Cosmology and Nongalactic Astrophysics
Cosmic Birefringence from Large Lepton Asymmetry
Kentaro Kasai and Ippei Obata
Cosmic birefringence, the rotation of the polarization plane of CMB
photons, provides an intriguing probe of physics beyond the Standard Model. In this paper, we investigate a vector Chern-Simons coupling between the
neutrino current and
photons as a possible origin of isotropic
cosmic birefringence. Firstly, we perform the first analysis of anisotropic
cosmic birefringence induced by the
neutrino current. We find that the anisotropic signal sourced by lepton-number isocurvature fluctuations can be important on large scales, whereas on small scales, the anisotropic signal induced by gravitational effects becomes dominant comparing with the
axion-like particle (ALP) scenario. Secondly, we show that a large lepton asymmetry generated through the Affleck-Dine mechanism can successfully account for the observed isotropic birefringence, and discuss its possible connection to the recently reported low helium-4 abundances in Subaru EMPRESS and Atacama Cosmology
Telescope (ACT).
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Instrumentation and Methods for Astrophysics
A Deep Learning Approach to Improving the Arrival-Direction Accuracy of Gamma-Ray Air-Shower Measurements with the Tibet-III Air-Shower Array
S. Okukawa, M. Kobayashi, K. Hara et al. (+8 more)
The Tibet AS$\gamma$ experiment observes
cosmic gamma rays from several teraelectron volts to the petaelectron volt range using an array of ground-based surface air-shower detectors (Tibet-III), which is composed of plastic scintillators. The arrival direction is conventionally reconstructed by fitting the shower front to detector hit-timing data, and the resulting angular resolution is approximately 0.5$^\circ$ to 0.2$^\circ$ in the energy range from 10 to 100 TeV. In this study, to further improve the directional reconstruction accuracy, we developed a new arrival-direction reconstruction method that integrates a convolutional neural network (CNN) with the conventional method. Evaluations using gamma-ray events generated by Monte Carlo simulations show that the angular resolution is improved by approximately 10\,%–15\,% compared with that achieved with the conventional method. In addition, this performance improvement shows little dependence on the zenith angle up to 40$^\circ$. The proposed method provides a performance gain equivalent to increasing the Tibet-III array, which consists of approximately 600 detectors, by roughly 150 to 200 detectors.
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All authors S. Okukawa, M. Kobayashi, K. Hara, K. Hibino, Y. Katayose, K. Kawata, M. Ohnishi, T. K. Sako, T. Sako, A. Shiomi, M. Takita
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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Solar and Stellar Astrophysics
A Novel Approach to Identify Unseen Companions: An In-Depth study of GDR3 220_28
T. B. Cysil, Blesson Mathew, B. Shridharan et al. (+6 more)
Emission lines in the spectrum and the excess seen in the spectral energy distribution (SED) at infrared (IR) wavelengths serve as crucial indicators of
accretion activity and the presence of gas and dust in the circumstellar environment of a young
star. As
accretion ceases over several million years, the H$\alpha$ emission diminishes. However, IR excess combined with negligible H$\alpha$ emission could also extend to indicate certain close binary systems, where a circumbinary disk contributes to the observed excess. Additionally, in unresolved binaries, companion
stars can contribute to the photometry, leading to inaccuracies in parameter measurements and skewed positions in the color-magnitude diagram (CMD). We identify a photometrically peculiar object from a survey of the Cepheus
star-forming regions (SFRs), Gaia DR3 2205212960562531328, exhibiting a shift from its expected position in the CMD, as indicated by the Gaia optical photometry. This peculiarity drives our research, prompting an investigation through an approach that combines astrometric accuracy, photometric modeling and spectroscopic studies. The Gaia XP spectrum of the source shows a systematic blue excess, indicating an additional hotter unresolved component. Two-component SED modelling yields a companion with $T_{\rm eff}\sim27000$ K and $\log g\ge4.5$, consistent with sdB
stars. Further CMD and spectroscopic analyses support a binary system consisting of an A0-type PMS
star with an sdB companion. In addition, ZTF photometry shows tentative evidence for a periodicity of 0.498 days. This study paves the way for a systematic search of similar binary systems that would otherwise remain unidentified.
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All authors T. B. Cysil, Blesson Mathew, B. Shridharan, S. Muneer, Pramod Kumar S, S. Nidhi, Sreeja S Kartha, Suman Bhattacharyya, T. Robin
Astrophysics of Galaxies
Registering Gaia positions on VLBI images: constraining opacity-driven core shift in B 1038+528 / B 1038+529
M. H. Xu and Y. Y. Kovalev
Because of the complex and time-variable
radio structure of active galactic
nuclei (AGNs), absolute astrometric positions cannot generally be registered unambiguously with specific features in VLBI images. We aim to register Gaia optical centroids on VLBI images with about 0.1 mas accuracy and to locate the optical emission relative to the
radio core in the
quasar pair B 1038+528 and B 1038+529, in which the opacity-driven core shift was first discovered. We combine the relative position of the pair from phase-referenced VLBI at four epochs between 1981 and 1995, their Gaia optical positions, and the nearly orthogonal
radio-jet directions. Assuming that each Gaia–core displacement lies along the local jet axis, we register the Gaia centroids relative to the
radio cores with about 0.1 mas accuracy without using absolute VLBI positions. The Gaia centroids lie upstream of the 8.4-GHz
radio cores, by 293 {\pm} 93 {\mu}as in B 1038+529 and 169 {\pm} 96 {\mu}as in B 1038+528. The latter agrees with the 260 {\mu}as 8.4-GHz core shift implied by the 2.3–8.4 GHz shift measured for this
quasar four decades ago. The absolute astrometric position of B 1038+528 with VLBI lies about 0.6 mas downstream of the core, and its geodetic position time series drifts along the jet by more than 1 mas, showing that the 0.8–1.6 mas Gaia–VLBI offsets of the pair are dominated by source structure rather than by core shift. Gaia positions can be registered on VLBI images with an accuracy set by the astrometry uncertainties. If the Gaia centroids mark the jet base, the measured separations are absolute 8.4-GHz core shifts, corresponding to projected distances of about 1.2 pc and about 2.5 pc from the jet base; a contribution from an optical jet emission would make them lower limits. The method is applicable to AGNs with Gaia detection and phase-referenced VLBI astrometry.
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Cosmology and Nongalactic Astrophysics
LazyTB: Extended LTB-solutions with Multiple Interacting Fluids
Kimmo Kainulainen, Enrico D. Schiappacasse, Linda Tenhu et al. (+1 more)
We present a low entry barrier numerical code LazyTB that solves the Einstein equations in the ADM-formulation for a general spherically symmetric problem with an arbitrary number of fluids with user defined equations of state and optional interactions between the fluids mediated by drag-vectors. We present the ADM equations in terms of carefully chosen dimensionless variables that avoid some numerical instabilities by analytic cancellation of potentially diverging terms. We demonstrate the code by applying it to a number of examples including the standard LTB model, a model with radiation and dark matter, a polytropic dark energy model with a non-vanishing sound speed and a model with coupled interacting dark matter and dark energy. We also implement a suite of diagnostic tools, including ray-tracing
algorithms, that allow extracting physical observables such as luminosity distances for observers at arbitrary locations from the simulations. The code is built to be easy to extend also beyond the late-universe applications considered in the examples of this paper. LazyTB is available at https://github.com/ollijvaisanen/LazyTB.
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All authors Kimmo Kainulainen, Enrico D. Schiappacasse, Linda Tenhu, Olli Väisänen
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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Earth and Planetary Astrophysics
A comparison between particle and fluid approaches for streaming instability in global patch simulations
Dominik Hetzel Ostertag, Mario Flock, Paul Rosendahl et al. (+3 more)
Identifying the process relevant to
planet formation is still a major task. Mechanisms have been identified that can trap dust, preventing it from migrating inwards. A particular hydrodynamic instability, known as the streaming instability, is of high interest because it may be responsible for the formation of dust clumps from which the first planetesimals form. We aim to compare the outcome of non-linear streaming instability simulations using both fundamental approaches in a more realistic global disk model. The results will not be used to identify or disentangle the effect of, e.g., numerical
diffusion in fluid simulations or the possibility of having crossing particle streams. Instead, they will enable us to determine whether the Lagrangian particle approach is the most suitable for numerical studies of the streaming instability. Methods: We perform 2D, axisymmetric, global, and isothermal hydrodynamic simulations using the two codes FARGO3D (dust fluid) and PLUTO (fluid, particles) to investigate the temporal
evolution of dust scale height, midplane dust-to-gas ratio, and maximum dust density as well as to study velocity distributions and vorticities. For PLUTO particle simulations, we also investigate the influence of the gas Riemann solver and spatial reconstruction orders. Our results from a resolution study up to 5120
cells per gas scale height indicate convergence between PLUTO fluid and FARGO3D. Both yield a Gaussian profile dominated by small-scale, dust-depleted voids. Particle simulations with PLUTO have not yet converged, as a transient midplane dust component persists, indicating the need for even higher resolution or more particles. Using a more accurate Riemann solver or a higher-order spatial reconstruction scheme helps to bring the dust density profile closer to the converged structure.
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All authors Dominik Hetzel Ostertag, Mario Flock, Paul Rosendahl, Andrea Mignone, Jeonghoon Lim, Jacob B. Simon
Astrophysics of Galaxies
SAGE26 Paper I: Modelling the baryon cycle from cosmic dawn to the present day
Michael Bradley, Darren J. Croton, Robert Adriel Mostoghiu Paun et al. (+2 more)
We present SAGE26, a comprehensive update to the Semi-Analytic
Galaxy Evolution (SAGE) model. SAGE26 integrates five observationally motivated improvements: (1) distinct circumgalactic medium and hot-
halo gas physics, (2) molecular hydrogen for
star formation, (3) velocity- and
redshift-dependent
supernova feedback, (4) bulge radii tracking through time, and (5) feedback-free burst
galaxies. Each improvement works to regulate
galaxy formation from
cosmic dawn to the present. Most significantly, the addition of feedback-free burst
galaxies provides a pathway to produce high-
redshift massive
galaxies at the abundances observed by the James Webb Space
Telescope. Furthermore, the new circumgalactic medium framework improves the treatment of baryons by distinguishing between free-fall-limited inflow in low-mass
haloes and hydrostatic cooling in massive groups and clusters using a characteristic
halo shock mass. SAGE26 reproduces key observational constraints across
cosmic time, including the stellar mass function and
cosmic star formation rate density, while retaining the computational efficiency and flexibility of its predecessor. The code and data products are open-source and publicly available.
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All authors Michael Bradley, Darren J. Croton, Robert Adriel Mostoghiu Paun, Dhruba Dutta Chowdhury, Jayde Willingham
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
High Energy Astrophysical Phenomena
Evidence for a Similar Physical Origin Between Prompt Emission and X-Ray Flares of Gamma-Ray Bursts
Wen-Yuan Yu, Wen-Long Zhang and Hou-Jun Lü
Statistical properties of X-ray
flares are highly similar to those of the
prompt emission in gamma-ray bursts (GRBs), suggesting that they may share a common physical origin (e.g., central engine activity). However, comparable statistical studies of afterglow X-ray
flares and
prompt gamma-ray pulses that are not observed simultaneously remain incomplete. In this paper, by systematically searching for gamma-ray emission simultaneous with X-ray
flares, we identify 38 GRBs containing 50 pulse–
flare pairs, 19 of which have measured
redshifts. We perform joint temporal and spectral analyses of the gamma-ray pulses and X-ray
flares over their overlapping time intervals, and find that the spectrum of each pair can be well described by an absorbed power-law (PL), PL plus blackbody (PL+BB), cutoff power-law (CPL), and Band function. More importantly, by applying self-organized criticality (SOC) analysis to both the
prompt gamma-ray pulses and the simultaneously observed X-ray
flares, we find that the temporal parameters (e.g., waiting time, rise time, peak time, decay time, and duration) of the two components follow similar power-law distributions, with the largest absolute difference of $\Delta\alpha\sim0.36$ for the waiting time and much smaller differences ($\Delta\alpha\lesssim0.12$) for the other four parameters. For the isotropic energy derived from the subsample with measured
redshifts, the difference between the two components is $\Delta\alpha\sim0.54$. These results suggest that the X-ray
flares and the
prompt emission are likely to share the same physical origin.
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Solar and Stellar Astrophysics
Light curve spectral energy distribution fitting of 13 carbon-rich Mira variables stars in the Small Magellanic Cloud
A. Liberatori, M. Trabucchi, D. Hatzidimitriou et al. (+8 more)
Carbon-rich Mira variables are highly dynamic
stars characterized by large-amplitude radial pulsations and intense mass-loss. Standard spectral energy distribution (SED) fitting often relies on time-averaged observations, which flatten intrinsic variability and yield only approximate stellar and circumstellar parameters. To overcome these limitations, we dynamically trace the physical parameters of 13 carbon-rich Miras in the Small Magellanic Cloud (SMC) throughout their pulsation cycles. Combining time-series photometry from $Gaia$, WISE, VMC, 2MASS, and OGLE, we reconstruct simultaneous multi-band light curves. We extract independent SEDs at 14 distinct phases and model them using the ATHENA-C grid. This phase-resolved approach traces cyclic variations in effective temperature ($T_{\rm eff}$), optical depth ($\tau_{0.55}$), and bolometric luminosity. We find peak-to-peak $T_{\rm eff}$ variations ranging from $\sim 100$ K to $>1000$ K. A strong anti-correlation between $\tau_{0.55}$ and bolometric luminosity indicates the dust condensation zone is pushed outward at maximum brightness, rendering the circumstellar environment optically thinner. We also recover the classical Period-Luminosity relation and derive radial expansion velocities and radius variation amplitudes, which are consistent with independent measurements. Phase-resolved fitting reveals the true dynamic ranges of these parameters, which static fits fail to capture. Although limited by grid sampling and gaps where converged hydrostatic models are unavailable, explicitly accounting for time-dependent behavior provides a much more physically realistic picture of the interplay between stellar pulsation and dust formation. This highlights the critical need for time-resolved methodologies when studying large-amplitude variables.
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All authors A. Liberatori, M. Trabucchi, D. Hatzidimitriou, M.A.T. Groenewegen, A. Bressan, G. Pastorelli, L. Girardi, K. Shepherd, B. Aringer, A. Udalski, A. Nunnari
Solar and Stellar Astrophysics
ASTRAL: A Framework for Optimal Placement and Emulation of Stellar Evolution Models
Trevor Gravely, Marko Ristić and Brandon Barker
Generating dense grids of detailed stellar
evolution simulations is computationally prohibitive, as resolving progressively complex nucleosynthesis chains and dynamical timescales incurs a rapidly increasing computational cost that scales with dimensionality and resolution. Existing simulation grids are sparse, heterogeneous, and often cover disparate regions of parameter space, limiting (direct) systematic comparison and broader generalization to downstream science analyses. We present ASTRAL (Active Simulation Tuning and Regression for Astrophysical Libraries), a publicly available framework for optimal placement and emulation of stellar
evolution models. The ASTRAL framework builds surrogate models that provide a continuous representation of the parameter space which is populated by sparse simulation grids. Furthermore, ASTRAL interpolates across the simulation parameter space (e.g. zero-age-main-sequence mass) to predict stellar properties of
stars with associated uncertainty estimates for systems not explicitly simulated. We show that ASTRAL significantly outperforms uniform placement in constructing emulation-ready model libraries that capture both global and local variability in the output features. We showcase the performance of our framework in the context of a simple, one-dimensional MESA model. Finally, we examine how our methodology can inform downstream applications by analyzing our emulator's predictive confidence when applied to
supernova simulations. We emphasize that, while developed for stellar
evolution, the ASTRAL framework is directly extensible to other simulation-driven domains where model evaluations are expensive and parameter spaces are high-dimensional.
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High Energy Astrophysical Phenomena · Cross-listed from physics.hist-ph
The contribution of Soviet scientists to the emergence of high-energy neutrino astrophysics: from the Cherenkov effect to underwater and in-ice telescopes
Bakhromzod Rizoi
The 2026 Nobel Prize in Physics was awarded to Francis Halzen @for decisive contributions to the IceCube
Neutrino Observatory and the discovery of high-energy
neutrinos of astrophysical origin.@ This article examines the Soviet contribution to the broader historical development of high-energy
neutrino astrophysics. Four interconnected strands are distinguished: the physics of Cherenkov radiation and its application to
cosmic rays; early proposals for large-scale
neutrino detection underground and in natural water; the theory of atmospheric, cosmogenic, and astrophysical
neutrinos; and the development of underground, underwater,
radio, and in-ice detection techniques. The article shows that M. A. Markov s 1960 proposal was concerned primarily with atmospheric
neutrinos and with detecting charged secondary particles through Cherenkov radiation in water, whereas the use of Antarctic ice was initially developed in the Soviet Union in the
radio domain. The optical concept of a detector in deep polar ice was formulated by F. Halzen and J. G. Learned in 1988. The Pamir
cosmic-ray school, the theoretical work of G. T. Zatsepin, V. A. Kuzmin, and V. S. Berezinsky, the Baksan
Neutrino Observatory, Soviet participation in DUMAND, and the formation of the Lake Baikal
neutrino program are considered separately. The Pamir material is included as historical and methodological context; it does not imply a direct experimental lineage from Pamir installations to IceCube. The historical links of these strands to IceCube are therefore different in character: conceptual and citation-based in some cases, methodological or institutional in others. This distinction helps separate documented continuity of ideas from the broader scientific context in which modern high-energy
neutrino astronomy emerged.
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Astrophysics of Galaxies
Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) V. Tracing cavity walls and shocked knots with nonthermally desorbed CH$_3$OH in BHR71-IRS1
H.-S. Yun, J.-E. Lee, J. K. Jørgensen et al. (+16 more)
Complex organic molecules are detected in both cold molecular clouds and the warm inner regions of protostars. Whether they are eventually inherited by protostars and
planets or whether their chemistry is reset by in situ reactions remains unclear. Understanding the desorption mechanisms of these molecules is essential for tracing their chemical
evolution. As part of the ALMA Large Program Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS), we investigate the CH$_3$OH emission that extends beyond the hot corino of BHR71-IRS1. We identified extended CH$_3$OH emission around BHR71-IRS1. We assessed characteristic emission structures by applying a principal component analysis. The physical properties of their line-emitting gas were derived under the local thermodynamic
equilibrium (LTE) assumption through both rotational diagram and spectral modeling analyses. In total, 26 extended CH$_3$OH lines have been detected. These emission lines are distributed across (1) X-shaped cavity walls, (2) northern and southern shocked
knots, and (3) eastern and southeastern components. The cavity walls exhibit excitation temperatures of $\sim$ 25~K and column densities of $\sim$ 10$^{14}$~cm$^{-2}$, while the
knots show relatively higher excitation temperatures (70$-$120~K) and column densities ($10^{14}-10^{16}$~cm$^{-2}$). The physical properties of eastern and southeastern components are not well constrained under LTE conditions. Plausible desorption mechanisms are evaluated by comparing the physical parameters with model predictions. The low excitation temperatures and narrow line widths in the cavity walls favor a reactive desorption scenario driven by far-ultraviolet irradiation from IRS1. In contrast, the
knots with higher excitation temperatures and column densities support the dust sputtering scenario induced by C-type shocks.
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All authors H.-S. Yun, J.-E. Lee, J. K. Jørgensen, A. Coutens, A. L. Plunkett, M. N. Drozdovskaya, A. Belloche, J.-H. Jeong, P. Nazari, Y.-L. Yang, M. L. R. van 't Hoff, S. Maret, F. Cruz-Sáenz de Miera, D. Harsono, C.-H. Kim, Á. Kóspál, M. Lützen, S. Spezzano, S. Zeng
High Energy Astrophysical Phenomena
Axion effects on hybrid stars: earlier hadron-quark phase transition and larger quark-matter cores
Yu-Heng Liu, He Liu, Hong-Ming Liu et al. (+3 more)
We investigate the effects of
axions on the properties of hybrid
stars and the possible
hadron-
quark phase transition in their cores within the Maxwell construction. The hadronic phase is described by the improved isospin- and momentum-dependent interaction (ImMDI) model, while the
quark phase is modeled by a three-flavor Nambu–Jona-Lasinio (NJL) model with an
axion-modified Kobayashi-Maskawa-'t Hooft (KMT) interaction. Based on the obtained equation of state, we analyze the properties of hybrid-
star matter, including the speed of sound and the polytropic index, as well as the structural characteristics of hybrid
stars, including the mass-radius relation and the properties of the
quark core. Our study shows that the
axion field $a/f_a$ slightly stiffens the equation of state of
quark matter but significantly shifts the
hadron-
quark phase transition to lower baryon densities. These two effects compete in influencing the equation of state of hybrid-
star matter, and may even soften it, which is different from the effect of the vector coupling interaction. Consequently, the
axion field has only a weak influence on the maximum mass of hybrid
stars, but significantly increases the mass and radius of the
quark core, especially for weak vector coupling. These results indicate that, within the present Maxwell construction, the primary role of the
axion is to promote an earlier
hadron-
quark transition at low densities and to enlarge the
quark core.
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All authors Yu-Heng Liu, He Liu, Hong-Ming Liu, Min Ju, Xu-Hao Wu, Peng-Cheng Chu
High Energy Astrophysical Phenomena
Polarised X-ray emission from the accreting millisecond pulsar SAX J1808.4-3658 during an X-ray outburst and a type-I X-ray burst
A. Papitto, A. Di Marco, M.C. Baglio et al. (+22 more)
Modelling the X-ray pulse profiles of accreting millisecond
pulsars (AMSPs) is a key technique to constrain the masses and radii of
neutron stars. To this end, measuring the properties of polarised X-rays provides crucial information on the system geometry. We present the significant detection by IXPE of linearly polarised 2-8 keV X-rays during the decay of the 2025 outburst of the 2.5 ms AMSP SAX J1808.4-3658, with an average polarisation degree of $2.9\%\pm0.5\%$ and a polarisation angle of $46{\deg}\pm5{\deg}$, broadly consistent with the direction observed in the optical band. This level of polarisation is similar to that found in the only other source of this class that has been targeted so far. By using a pulse-phase timing solution obtained with IXPE and NuSTAR, which confirms the long-term
evolution of the
pulsar spin and orbit, we did not find any significant pulse phase-resolved variability in the Stokes parameters. Interpreting the polarised X-rays as originating from hot spots on the NS surface indicates they are almost aligned with the
spin axis. We also report an indication for linearly polarised emission with a degree of $16\%\pm6\%$ (2-6 keV) during the peak of a type-I thermonuclear X-ray burst, with an angle almost orthogonal to that of the persistent emission. If confirmed, this level would exceed expectations for X-rays reflected by a thin
accretion disc.
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All authors A. Papitto, A. Di Marco, M.C. Baglio, D. Baldi Tomei, C. Ballocco, G. Illiano, R. La Placa, C. Malacaria, A. Bobrikova, J. Poutanen, T. Salmi, A. Sanna, A. Veledina, F. Ambrosino, L. Burderi, S. Campana, F. Coti Zelati, D. de Martino, T. Di Salvo, G.L. Israel, F. La Monaca, A. Marino, A. Miraval Zanon, M. Pilia, L. Stella
High Energy Astrophysical Phenomena
The Prompt Emission of Gamma-Ray Bursts from Population III Stars
Nathan Walker, Davide Lazzati, Tyler Parsotan et al. (+4 more)
Population (Pop) III
stars, the first generation of
stars in the Universe, have eluded direct detection for decades. The advent of the James Webb Space
Telescope, with its unprecedented sensitivity to high-
redshift sources, has renewed interest in the prospects for identifying these primordial
stars. While previous studies have explored the direct detection of Pop III
stars through gravitational
lensing, here we investigate an alternative approach: identifying Pop III
stars through gamma-ray bursts (GRBs) produced by the deaths of massive Pop III progenitors. We generate source-frame mock observables and find that, although Pop III and Pop I GRBs share many common properties, several observables can potentially distinguish between the two populations. In particular, light-curve timescales, isotropic-equivalent energies, peak energies, and the locations of GRBs in empirical
prompt-emission correlations depend sensitively on the mass and explosion energy of the progenitor. We find that Pop III GRBs tend to exhibit longer characteristic timescales and higher isotropic-equivalent peak energies than their Pop I counterparts. Finally, we show that Pop III GRBs may provide a promising origin for the population of soft X-ray
prompt emission recently detected by the Einstein Probe, offering a complementary avenue for probing the first generations of
stars.
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All authors Nathan Walker, Davide Lazzati, Tyler Parsotan, Rosalba Perna, Matteo Cantiello, Jake Hassan, Brian Morsony
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Anisotropic Fluid Model for Massive Boson Stars: Spherically Symmetric Configurations
Ethan McKeever, Victor Guedes, Shu Yan Lau et al. (+1 more)
Boson stars are among the best-studied exotic compact objects and constitute one of the few black-hole mimickers for which both the formation mechanism and dynamical behavior can be investigated from first principles. Constructing
boson star solutions is generally more challenging than constructing
neutron star solutions, often requiring either extremely accurate initial guesses for the oscillation frequency or the use of computationally intensive relaxation methods to solve the field equations. In the strong-coupling limit, massive
boson stars can be described effectively as perfect- fluid configurations, enabling their construction using methods analogous to those developed for
neutron stars. In this paper, we relax the strong-coupling assumption and derive a phenomenological anisotropic-fluid model for spherically symmetric massive
boson stars that remains applicable over a broad range of model parameters. We provide ready-to-use fits for both the anisotropy profile and the effective equation of state in terms of the central scalar-field amplitude and the ratio of the self-interaction coupling constant to the squared scalar-field mass. To validate the model, we compute the masses and radii of
boson stars using a standard
neutron-
star construction approach and compare the results against full
boson-
star calculations. We find excellent agreement between the two methods, while the new approach reduces the computational cost by a factor of about 10. Our phenomenological fluid framework offers an efficient and accurate method for constructing spherically symmetric massive
boson stars and computing their observable properties. We provide a ready-to-use Python implementation of the Tolman-Oppenheimer-Volkoff solver, allowing users to construct
boson star configurations within the proposed framework.
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All authors Ethan McKeever, Victor Guedes, Shu Yan Lau, Kent Yagi
Instrumentation and Methods for Astrophysics
The Fast Transient Foreground Fog of LSST
G. Megias Homar, S. R. Kulkarni, C. S. Lage et al. (+3 more)
Studies with 1-m class
telescopes have now established that glints from satellites and space debris are the main obstacles to searches for sub-minute optical transients. Here, we report a study of this foreground using observations from the Legacy Survey of Space and Time (LSST) with the Simonyi 8.4-m
telescope at the Vera C. Rubin Observatory. From 2,955 pairs of consecutive 30-s exposures, spanning 4,755 deg$^2$ of unique sky area and an area-time coverage of 472 deg$^2$ hr, we identified a population of faint, PSF-like, sub-minute events, including isolated events with no linear sequences of repeated flashes (tracklets) indicative of a tumbling satellite. We extended measurements of this population to the faintest magnitudes yet probed, finding a lower-bound to the occurrence rate of $(9.55 \pm 0.14) \times10^6\,$sky$^{-1}\,$day$^{-1}$ to $\sim23$ mag (median of $\sim22.3$ mag). The large aperture of the Simonyi 8.4-m
telescope provides an additional discriminator: orbital debris in low-Earth orbit are sufficiently out of focus that they are filtered out. We find that, for a $1^{\prime\prime}$ PSF, unresolved events are constrained to distances beyond 1,732 km, and identify a previously unreported population of glints consistent with medium-Earth-orbit altitudes with estimated sizes of order $\sim1$-3 mm. We also report two isolated events observed along sight lines deep within the Earth's shadow, for which a sunlit-debris origin is disfavored. The scant evidence is consistent with an eclipsing binary and a stellar
flare. The robust way forward for sub-minute transients is simultaneous observations of the same field by two (or more) widely separated
telescopes. The resulting measurement of parallax will cleanly eliminate local (primarily orbital) debris, clarifying the nature of this transient foreground. This understanding will help astronomers probe truly celestial optical bursts.
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All authors G. Megias Homar, S. R. Kulkarni, C. S. Lage, J. A. Tyson, S. M. Kahn, F. A. Harrison
Cosmology and Nongalactic Astrophysics
Scalar Perturbations and Induced Gravitational Waves from First-Order Phase Transitions in Lattice Simulations
Yuan-Jie Li, Jing Liu, Zhuan Ning et al. (+2 more)
Cosmological first-order phase transitions can generate
curvature perturbations through inhomogeneous
quantum tunneling, as studied previously on superhorizon scales. In this work, we for the first time conduct three-dimensional
lattice simulations that incorporate scalar metric perturbations and radiation perturbations, covering a range from bubble wall scales to super-horizon scales. We obtain the precise scalar perturbation power spectrum and the probability density function of energy density perturbations. Furthermore, we simulate the gravitational-wave energy spectra generated by each source during the first-order phase transition, including the scalar field itself, scalar metric perturbations, and the energy density and velocity perturbations of radiation. For gravitational waves, the contribution from other sources can exceed $1/3$ of that from the scalar field at superhorizon scales. Additionally, we compare the effects of different values of the transition strength and rate on the results. This paper provides more accurate numerical results for research aimed at detecting or constraining first-order phase transitions via gravitational waves and
curvature perturbations.
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All authors Yuan-Jie Li, Jing Liu, Zhuan Ning, Xiang-Xi Zeng, Zong-Kuan Guo
Instrumentation and Methods for Astrophysics
Constrained Diffusion for Data-Scarce Orbital Monte Carlo in Constellation Tasking
Omar Ramadan, Sam Siavoshian, Amir Kashif Saeed et al. (+3 more)
Constellation Monte Carlo results depend on the orbital population used to evaluate a tasking policy. With scarce reference trajectories, replay limits
geometric diversity, while independent orbital-element jitter can violate physical constraints. We study constrained
diffusion for orbital-population augmentation. A force-conditioned
diffusion model learns a 13-dimensional orbital prior, recovering semimajor axis from perigee altitude and eccentricity; Basilisk propagates each sample under one of five force-model tiers. Using 800 reference trajectories, we compare
diffusion with jittered bootstrap, per-tier Gaussian mixtures, and a conditional variational autoencoder, and evaluate distributional fidelity, support shift, classical astrodynamics diagnostics, and 4,000 paired GoDSAT-compatible campaigns. The largest
diffusion model achieves held-out trajectory MMD of 0.0171 +/- 0.0239, similar to bootstrap (0.0170) and the mixture (0.0198), but samples farther from training priors (median nearest-training distance 2.7 versus 0.10 standardized units). All generators fail to match the shifted-blind population (classifier AUC 0.991-0.998). Endpoint-conditioned samples satisfy Lambert boundaries but have greater interior error than the matched Lambert reference (29.2 versus 5.7 km mean RMSE). Residual
diffusion improves selected sparse forecasts and catalog-mode recall but does not outperform classical
estimators on custody ranking. In a fixed 16-satellite configuration,
diffusion yields similar mean custody to replay and bootstrap, while the force-tier mixture shifts custody by about 5.5 percentage points. Constrained
diffusion supports local, in-support augmentation but cannot replace orbital dynamics or serve as an operational posterior. Orbital-population construction is a consequential source of uncertainty in constellation analysis.
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All authors Omar Ramadan, Sam Siavoshian, Amir Kashif Saeed, Benjamin A. Johnson, Amin M. E.-A. Diab, Benjamin M. Rodriguez