-angle anisotropy reverses sign across the same confinement–escape transition at 2–5~MeV. The rigidity to which a shock precursor can confine particles is therefore set, in-situ, by how quickly the streaming population builds its own confining waves.
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All authors A. Kouloumvakos, I. C. Jebaraj, M. A. Malkov, G. D. Berland, M. Gedalin, N. Wijsen, C. M. S. Cohen, O. V. Agapitov, D. J. McComas, D. G. Mitchell, E. Husidic, S. Yadav, M. E. Hill, R. McNutt
High Energy Astrophysical Phenomena
Composition $g$-modes and $f$-modes of neutron stars with gravitationally coupled dark matter: degeneracy with the symmetry energy
Probit J Kalita, Bharat Kumar and H C Das
Composition
gravity ($g$) modes have been proposed as a probe of dark matter in
neutron stars, on the basis of one-fluid models in which the dark matter is in chemical
equilibrium with the
neutrons. We computed in full general relativity the $g$- and $f$-modes of
stars in which fermionic dark matter couples to the nucleons only through
gravity, and inferred the dark mass fraction from NICER and GW170817 data with a prior that comprises $63$ nucleonic equations of state (EOSs) from Skyrme and relativistic mean-field functionals, among them the models of Dutra et al. and of the compilation of Sun, Bhattiprolu and Lattimer. The data do not prefer dark matter and bound its mass fraction at $0.12 - 0.26$ ($95\%$) for particle masses of $0.5$–$2$~GeV. Coupled only through
gravity, the dark matter reaches the buoyancy that restores a $g$-mode only through the gravitational field: a dark core holding $10\%$ of the mass raises the fundamental $g$-mode by $10-15\%$ at fixed EOS, several times less than in the one-fluid picture. The larger effect is indirect: a
star with a dark core needs a stiffer nucleonic EOS, and since the $g$-mode is set by the density dependence of the
symmetry energy, its frequency reflects the EOS that the
star requires rather than the dark matter it contains; at fixed radius and
symmetry energy a core holding $10\%$ of the mass is equivalent to a change of the slope $L$ of $9-12$~MeV. The $f$-mode, which follows the mean density, behaves differently: a dark core breaks the relation between the $f$-mode and the tidal deformability by about one percent, far outside the posterior spread of nucleonic
stars.
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High Energy Astrophysical Phenomena
Dynamical pairing in gravitational wave populations
Fabio Antonini, Aleksandra Olejak, Isobel Romero-Shaw et al. (+1 more)
Binary black holes formed dynamically in
star clusters are often associated with nearly equal-mass pairing. We show that this expectation is incomplete. The more general prediction of dynamical assembly is an ordered-pairing relation in which the two black holes are drawn independently from a common one–body mass spectrum and then ordered by mass. Once the primary-mass distribution is known, this relation fixes the conditional companion distribution $p(m_2\mid m_1)$. We validate this picture with cluster simulations. In these models, the ordered-draw relation reproduces the morphology of the mass-pairing distribution, including fixed-$m_2$ ridges that are inherited from the one–body mass spectrum: features at nearly constant secondary mass that extend over a range of primary masses and therefore correspond to decreasing $q=m_2/m_1$ as $m_1$ increases. Thus, the geometry of the mass distribution itself becomes a formation-channel diagnostic. Dynamical pairing maps peaks of the primary-mass spectrum into fixed-$m_2$ ridges with predictable amplitude, whereas isolated binary
evolution more naturally produces structures at fixed $q$. Applying this test to a non-parametric reconstruction of the gravitational-wave population, we find a tentative enhancement at $m_2\simeq 30~ M_\odot$ for $m_1\gtrsim 40~ M_\odot$ that is broadly compatible with ordered pairing. Finally, we show that dynamical formation naturally explains the rapid decline of the secondary-mass distribution above $\sim30~M_\odot$ recently reported by the LIGO–Virgo–KAGRA Collaboration.
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All authors Fabio Antonini, Aleksandra Olejak, Isobel Romero-Shaw, Matthew Mould
High Energy Astrophysical Phenomena
Gamma-rays from black hole coronae: disentangling the leptonic and hadronic contributions
Sébastien Le Bihan, Daniel Grošelj and Martin Lemoine
IceCube detections of TeV
neutrinos from nearby Seyfert
galaxies imply, if confirmed, that the accompanying gamma rays are substantially reprocessed down to the MeV range by electromagnetic cascades, which places multi-messenger constraints on
neutrino emission models. However, gamma rays can also be produced by the primary nonthermal electrons of the X-ray corona, as recent radiative kinetic simulations of magnetized
turbulence have shown. To disentangle the leptonic and hadronic contributions, we self-consistently combine results from these simulations with predictions for hadronically induced electromagnetic cascades. Focusing on NGC 4151, we find that the leptonic channel dominates the MeV band, and remains comparable to the hadronic cascade up to $\lesssim$0.1 GeV, when the nonthermal electron tail carries $\gtrsim5\%$ of the electron energy. The leptonic contribution may be detectable in soft gamma rays by the upcoming Compton Spectrometer and Imager (COSI) mission, whereas our predicted hadronic component falls below its sensitivity. We also find that the same nonthermal electron population can account for both the $0.1-1$ MeV excess of the
cosmic X-ray background, generally attributed to AGNs, and the MeV tail of Cygnus~X-1 in the hard state, lending observational support to our model. Our results offer a refined view of multi-messenger signals from black-hole coronae, grounded in their nonthermal kinetic physics.
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High Energy Astrophysical Phenomena
Remnant-wide mapping of heterogeneous presupernova burning histories in the O-rich ejecta of Cassiopeia A
Yui Kuboike, Toshiki Sato and Kai Matsunaga
Late-stage shell interactions, including shell mergers, can mix material across adjacent burning shells and imprint chemically inhomogeneous structures in the hydrostatic O-rich layers before core collapse. Whether such inhomogeneities survive the explosion and remain observable in the resulting
supernova remnant is not well understood. We investigate the spatial and compositional structure of the O-rich ejecta in Cassiopeia A using deep Chandra X-ray observations. Principal component analysis of narrow-band images reveals O-rich ejecta spanning a compositional sequence from Ne-rich through NeSi-rich to O/Si-rich material. Spatially resolved spectroscopy of 20 representative regions confirms systematic depletion of Ne and Mg toward the O/Si-rich component. Notably, the composition of the O/Si-rich ejecta over a broad elemental range from O to Ca is simultaneously well reproduced by O-layer material in 17-20 $M_\odot$ one-dimensional presupernova models that experienced strong shell-merger-induced burning and mixing. In contrast, the Ne-rich ejecta retain substantially higher Ne and Mg abundances and resemble less strongly processed O-shell material. These chemically distinct components are spatially segregated on remnant-wide scales, with Ne-rich ejecta preferentially concentrated toward the north and O/Si-rich ejecta more prominent in the southeast. Our results suggest that late-stage shell burning and mixing in the progenitor of Cassiopeia A were spatially heterogeneous. The remnant-wide distribution of ejecta with different burning histories further indicates that part of this asymmetric presupernova structure may have survived the explosion and remains imprinted in the present-day remnant.
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High Energy Astrophysical Phenomena
$r$-Process as Production of Lanthanides from Compact Object Mergers
Li-ming Zhuo, Hou-Jun Lü, Xiao-Xuan Liu et al. (+2 more)
The relative contributions of binary
neutron star (BNS) and black hole-
neutron star (BH-NS) mergers to the Galactic enrichment from the rapid
neutron-capture process ($r$-process) remain an open question in nuclear astrophysics. In this paper, we present end-to-end numerical simulations of binary population synthesis, nucleosynthesis, and Galactic chemical
evolution (GCE) via Compact Object Mergers Population Astrophysics and Statistics (COMPAS), SkyNet network, and a
stochastic multi-zone model, respectively. By considering five equations of state (EOS) of NS and different
spins of BH for BNS and BH-NS mergers, it is found that the averaged abundance seems not to be significantly dependent on the EOS of NS and
spin of BH for BNS and BH-NS mergers. For given EOSs of NS, the abundance of BNS is higher than that of BH-NS mergers before the second peak (for mass number $A\sim$130), but it is lower when the mass number is greater than 130. Moreover, the lighter $r$-process elements ($A<$130) are predominantly produced by the disk wind, while the heavier elements ($A>$130) originate mainly from the dynamical ejecta. The purely BNS mergers, on the other hand, are the dominant contributors to the production of lanthanides like europium (Eu) at the later stage of GCE, but BH-NS mergers make only a small contribution of heavy $r$-process elements for the $\rm Eu/Fe$ evolutionary tracks in the Milky Way.
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All authors Li-ming Zhuo, Hou-Jun Lü, Xiao-Xuan Liu, Meng-Hua Chen, En-Wei Liang