High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Shadow, Sparsity of Radiation and Energy Emission Rate in Skyrmion Black Holes
Faizuddin Ahmed, Ahmad Al-Badawi and İzzet Sakall{\i}
We examine several observable optical properties of a Skyrmion black hole (BH), focusing on the
photon sphere, BH shadow, and
photon trajectories. The Skyrme term, along with other
geometric parameters of the spacetime, determines the
photon sphere location and shapes the resulting BH shadow. Parameter variations produce observable departures from standard BH geometries, offering potential signatures of nonlinear field effects. We also analyze the sparsity of Hawking radiation and the associated energy emission spectra, showing how these quantities respond to the Skyrme coupling and background parameters. Our findings illuminate the connection between nonlinear field contributions and BH optics, with implications for observational and theoretical studies of modified
gravity scenarios.
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Solar and Stellar Astrophysics
Learning the Stellar Structure Equations via Self-supervised Physics-Informed Neural Networks
Manuel Ballester, Santiago Lopez-Tapia, Seth Gossage et al. (+9 more)
Stellar astrophysics relies critically on accurate descriptions of the physical conditions inside
stars. Traditional solvers such as MESA (Modules for Experiments in
Stellar Astrophysics), which employ adaptive finite-difference methods, can become computationally expensive and challenging to scale for large
stellar population synthesis ($>10^9$
stars). In this work, we present an self-supervised physics-informed neural network (PINN) framework that provides a mesh-free and fully differentiable approach to solving the
stellar structure equations under hydrostatic and thermal
equilibrium. The model takes as input the
stellar boundary conditions (at the center and surface) together with the chemical composition, and learns continuous radial profiles for mass $M_r(r)$, pressure $P(r)$, density $\rho(r)$, temperature $T(r)$, and luminosity $L_r(r)$ by enforcing the governing structure equations through physics-based loss terms. To incorporate realistic microphysics, we introduce auxiliary neural networks that approximate the equation of state and opacity tables as smooth, differentiable functions of the local thermodynamic state. These surrogates replace traditional tabulated inputs and enable end-to-end training. Once trained for a given
star, the model produces continuous solutions across the entire radial domain without requiring discretization or interpolation. Validation against
benchmark MESA models across a range of
stellar masses yields a Mean Relative Absolute Error of $3.06\%$ and an average $R^2$ score of $99.98\%$. To our knowledge, this is the first demonstration that the
stellar structure equations can be solved in a fully self-supervised and data-free fashion employing PINNs. This work establishes a foundation for scalable, physics-informed emulation of
stellar interiors and opens the door to future extensions toward time-dependent
stellar evolution.
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All authors Manuel Ballester, Santiago Lopez-Tapia, Seth Gossage, Patrick Koller, Philipp M. Srivastava, Ugur Demir, Yongseok Jo, Almudena P. Marquez, Christoph Wuersch, Souvik Chakraborty, Vicky Kalogera, Aggelos Katsaggelos
Cosmology and Nongalactic Astrophysics
Measurement of the minimum cold dark matter halo mass with strong gravitational lensing
A. M. Nierenberg, D. Gilman, T. Treu et al. (+19 more)
We explore the lowest mass limit that can be placed on the
halo mass function in CDM using 28 strong gravitational lenses. For this purpose, we study an extreme model in which the
halo mass function and mass-concentration relation follow CDM, with a sharp cutoff at some mass scale, $m_{\rm{low}}$.
Lensing provides a unique window into this quantity as it does not depend on the presence of
baryons in dark matter
halos and also allows the detection of low mass
halos at cosmological distances, both in the lens
galaxies and along the line-of-sight. Our model incorporates the effects of
tidal stripping of subhalos, leading to the presence of many subhalos below a given model cutoff scale. We place an upper limit on the low-mass cutoff of the
halo mass function of $m_{\rm{low}}<10^{8.3}$ M$_\odot$ at 10:1 odds using a prior for the normalization of the subhalo mass function from the semi-analytic model galacticus and $m_{\rm{low}}<10^{8.2}$ M$_\odot$ at 10:1 odds using a prior from $N$-body simulations. These limits are comparable to, or stronger than, existing constraints based on Milky Way satellite
galaxies. Based on these results, we forecast more than an order of magnitude improvement with a sample of 200 quadruply imaged
quasar lenses. This number represents a small subset of the thousands that are anticipated to be discovered by Rubin, Euclid, and Roman. Furthermore, with this larger sample of lenses we expect to directly constrain the normalization of the subhalo mass function, thereby eliminating a major source of uncertainty in our current measurements.
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All authors A. M. Nierenberg, D. Gilman, T. Treu, X. Du, C. Gannon, H. Paugnat, S. Birrer, A. J. Benson, K. N. Abazajian, T. Anguita, S.G. Djorgovski, S. F. Hoenig, R. E. Keeley, A. Kusenko, H. R. Larsson, L. A. Moustakas, P. Mozumdar, W. Sheu, D. Sluse, D. Stern, D. Williams, K. C. Wong
Earth and Planetary Astrophysics
The JWST Search for Earth-Luna Analogs: Upper Limits on Exomoons and Refined Ephemerides for TOI 700 d and e
Emily K. Pass, David Charbonneau, Andrew Vanderburg et al. (+1 more)
While no conclusive detections of exomoons have been reported to date,
planet formation theories predict that Luna-analog satellites should be a common outcome of the collisional dynamics in early extrasolar systems. Such satellites have the potential to unlock new avenues to learn about
exoplanet systems, speaking to topics of habitability,
tidal heating,
planet formation, late-stage growth, planetary compositions, and more. Here we describe the results of our JWST program to search for Luna-analog exomoons around the rocky, habitable-zone M-
dwarf planets TOI 700 d and e. We refine the ephemerides of both worlds, providing an order-of-magnitude improvement in period precision and a factor of 2-3 improvement in planetary radii. We identify a strong correlated noise signal with a timescale of $16\pm4$ minutes and an amplitude of $46\pm4$ ppm; similar signals have been observed in previous JWST analyses of other
stars and have been ascribed to
stellar granulation. This noise source inflates our error by a factor of 4 relative to
photon-noise expectations in 10-minute bins and limits our sensitivity to moons: we determine that our observations are sensitive mainly to moons larger than Ganymede on periods longer than 2 days (i.e., moons larger than our
solar system's natural satellites). If this noise could be corrected, we would be sensitive to Luna-analog moons. Future work to address this noise source will thus be critical for detecting exomoons in
stellar transits, as well as for all other science cases that hope to take advantage of JWST white-light curves in the
photon-noise limit.
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All authors Emily K. Pass, David Charbonneau, Andrew Vanderburg, Jacob L. Bean
Solar and Stellar Astrophysics
A Chemical Mismatch Between Young Stars and Their Inner Disks
Diogo Souto, Ilaria Pascucci, Katia Cunha et al. (+1 more)
We present the first
stellar elemental abundance study for two very low-mass
stars, similar in mass to TRAPPIST-1, in the $\sim5-10$\,Myr-old Upper-Sco association. Their mid-infrared JWST/MIRI spectra, like those of many very low-mass
stars, are hydrocarbon-rich, indicating C/O ratios greater than unity in the inner disk gas inside their snowlines. By fitting synthetic spectra to high-resolution APOGEE near-infrared
stellar spectra, we show that, unlike their inner disks, both
stars have
solar C/O ratios. Their Fe, C, O, Mg, and Ca abundances are likewise consistent with
solar values, placing them within the
Galactic thin-disk population, as expected for nearby
star-forming regions. This contrast between
stellar and inner disk C/O ratios provides the first direct evidence that the inner disk's supersolar values are not inherited from the natal cloud but arise from disk processes. If these enhanced C/O ratios are primarily driven by inward drift of icy pebbles, there are major implications for disk
evolution and
planet formation, which we also discuss.
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All authors Diogo Souto, Ilaria Pascucci, Katia Cunha, Shubham Kanodia
Cosmology and Nongalactic Astrophysics
Local primordial non-Gaussianity using cross-correlations of DESI tracers
A. J. Rosado-Marín, A. J. Ross, H. Seo et al. (+46 more)
We constrain local
primordial non-Gaussianity by a combined analysis of auto and cross-correlations of DESI DR1 tracers, leveraging LRGs and QSOs as well as ELGs between $0.8<z<3.1$. By cross-validating the signal across different clustering tracers within the same
redshift range, we evaluate potential systematics in the $f^\mathrm{loc}_\mathrm{NL}$ measurements, capitalizing on the reduced susceptibility of cross-correlations to non-common systematics. We find that the cross-correlation between LRG and
quasars can robustly improve the DESI DR1 $f^\mathrm{loc}_\mathrm{NL}$ constraints, by $\sim9\%$ to a measurement of $f^\mathrm{loc}_\mathrm{NL}=2.1_{-8.3}^{+8.8}$ at 68% confidence. On the other hand, we do not find a clear improvement when including the DESI DR1 ELG sample. Mock tests predict an additional $\sim8\%$ gain with statistical scatter, and the lack of improvement in the data remains consistent with this expectation. This project serves as an exploratory analysis of DESI ELG clustering for $f^\mathrm{loc}_\mathrm{NL}$ through its cross-correlation in preparation for future DESI data analyses.
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All authors A. J. Rosado-Marín, A. J. Ross, H. Seo, E. Chaussidon, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, A. Cuceu, A. de la Macorra, A. de Mattia, R. Demina, B. Dey, P. Doel, S. Ferraro, A. Font-Ribera, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, G. Gutierrez, C. Hahn, H. K. Herrera-Alcantar, D. Huterer, M. Ishak, R. Joyce, D. Kirkby, A. Kremin, O. Lahav, C. Lamman, M. Landriau, M. E. Levi, M. Manera, A. Meisner, R. Miquel, S. Nadathur, J. A. Newman, N. Palanque-Delabrouille, W. J. Percival, F. Prada, I. Pérez-Ràfols, G. Rossi, E. Sanchez, D. Schlegel, J. Silber, G. Tarlé, B. A. Weaver, C. Zhao, H. Zou
High Energy Astrophysical Phenomena
Approximating the Fourier Transform of Ring-Like Images: the Focal Expansion
Filip Niewiński and Michael D. Johnson
We derive and showcase a novel approach to approximating
Fourier transforms in higher dimensions, focusing specifically on the case of 2D radially concentrated ('ring-like') functions. We first reduce the problem to that of evaluating the Hankel transforms of each angular mode of the image and then use our focal expansion to approximate these remaining Hankel transforms. Our method provides a single approximation that remains accurate from small to large spatial frequencies, bridging regimes where moment-based or large-frequency asymptotic expansions are individually reliable. We explore a series of examples, showing that the leading focal term provides an accurate global approximation for a broad range of functions. We demonstrate the utility of this approximation by examining the interferometric response for toy models of a black hole's "
photon ring," highlighting the application to experiments designed to measure this feature such as the Black Hole Explorer.
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Earth and Planetary Astrophysics
Gas Giants Formed by Gravitational Instability May Accrete Atmospheres with Super-Stellar Carbon to Oxygen Ratios
Alan P. Boss
Characterizing the atmospheric compositions of
exoplanets, along with determining properties such as their mass, mean density, and orbital configuration, is thought to be an effective means for differentiating between various formation and
evolution scenarios.
Exoplanet atmospheric C/O ratios, when compared to host
star C/O ratios, have been advanced as discriminators of gas
giant formation and
evolution scenarios in the context of the core
accretion mechanism. Gas
giants formed by gas disk gravitational instability (GDGI), on the other hand, are thought to have atmospheres with C/O ratios identical to their host
stars. We examine this assumption through analysis of fully three dimensional radiative hydrodynamics models of the GDGI in the flux-limited
diffusion approximation. We show here that GDGI protoplanets may be able to form and accrete disk gas with super-
stellar C/O ratios, as a result of their formation and orbital
evolution in a disk with midplane temperatures in the range of the evaporation temperatures of water ($\sim$ 135 K) and CO$_2$ ($\sim$ 47 K) ices. Solids that avoid fragmentation and grow rapidly to cm-size could be transported inward to the central protostar or outward to the edge of the disk considerably faster than the disk gas is dissipated, leading to the preferential
accretion of C-rich disk gas compared to the O-rich ices, provided that the protoplanet's orbit remains outside $\sim$ 7 au from a
solar-mass protostar. Orbits inside $\sim$ 7 au, however, could result in the
accretion of disk gas with nearly
stellar C/O.
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Solar and Stellar Astrophysics
SN2023ixf: Radiative-transfer modeling of the photospheric phase evolution from the ultraviolet to the infrared
Luc Dessart, Wynn V. Jacobson-Galan, K. Azalee Bostroem et al. (+4 more)
SN2023ixf, a Type II
supernova (SN) showing early signs of interaction with circumstellar material (CSM), has been observed with unprecedented detail across the electromagnetic spectrum since shock breakout. Here, we present nonlocal thermodynamic
equilibrium time-dependent radiative-transfer calculations of its photospheric-phase
evolution (i.e., ~20 to ~120d), and for the first time encompassing from the ultraviolet (UV) to the infrared (IR). The explosion of a 15Msun progenitor
star, evolved with enhanced mass loss during the red-supergiant phase, yielding an ejecta of 7-8Msun, a kinetic energy of 1.2x10^51 erg, and a 56Ni mass of 0.05Msun, yields a satisfactory match to the photospheric-phase duration and brightness. Prolonged interaction with a decreasing CSM density is required to match a number of salient features of SN2023ixf during the photospheric phase, including the persistent UV continuum and line fluxes, the optical brightness and line profiles (in particular Halpha), as well as the IR flux (interaction boosts the free-free emission at long wavelengths). The presence of a cold dense shell (CDS), which is hard to infer at early times when the CDS and photosphere lie at similar velocities, becomes evident at later times and more so in the IR - we find no evidence for material faster than the CDS at ~8000km/s. Exploratory two-dimensional radiative-transfer calculations based on axially symmetric CSM or ejecta suggest that asymmetry can produce a diversity of profile shapes, with absorption troughs exhibiting a flat bottom or notches at any Doppler velocity. We emphasize the complexity of UV spectra influenced by complex metal-line blanketing at these phases. We document the sensitivity of model results to the adopted clumping in the CDS, though the largest offset is obtained here in the unlikely case of a smooth CDS.
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All authors Luc Dessart, Wynn V. Jacobson-Galan, K. Azalee Bostroem, Alexei V. Filippenko, WeiKang Zheng, Thomas G. Brink, Stefano Valenti
Astrophysics of Galaxies
Identifying signatures of inflow onto face-on galaxies using the Balmer decrement
Meghna Sitaram, Hui Li, Yong Zheng et al. (+4 more)
Isolated
star-forming
galaxies require inflows of fresh gas from the surrounding medium to sustain episodes of
star formation over time. However, there are very few direct detections of
accretion onto external
galaxies. Studies in absorption can only observe along limited sightlines, while those in emission can have difficulty distinguishing inflowing gas in the foreground of the
galactic disk from similarly Doppler-shifted outflowing gas in the background. We explore the possibility of using the Balmer decrement (H$\alpha$/H$\beta$) in low-inclination systems as a diagnostic for disentangling the flow geometry in disk-like
galaxies. We leverage mock spatial-spectral observations of an isolated Milky Way-mass
galaxy simulated using the radiation-hydrodynamics code AREPO-RT and post-processed with the Monte Carlo radiative transfer code COLT. We find that gas components located in front of the disk exhibit systematically lower Balmer decrements than gas embedded in or behind the disk, with a mean front-back offset of $\Delta(\text{H}\alpha/\text{H}\beta) \approx -0.14$. The ability to differentiate between the disk and far-side components is limited by the extremely clumpy, multiphase
dust distribution along the line of sight introducing substantial scatter. Overall, the results provide a useful observational diagnostic of inflow and
outflow in dusty face-on
galaxies.
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All authors Meghna Sitaram, Hui Li, Yong Zheng, Greg L. Bryan, Mary Putman, Aaron Smith, Rahul Kannan
High Energy Astrophysical Phenomena
Identification of a Radio Counterpart to SN 2025ulz in the S250818k Localization Area
Tanner O'Dwyer, Alessandra Corsi, Deepika Yadav et al. (+12 more)
On 2025 August 18, the LIGO-Virgo-KAGRA collaboration reported S250818k, a sub-threshold gravitational-wave (GW) candidate consistent with a binary
neutron star (NS) merger potentially involving a sub-
solar-mass NS. Optical follow-up by the Zwicky Transient Facility identified AT2025ulz, a transient temporally coincident with the GW trigger that initially resembled a kilonova but was later classified as a young stripped-envelope Type IIb
supernova (SN), dubbed SN 2025ulz. A key question is whether SN 2025ulz harbors fast, possibly collimated, non-thermal ejecta indicative of a central engine, as invoked in "superkilonova" scenarios linking sub-
solar-mass NSs to
accretion-disk fragmentation or core fission. We present early-to-late-time multi-band
radio observations of SN 2025ulz obtained with the Karl G. Jansky Very Large Array as part of the JAGWAR program, complemented by observations with the upgraded
Giant Metrewave
Radio Telescope and MeerKAT. We detect a faint but significant
radio counterpart to SN 2025ulz at 6-10 GHz. The data are consistent with non-thermal emission from SN ejecta interacting with circumstellar material, favoring a compact progenitor and relatively fast ejecta akin to those of Type cIIb SNe. Our data are also consistent with emission from an off-axis jet peaking at about 50-100 days after the GW trigger. Overall, our
radio detection is compatible with a superkilonova scenario and would motivate future systematic multi-wavelength follow-up of core-collapse events coincident with sub-
solar NS GW candidates, should the association between S250818k and SN 2025ulz be supported by offline GW analyses.
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All authors Tanner O'Dwyer, Alessandra Corsi, Deepika Yadav, Kunal P. Mooley, Raphael Baer-Way, Poonam Chandra, Gregg Hallinan, Mansi M. Kasliwal, Lauren Rhodes, Oleg M. Smirnov, Davide Lazzati, Joeri van Leeuwen, Adam Deller, Pikky Atri, Tanazza Khanam
Earth and Planetary Astrophysics
Azimuthal Dust Polarization from Aerodynamically Aligned Grains as Evidence for the Streaming Instability in Protoplanetary Disks
Zhe-Yu Daniel Lin, Jeonghoon Lim, Jacob B. Simon et al. (+8 more)
(Sub)millimeter
dust polarization in protoplanetary disks has revealed the presence of large (~ 100 um)
dust grains that are aligned along their long axis following the azimuthal direction of the disk. The novel badminton birdie-like aerodynamic
alignment predicts large grains to align with their long axes following the direction of gas flow experienced by the
dust, denoted as the A-field. With 3D streaming instability (SI) simulations, we find that the A-field is predominantly in the radial direction in regions of low
dust-to-gas ratio, but in the azimuthal direction in regions of high
dust-to-gas ratio. Through polarized radiation transfer, we find that the resulting polarization angle indeed follows the disk azimuthal direction in the high
dust density regions. Therefore, ongoing SI is an attractive candidate for producing the azimuthal
dust polarization pattern as observed in an increasing number of disks.
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All authors Zhe-Yu Daniel Lin, Jeonghoon Lim, Jacob B. Simon, Zhi-Yun Li, Daniel Carrera, Manuel Fernández-López, Rachel Harrison, Rixin Li, Leslie W. Looney, Ian W. Stephens, Haifeng Yang
Solar and Stellar Astrophysics
A Comprehensive Atmospheric Retrieval Analysis of 22 James Webb Space Telescope Spectral Energy Distributions of Cool Brown Dwarfs
Harshil Kothari, Michael C. Cushing, Samuel A. Beiler et al. (+5 more)
We present a uniform atmospheric
retrieval analysis of 22 late-T and Y-type brown
dwarfs within 20 pc, observed with the James Webb Space
Telescope NIRSpec PRISM and MIRI LRS. This
dataset provides the first continuous 0.95-12 um spectroscopic coverage of late-T and Y-type brown
dwarfs, which in turn enables precise constraints on their thermal structures and volume mixing ratios (VMRs) of H2O, CH4, CO, CO2, NH3, H2S, K, Na, and PH3. We find positive correlations between the VMR of H2O and CH4, and CO and CO2, consistent with thermochemical
equilibrium chemistry. Using the VMRs, we derive atmospheric
metallicity, which is positively correlated with H2O and CH4, showing H2O and CH4 trace oxygen and carbon content, respectively, allowing us to effectively measure (O/H)bulk and (C/H)bulk. We also report tentative PH3 detections in roughly half the sample, suggesting potential vertical mixing or non-
equilibrium chemistry. Apart from chemical properties, we retrieve masses and radii spanning approximately 6-77 M_Jup and 0.66-1.53 R_Jup, respectively. We compare the derived log10(g) values of about 4-5.5 cm s^-2 and Teff values of about 350-1100 K with Sonora Bobcat evolutionary models and find an age range of 0.4 to 10 Gyr across the sample. Comparing our retrieved thermal profiles with the Elf-Owl forward-model thermal profiles, we find a systematic difference between the two, likely arising from differences in chemistry treatment.
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All authors Harshil Kothari, Michael C. Cushing, Samuel A. Beiler, Channon Visscher, Mark S. Marley, Ben Burningham, Adam C. Schneider, J. Davy Kirkpatrick
Cosmology and Nongalactic Astrophysics
A generic $\omega_b$ tension in early-time solutions to the Hubble tension
Cara Giovanetti
I show that early-time (pre-recombination) solutions to the Hubble tension are generically expected to increase the preferred
baryon density $\omega_b$. This puts these models in tension with Big Bang Nucleosynthesis (BBN), as measurements of
primordial deuterium constrain $\omega_b$ at percent level. I show that existing analyses are in tension with the BBN determination of $\omega_b$, and that including a likelihood component for
primordial deuterium deters two representative models from recovering a high $H_0$.
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Cosmology and Nongalactic Astrophysics
Great Walls of Cosmic Baryons in the Northern Sky
Vikram Ravi, Kritti Sharma and Liam Connor
The dispersion measures (DMs) of fast
radio bursts (FRBs) encode the total ionized-gas column densities along their sightlines. Most observed FRBs originate at distances where the cosmological principle applies. Thus, variations in the DM distribution of FRBs observed in different regions on the sky trace local sources of anisotropy, such as the warm ionized medium and circum-
galactic medium of the Milky Way, and local large-scale structure. We present a map of extragalactic DM variations across the Northern sky using a few thousand FRBs from the second \chime{} catalog. We detect a $\gtrsim 4\sigma$ excess of $\sim$150 pc cm$^{-3}$ above the global mean, extended over $\sim$30$^\circ$ scales and centered near $\alpha \approx$ $12^{\rm h}$, $\delta \approx$ $55^\circ$. This excess, termed Wall 1, is robust to variations in sample selection and jackknife resampling, and cannot be explained by
Galactic-disk DM-model uncertainties. The excess is likely too large to correspond to anisotropy in the Milky Way
halo. The signal spatially coincides with the Ursa Major supercluster and associated large-scale structures. A secondary, more tentative Wall 2 near $\alpha \approx 2^{\rm h}$, $\delta \approx$ $45^\circ$ is spatially coincident with the Perseus-Pisces supercluster. Although the spatial coincidences suggest that the Walls may correspond to
baryons in the local large-scale structure, the probability of chance coincidence is likely too high ($\sim10-20\%$) to claim confident associations. These results highlight the potential of using FRB DMs to detect
baryon overdensities associated with local large-scale structure, and have important implications for near-field
baryon mapping and FRB cosmology.
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High Energy Astrophysical Phenomena
A New Approach for Testing Einstein's Theory of Gravity Close to Rapidly Spinning Black Holes
Shravan Vengalil Menon, Kun Hu and Henric Krawczynski
The Penrose process and the collisional Penrose process involve particles decaying or interacting very close to a spinning black hole, respectively, during which some particles are pushed into negative energy trajectories and fall into the black hole while others gain that energy and escape the system. These two processes are difficult to observe as they occur very rarely by chance. Here we report a new observational signature of similar, but less extreme processes occurring in and near the ergospheres of rapidly spinning black holes. We find that the reflection of the thermal emission from a geometrically thin, optically thick
accretion disk can lead to the formation of a power-law component, even in the absence of a corona. Unlike the well-known Penrose processes, the scattering particles lose a good fraction of their energy, but are not pushed into negative energy trajectories. We emphasize that this new component has distinct spectral and polarimetric properties that can be used for its identification as long as it out-competes other power-law emission components. We emphasize that the new component needs to be taken into account when interpreting spectral and spectropolarimetric observations of black holes. The detection and unambiguous identification of the new component with current or future broadband X-ray spectral and spectropolarimetric missions can open a new window into testing Einstein's theory of
gravity close to the edges of black holes, and opens up new opportunities to constrain black hole
spins and inclinations.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Gravitational Waves from Matter Perturbations of Spectator Scalar Fields
Marcos A. G. Garcia, Angel Garcia-Vega and Sarunas Verner
We compute the
stochastic gravitational wave background sourced at second order by a spectator scalar field $\chi$ coupled to the inflaton $\phi$ through a portal interaction $\sigma\phi^2\chi^2$ and with quartic self-interaction $\lambda_\chi\chi^4/4!$. In the large portal coupling regime ($\sigma/\lambda \gg 1$, with $\lambda$ the inflaton normalization), parametric resonance during reheating amplifies the spectator power spectrum by many orders of magnitude near the resonance band until Hartree backreaction from the quartic coupling detunes the instability, while the large inflationary effective mass suppresses superhorizon power and ensures compatibility with CMB isocurvature bounds. We focus on the direct field-gradient source $\partial_a\chi\,\partial_b\chi$ in the second-order Einstein equations and derive a master formula that factorizes into a spectral integral over the frozen,
vacuum-subtracted spectator spectrum and a time integral encoding the post-inflationary expansion history. For our
benchmark reheating history we obtain analytic scaling relations, including a peak amplitude $\Omega_{\rm GW}\propto T_{\rm reh}^{8/3}$, strong dependence on the portal strength, and weak sensitivity to $m_\chi$. We validate the framework against nonlinear
lattice simulations, demonstrating complementarity: the Hartree treatment captures superhorizon
evolution inaccessible to the
lattice, while the
lattice resolves rescattering and fragmentation near the spectral peak. For $\sigma/\lambda \simeq 10^4$ and $T_{\rm reh}=2 \times 10^{14}\,\mathrm{GeV}$, the signal reaches $\Omega_{\rm GW}h^2\sim 10^{-11}$ at $f\sim10^{7}$-$10^{8}\,\mathrm{Hz}$. Increasing $\lambda_\chi$ at fixed $\sigma$ has a non-monotonic effect: small values enhance the signal via rescattering, whereas larger values suppress it by detuning the resonance.
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High Energy Astrophysical Phenomena
Implications of low neutron star merger rates for gamma-ray bursts, r-process production and Galactic double neutron stars
Maya Fishbach, Alexander P. Ji, Wen-fai Fong et al. (+4 more)
The first multimessenger discovery of a binary
neutron star (BNS) merger, GW170817, proved that such mergers can source short gamma-ray bursts (SGRBs) and produce r-process elements. The initial merger rate from this single event was found to be broadly consistent with the SGRB rate, the Milky Way (MW) r-process mass, and the
Galactic population of double
neutron star (DNS) systems that will merge in a Hubble time. However, only one additional BNS merger has been detected since, and the BNS merger rate has been consistently revised downwards with recent gravitational wave (GW) catalog updates. Analyzing GWTC-4, we find a total BNS merger rate of $28$–$300\,\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$ consisting of $53^{+176}_{-49}\,\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$ in GW170817-like $\sim(1.3,1.3)\,M_\odot$ BNSs (90% credibility). We revisit the consistency of the BNS merger rate with SGRBs, r-process and
Galactic DNSs. In all cases, there is an emerging tension with the BNS (and EM-bright
neutron star–black hole, NSBH) merger rate. Comparing to a BNS merger rate of $100\,\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$, the cosmological SGRB rate is a factor of 3.6–18 higher (despite kilonova followup of SGRBs implying a significant fraction of SGRBs are of BNS origin), the r-process rate is a factor of 0.9–4.1 higher (even though we consider only r-process elements above the second peak), and the rate inferred from
Galactic DNSs is a factor of 2.3–5.1 higher than the BNS rate. We discuss how various uncertainties in the inferred rates either alleviate or exacerbate this tension, which point to the various physical processes that can be constrained by such rate comparisons.
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All authors Maya Fishbach, Alexander P. Ji, Wen-fai Fong, Tom Y. Wu, Jillian C. Rastinejad, Aditya Vijaykumar, Hsin-Yu Chen
High Energy Astrophysical Phenomena
Kinetic magnetohydrodynamics and Landau fluid closure in relativity
Abhishek Hegade K. R. and James M. Stone
Diffuse
accretion flows near a supermassive black hole are fundamentally weakly collisional. In such weakly collisional
plasmas, the ion and electron distribution functions can deviate significantly from thermal
equilibrium, and particle kinetic effects can influence large-scale fluid motion by driving pressure anisotropy, heat conduction, and
plasma instabilities. Modeling these
plasma effects in highly relativistic flows could be important for interpreting horizon-scale observations of black hole images. In this paper, we present a theoretical framework for understanding weakly collisional
plasmas in general relativity by deriving the relativistic drift kinetic equations from the Vlasov-Maxwell equations. We present the
evolution equations for the moments of the gyroaveraged distribution function and introduce a new analytic Landau fluid closure to capture anisotropic heat flow in relativistic
plasmas. Unlike standard (collisional) general relativistic magnetohydrodynamics or extended magnetohydrodynamics, our model does not rely on strong collisions to enforce thermal
equilibrium and consistently incorporates Landau damping in a fluid closure. The model introduced in this work provides a complementary approach to fully kinetic simulations in understanding weakly collisional effects in low-luminosity relativistic black hole
accretion disks.
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High Energy Astrophysical Phenomena
Photon-Count Statistics of Crab X-ray Pulses: Skellam Behavior and Excess Variance in the Main Pulse
Max Worchel, Margaret M.Ferris, Sasha Levina et al. (+15 more)
The Crab
pulsar (PSR B0531+21) provides an unusually rich test bed for statistical studies of high-energy
photon-counting data, owing to its extreme brightness and the contrasting behavior of its main pulse (MP) and interpulse (IP) components. Using 78.8 ks of
Neutron star Interior Composition Explorer (NICER; Gendreau and Arzoumanian 2017) data-over two million individual X-ray pulses- we construct the single-pulse
photon-count distributions of the MP and IP at keV energies. We find that the IP is well described by the Skellam distribution expected for the difference of two Poisson processes, providing a rare, high-statistics empirical demonstration of Skellam behavior in an astrophysical
photon-counting context. The MP also shows pulse-by-pulse variability best described by a Skellam framework when compared to Gaussian alternatives, but exhibits a significant excess variance driven by high-count events. When
photon counts are summed over successive pulses, this excess averages out and the MP distribution becomes consistent with Skellam expectations, indicating that the enhanced variability does not persist across rotations. We further search for short-lag (
memory) correlations between successive X-ray pulses and find no statistically significant lag-1 correlation. Although
giant radio pulses occur in the MP phase window, their contribution is insufficient to account for the observed excess variability. Together, these results highlight a clear statistical distinction between the MP and IP and underscore the importance of using statistically appropriate models for high-energy
photon-counting analyses. The distributional fits and
memory limits reported here provide quantitative constraints on
pulsar emission models and illustrate the broader utility of Skellam-based approaches.
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All authors Max Worchel, Margaret M.Ferris, Sasha Levina, Iris Horn, Mac Tygh, Andrea N. Lommen, Kent S. Wood, Paul S. Ray, Julia S. Deneva, Natalia Lewandowska, Matthew Kerr, Jeffrey S. Hazboun, David A. Howe, Zaven Arzoumanian, Slavko Bogdanov, Craig B. Markwardt, Teruaki Enoto, Keith C. Gendreau