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Physics Astrophysics

astro-ph Watch Astrophysics

Thursday 30 October

Solar and Stellar Astrophysics

The light curve model fitting of LMC Cepheids: MESA-RSP versus Stellingwerf's code predictions

M. Deka, M. Marconi, R. Molinaro et al. (+9 more)

A major challenge in modeling classical Cepheids is the treatment of convection, particularly its complex interplay with pulsation. This inherently three-dimensional process is typically approximated in one-dimensional hydrocodes using dimensionless turbulent convection (TC) free parameters. Calibrating these parameters is essential for reproducing key observational features such as light-curve amplitudes, secondary bumps, and the red edge of the instability strip. In this work, we calibrate TC parameters adopted in the publicly available MESA-RSP code through comparison with both observational data of classical Cepheids and stellar parameter constraints from the Stellingwerf code. We compute multi-band (V, I, and Ks) MESA-RSP light curves for 18 observed Large Magellanic Cloud Cepheids, using stellar parameters determined from the Stellingwerf code. By fine-tuning the mixing-length and eddy viscosity parameters, we calibrate the TC treatment in MESA-RSP. We then compare the resulting period-luminosity (PL), period-radius (PR), and period-mass-radius (PMR) relations with predictions from the Stellingwerf models. We successfully reproduce multi-band light curves and obtain PL, PR, and PMR relations consistent with Ragosta et al. (2019). While in broad agreement with previous work, we explicitly identify distinct mass-luminosity (ML) relations for fundamental-mode and first-overtone Cepheids for the first time. This suggests that the macroscopic processes affecting the ML relation depend on stellar mass and/or effective temperature range. Although our study focuses on the calibration of TC parameters, we do not find a single set of parameter values that reproduces all light curves. No statistically significant correlation is found between stellar properties and convection parameters, although subtle trends with period and effective temperature may be present.
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All authors M. Deka, M. Marconi, R. Molinaro, G. De Somma, A. Bhardwaj, E. Trentin, S. Deb, T. Sicignano, I. Musella, V. Ripepi, E. Luongo, Shashi M. Kanbur

Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Breaking Eternal Inflation: Empirical Viability of a Spontaneous Collapse Scenario

María Pía Piccirilli, Gabriel León, Rosa-Laura Lechuga-Solis et al. (+1 more)

We revisit an inflationary scenario in which primordial inhomogeneities arise from a quantum collapse, a stochastic mechanism described in the context of quantum collapse theories in its continuous version and within semiclassical gravity. The predictions of the model show a non-conventional scalar spectrum governed by two new parameters in the collapse rate, whose aim is twofold: on one side, to account for the primordial cosmic structure, and on the other to explain the suppression amplitude associated with long-wavelength modes, thereby eliminating the occurrence of eternal inflation. Furthermore, this model can contribute to accounting for the lack of power anomaly in the low $l$ angular power spectra of the Cosmic Microwave Background (CMB). Using the latest data from the Planck (2018) collaboration, we establish observational constraints on the model parameters, which produce a characteristic low-$\ell$ suppression in the cosmic microwave background spectrum. We conclude that the Planck data support the solution presented in the previous works, in other words, that the model allows us to solve simultaneously the emergence of the cosmic structure and, at the same time, avoid the eternal inflation scenario.
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All authors María Pía Piccirilli, Gabriel León, Rosa-Laura Lechuga-Solis, Daniel Sudarsky

Cosmology and Nongalactic Astrophysics

$\Lambda$vCF: Extending $\Lambda$CDM into a Unified Model with Particle Creation

Vishnu A Pai and Titus K Mathew

We present a novel extended version of the $\Lambda$CDM model that provides analytical solution for Hubble parameter uniting all epochs of cosmic evolution starting from inflation to late-acceleration, with intermediate radiation and matter-dominated epochs. This is achieved by relaxing the perfect fluid assumption in the standard model and considering a general viscous cosmic fluid (vCF) with non-zero particle creation rate and evolving adiabatic equation of state. Transition points of the Universe and the finite boundary connecting them is exactly determined. We then propose a novel method to determine the early-time viscous coefficient and inflation energy scale using the Cosmic Mode Index value postulated by Padmanabhan. Considering the data from the Planck 2018 analysis, this yields an inflationary Hubble parameter of $H_{I}\approx10^{13}$\,GeV. An equivalent scalar-field description for the inflationary epoch is then constructed and inferences are made regarding the nature of inflation. Notably, we find that the model describes an ultra-slow-roll hilltop inflation scenario with a graceful exit to radiation-dominated epoch. Subsequently, we show that bulk viscosity in this model can be expressed as Israel-Stewart equation in relativistic dissipative hydrodynamics with an appropriate underlying viscous coefficient and relaxation time that satisfy the causality constraint in its extreme limit. Finally, by comparing the evolution of this causal relation and its Navier-Stokes counterpart, we infer that the evolution from inflation to radiation era signifies a fluid transitioning from viscoelastic to pseudoplastic behavior.
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Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc

Mapping Anisotropies in the Stochastic Gravitational-Wave Background with space detector networks

Zhi-Yuan Li, Zheng-Cheng Liang, Cong-mao Zhang et al. (+2 more)

Future space-based gravitational-wave detectors such as TianQin, LISA, and Taiji are expected to conduct joint observations. Such a multi-detector network will provide complementary viewing angles for the anisotropic stochastic gravitational-wave background (SGWB), thereby significantly enhancing the capability to reconstruct and localize its spatial distribution. In this paper, we have established the first dedicated data analysis pipeline for the anisotropic stochastic gravitational-wave background using a joint network of TianQin, LISA, and Taiji. Our analysis incorporates both Gaussian, stationary, and unpolarized point sources from diverse sky locations as well as a random sky map. We have performed full-sky map reconstruction in pixel space using maximum likelihood estimation to extract the angular distribution of the SGWB. The results demonstrate that, when considering the detector noise, the TianQin+LISA+Taiji detector network can reconstruct the angular power spectrum of the stochastic background up to a maximum multipole moment of $l = 14 $, which can provide valuable information for studies on the spatial distribution of galactic compact binaries and physical imprints from the early Universe.
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All authors Zhi-Yuan Li, Zheng-Cheng Liang, Cong-mao Zhang, Jian-dong Zhang, Yi-Ming Hu

Cosmology and Nongalactic Astrophysics

Model-independent late-universe measurements of $H_0$ and $\Omega_K$ with the parametrization based on cosmic age-improved inverse distance ladder

Guo-Hong Du, Tian-Nuo Li, Jia-Le Ling et al. (+3 more)

The standard $\Lambda{\rm CDM}$ model has encountered serious challenges and the $H_0$ tension has become more significant with increasingly precise cosmological observations. Meanwhile, inconsistencies in measurements of the curvature parameter $\Omega_\mathrm{K}$ between different datasets also have emerged. In this work, we employ two global and cosmic age-based parameterizations, PAge and MAPAge, to perform model-independent measurements of the Hubble constant $H_0$ and $\Omega_\mathrm{K}$ by utilizing the inverse distance ladder (IDL). To construct the PAge-improved IDL, we utilize the strong gravitational lensing (SGL), cosmic chronometers (CC), and gamma ray bursts (GRB) data to calibrate the latest DESI DR2 baryon acoustic oscillation and DESY5 or DES-Dovekie type Ia supernova data. Our analysis indicates that DESI+DES-Dovekie+SGL+CC+GRB gives $H_0=72.20\pm 1.00\,{\rm km}~{\rm s}^{-1}~{\rm Mpc}^{-1}$ in the MAPAge model, reducing the $H_0$ tension to the $0.6\sigma$ level. Extending to the MAPAge$+\Omega_{\rm K}$ model, we obtain $\Omega_\mathrm{K}=0.005\pm 0.037$, which suggests that current late-time data are consistent with a flat universe. Finally, the Bayesian analysis indicates that the present late-universe data provide weak to moderate evidence in favor of PAge and MAPAge relative to $\Lambda{\rm CDM}$.
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All authors Guo-Hong Du, Tian-Nuo Li, Jia-Le Ling, Yan-Hong Yao, Jing-Fei Zhang, Xin Zhang

Astrophysics of Galaxies

oMEGACat. VIII. A Subpopulation Census of {\omega} Centauri

C. Clontz, A. C. Seth, Z. Wang et al. (+8 more)

An understanding of the assembly history of the complex star cluster Omega Centauri has long been sought after, with many studies separating the stars on the color-magnitude diagram into multiple groupings across small magnitude ranges. Utilizing the oMEGACat combined astro-photometric and spectroscopic dataset we parse 14 subpopulations from the upper red-giant branch to below the main-sequence turnoff. We combine our results with previous works to estimate the age and age spread of each population. We find that the chemically enhanced (P2) populations are all ~1 Gyr younger (~11.6 Gyr old) and have significantly higher intrinsic age spreads (0.6 Gyr) than the primordial (P1) populations (~12.6 Gyr old, 0.3 Gyr spread), with the intermediate (Im) populations falling in between the two. Additionally, we connect for the first time the Chromosome Diagram to the two-stream age-metallicity relation, allowing us to link the P1 and P2 stars to the distinct star formation tracks, proposed to be in-situ and ex-situ contributions to the cluster's assembly. Our results are consistent with some suggested formation models and rule out others but no current model can explain all observed features of the subpopulations.
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All authors C. Clontz, A. C. Seth, Z. Wang, M. Haeberle, M. S. Nitschai, N. Neumayer, P. J. Smith, M. Latour, A. Feldmeier-Krause, M. Libralato, A. Bellini

Instrumentation and Methods for Astrophysics

ampere: A tool to fit heterogeneous observations consistently

P. Scicluna, S. Zeegers, J. P. Marshall et al. (+5 more)

As astronomy advances and data becomes more complex, models and inference also become more expensive and complex. In this paper we present ampere, which aims to solve this problem using modern inference techniques such as flexible likelihood functions and likelihood-free inference. ampere\ can be used to do Bayesian inference even with very expensive models (hours of CPU time per model) that do not include all the features of the observations (e.g. missing lines, incomplete descriptions of PSFs, etc). We demonstrate the power of \ampere\ using a number of simple models, including inferring the posterior mineralogy of circumstellar dust using a Monte Carlo Radiative Transfer model. ampere\ reproduces the input parameters well in all cases, and shows that some past studies have tended to underestimate the uncertainties that should be attached to the parameters. ampere\ can be applied to a wide range of problems, and is particularly well-suited to using expensive models to interpret data.
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All authors P. Scicluna, S. Zeegers, J. P. Marshall, F. Kemper, S. Srinivasan T.E. Dharmawardena, L. Fanciullo, O. Morata, A. Trejo-Cruz

Astrophysics of Galaxies

Tracing the evolution of brightest galaxies and diffuse light in galaxy groups

B. Bilata-Woldeyes, J. D. Perea and J. M. Solanes

We present a suite of 100 cosmologically motivated, controlled N-body simulations designed to advance the understanding of the role of purely gravitational dynamics in the early formation of low-mass galaxy groups (~ 1-5 x 10^13 M_sun). In this work, we investigate the temporal evolution of key indicators of dynamical relaxation, with particular emphasis on the secular growth of the diffuse intragroup light (IGL), the four major group galaxies, and the mass distributions of their progenitors. We also assess the diagnostic power of several magnitude gaps between top-ranked galaxies as proxies for dynamical age. As in our previous study, we compare outcomes from three group classes defined by the number of brightest group galaxies (BGGs) present at the end of the simulations. The early assembly of galaxy groups is consistent with a stochastic Poisson process at an approximately constant merger rate. Various dynamical diagnostics - including galaxy pairwise separations, velocity dispersions, and the offset of the first-ranked galaxy from the group barycentre - indicate that single-BGG groups evolve more rapidly towards virialisation than double- and especially non-BGG systems. We further find that first-ranked group members and the IGL, follow distinct growth histories, with the IGL assembled from a more numerous and systematically lower-mass population than the central object. This distinction is particularly pronounced in non-BGG systems, where about one third of the IGL originates from small galaxies, each contributing less than 5% to this component. Among the tested magnitude gaps, the difference between the first- and fourth-ranked galaxies, $\Delta M4-1$, proves a more robust indicator of dynamical age for low-mass groups than the conventional $\Delta M2-1$ gap. The $\Delta M5-1$ and $\Delta M6-1$ gaps also perform well and may be preferable in certain contexts.
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Astrophysics of Galaxies

Shock-driven heating in the circumnuclear star-forming regions of NGC 7582: Insights from JWST NIRSpec and MIRI/MRS spectroscopy

Oscar Veenema, Niranjan Thatte, Dimitra Rigopoulou et al. (+24 more)

We present combined JWST NIRSpec and MIRI/MRS integral field spectroscopy data of the nuclear and circumnuclear regions of the highly dust obscured Seyfert 2 galaxy NGC 7582, which is part of the sample of AGN in the Galaxy Activity, Torus and Outflow Survey (GATOS). Spatially resolved analysis of the pure rotational H$_2$ lines (S(1)-S(7)) reveals a characteristic power-law temperature distribution in different apertures, with the two prominent southern star-forming regions exhibiting unexpectedly high molecular gas temperatures, comparable to those in the AGN powered nuclear region. We investigate potential heating mechanisms including direct AGN photoionisation, UV fluorescent excitation from young star clusters, and shock excitation. We find that shock heating gives the most plausible explanation, consistent with multiple near- and mid-IR tracers and diagnostics. Using photoionisation models from the PhotoDissociation Region Toolbox, we quantify the ISM conditions in the different regions, determining that the southern star-forming regions have a high density ($n_H \sim 10^{5}$ cm$^{-3}$) and are irradiated by a moderate UV radiation field ($G_0 \sim 10^{3}$ Habing). Fitting a suite of Paris-Durham shock models to the rotational H$_2$ lines, as well as rovibrational 1-0 S(1), 1-0 S(2), and 2-1 S(1) H$_2$ emission lines, we find that a slow ($v_s \sim 10$ km/s) C-type shock is likely responsible for the elevated temperatures. Our analysis loosely favours local starburst activity as the driver of the shocks and circumnuclear gas dynamics in NGC 7582, though the possibility of an AGN jet contribution cannot be excluded.
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All authors Oscar Veenema, Niranjan Thatte, Dimitra Rigopoulou, Ismael García-Bernete, Almudena Alonso-Herrero, Anelise Audibert, Enrica Bellocchi, Andrew J. Bunker, Steph Campbell, Francoise Combes, Ric I. Davies, Daniel Delaney, Fergus Donnan, Federico Esposito, Santiago García-Burillo, Omaira Gonzalez Martin, Laura Hermosa Muñoz, Erin K. S. Hicks, Sebastian F. Hoenig, Nancy A. Levenson, Chris Packham, Miguel Pereira-Santaella, Cristina Ramos Almeida, Claudio Ricci, Rogemar A. Riffel, David Rosario, Lulu Zhang

Cosmology and Nongalactic Astrophysics

Weak-Lensing Detection of Intercluster Filaments in Three Nearby Cluster Systems

Rahul Shinde and Ian Dell'Antonio

Direct detection of intercluster filaments is challenging due to their low surface density, resulting in a weak deflection field. We present weak-lensing detections of intercluster filaments using wide-field Dark Energy Camera (DECam) observations from the Local Volume Complete Cluster Survey (LoVoCCS). A matched-filter method was applied to identify filamentary structures in three nearby ($z < 0.1$) systems centered on Abell 401, Abell 2029, and Abell 3558. We discover two prominent filaments ($\geq 4\sigma$) in each system, with the strongest detections ($6.4\sigma - 7.3\sigma$) around Abell 401 and Abell 2029. In particular, we report the first robust weak-lensing detections $(>5 \sigma)$ of the intercluster bridges connecting the cluster pairs Abell 401/399, Abell 2029/2033, Abell 2029/SIG, and Abell 3558/3556. Adopting a filament convergence model motivated by numerical simulations, we infer the maximum convergence ($\kappa_0$) and characteristic width ($h_{\mathrm{c}}$) for all six filaments, yielding $\kappa_0 \sim 0.015 - 0.053$ and $h_{\mathrm{c}} \sim 0.11 - 0.45 \ \mathrm{Mpc}$. The performance of the matched-filter technique is validated using mock shear catalogs and further tested on a null field around Abell 2351. We also explore the potential of using the B-mode lensing signal of filaments to suppress cluster-induced shear contamination. These results demonstrate the feasibility of directly mapping dark matter filaments with current and future wide-field weak-lensing datasets.
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High Energy Astrophysical Phenomena · Cross-listed from physics.comp-ph

Generative Artificial Intelligence for Air Shower Simulation

C. Bozza, A. Calivà, A. De Caro et al. (+7 more)

The detailed simulation of extensive air showers, produced by primary cosmic rays interacting in the atmosphere, is a task that is traditionally undertaken by means of Monte Carlo methods. These processes are computationally intensive, accounting for a major fraction of the computational resources used in the large-scale simulations required by current and future experiments in the field of astroparticle physics. In this work, we present a novel approach based on Generative Adversarial Networks (GANs) to accelerate air shower simulations. We developed and trained a GAN on a dataset of high-energy proton-induced air showers generated with CORSIKA; our model reproduces key distributions of secondary particles, such as energy spectra and spatial distributions at ground level of muons. Once the model has been trained, which takes approximately 74 hours, the generation real time per shower is reduced by a factor of $10^4$ with respect to the full CORSIKA simulation, leading to a substantial decrease in both computational time and energy consumption.
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All authors C. Bozza, A. Calivà, A. De Caro, D. De Gruttola, S. De Pasquale, L.A. Fusco, G. Messuti, C. Poirè, S. Scarpetta, T. Virgili

Earth and Planetary Astrophysics

Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic Ray Processing

R. Maggiolo, F. Dhooghe, G. Gronoff et al. (+2 more)

Spectral observations of 3I/ATLAS (C/2025 N1) with JWST/NIRSpec and SPHEREx reveal an extreme CO2 enrichment (CO2/H2O = 7.6+-0.3) that is 4.5 sigma above solar system comet trends and among the highest ever recorded. This unprecedented composition, combined with substantial absolute CO levels (CO/H2O = 1.65+-0.09) and red spectral slopes, provides direct evidence for galactic cosmic ray (GCR) processing of the outer layers of the interstellar comet nucleus. Laboratory experiments demonstrate that GCR irradiation efficiently converts CO to CO2 while synthesizing organic-rich crusts, suggesting that the outer layers of 3I/ATLAS consist of irradiated material which properties are consistent with the observed composition of 3I/ATLAS coma and with its observed spectral reddening. Estimates of the erosion rate of 3I/ATLAS indicate that current outgassing samples the GCR-processed zone only (depth ~15-20 m), never reaching pristine interior material. Outgassing of pristine material after perihelion remains possible, though it is considered unlikely. This represents a paradigm shift: long-residence interstellar objects primarily reveal GCR-processed material rather than pristine material representative of their primordial formation environments. With 3I/ATLAS approaching perihelion in October 2025, immediate follow-up observations are critical to confirm this interpretation and establish GCR processing as a fundamental evolutionary pathway for interstellar objects.
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All authors R. Maggiolo, F. Dhooghe, G. Gronoff, J. de Keyser, G. Cessateur

Solar and Stellar Astrophysics

Apsidal Motion in O-Star Binaries: GENEC rotating binary models put to the k2-test

Sophie Rosu, Luca Sciarini, Sylvia Ekström et al. (+1 more)

Unveiling massive stars' internal structure and the physical origin and efficiency of the internal mixing processes? It is now possible using the apsidal motion rate in close eccentric binaries! The apsidal motion rate depends on the tidal interactions occurring between the stars and is proportional to k2, a measure of the star's inner density profile. Confronting standard stellar models with observations reveals the famous k2-discrepancy: models predict too high a k2 for the stars, that is to say, stars with too low a density contrast between their core and envelope. We built bespoke GENEC stellar evolution models including tidal mixing for the twin massive binary HD 152248. The models reveal the instabilities allowing to reproduce the stellar density profiles: advecto-diffusive models better reproduce k2 than magnetic models. A large overshooting is necessary to converge towards the observed k2, yet alone is not sufficient. While a change in metallicity or mass-loss rate has no significant impact on k2, a larger initial helium abundance allows us to better reproduce the k2. Yet, a super-solar helium abundance is not observationally supported. Our analyses highlight the need for a process in the stars that slows down the radial expansion.
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All authors Sophie Rosu, Luca Sciarini, Sylvia Ekström, Raphaël Hirschi

Astrophysics of Galaxies

The role of black hole feedback on galaxy star formation and the degeneracy with halo quenching

Hao Fu, Francesco Shankar, Feng Yuan et al. (+9 more)

The interplay between the accretion of supermassive black holes (SMBHs) and the stellar mass growth of the host galaxies is still a matter of hot debate. The accretion of the SMBHs is expected to release energy under the form of AGNs. This energy is believed to impact the star formation activity and contribute to the quenching of galaxies. Here, we address this key unsolved issue with our cosmological semi-empirical model DECODE. In DECODE, we grow galaxies with their SFR linked to halo accretion rate distributions via abundance matching. SMBHs are evolved following the stellar mass growth of their host galaxies by assigning an accretion rate at each redshift from the empirical Eddington ratio distributions and duty cycles. We test the assumption that galaxies permanently quench when their central SMBHs approach the limit imposed by the observed $M_{\rm BH} - \sigma_\star$ relation, as a proxy of SMBH disruptive feedback. We find that simply imposing the $M_{\rm BH} - \sigma_\star$ condition is sufficient to generate a fraction of quenched galaxies consistent with current data, including the newest ones from Euclid. In addition, our minimal, data-driven model, also predicts SMBH scaling relations consistent in slope and normalisation with those observed, and an $M_{\rm BH} - M_\star$ relation weakly evolving with redshift. The model also naturally generates SMBH accretion rates peaking within 1 Gyr of their host SFHs. We note that all the main predictions on galaxy quenched fractions and SMBH growth histories and scaling relations are degenerate with those expected in a halo quenching model. The comprehensive data-driven model presented in this work represents an invaluable tool to investigate SMBH demography across time and environments in an accurate, physically motivated manner, ideally suited to rapidly explore the implications from large surveys, such as Euclid and Rubin-LSST.
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All authors Hao Fu, Francesco Shankar, Feng Yuan, Daniel Roberts, Lumen Boco, Andrea Lapi, Pablo Corcho-Caballero, Mohammadreza Ayromlou, Antonis Georgakakis, Brivael Laloux, Iván Muñoz Rodríguez, Yingjie Peng

Astrophysics of Galaxies

The SOMA MM Survey. I. An Astrochemical Census of Massive Protostars

D. Gigli, P. Gorai, C.Y. Law et al. (+9 more)

During massive star formation, dense gas undergoes chemical evolution, producing both simple and complex organic molecules (COMs) characteristic of hot molecular cores. How this evolution depends on protostellar physical properties remains unclear. We investigate the chemical content of 22 well-studied massive protostars from the SOFIA Massive (SOMA) Star Formation survey, aiming to identify correlations between chemical and physical parameters. We analyzed Atacama Compact Array and Total Power 1.3 mm (Band 6) data, deriving column densities, line widths, and excitation temperatures of multiple molecular species by modeling detected lines under local thermodynamic equilibrium (LTE) using MADCUBA. Spectra show 35 species, from simple molecules (e.g., CO, SO, SiO) to complex organic molecules (COMs), with seven sources exhibiting high chemical complexity (> 100 transitions). Average excitation temperatures vary across the sample: $T_\text{ex}>100~\text{K}$ for eight sources, $50-100~\text{K}$ for four, and $T_\text{ex} < 50~\text{K}$ for the remainder. Sources with $T_\text{ex} < 50~\text{K}$ trace lukewarm, chemically simple gas, while those with $T_\text{ex}>100~\text{K}$ indicate the presence of typical hot cores where thermal desorption is efficient, resulting in line-rich spectra. Comparing these chemical properties with the bolometric luminosity to envelope mass ratio ($L_\text{bol}/M_\text{env}$), an evolutionary tracer, we find tentative correlations with line widths, excitation temperature, and column densities. These data provide important constraints for chemodynamical models of massive protostellar cores.
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All authors D. Gigli, P. Gorai, C.Y. Law, J. C. Tan, M. Bonfand, T. Rahman, Y. Zhang, K. Taniguchi, R. Fedriani, Z. Telkamp, V. Rosero, G. Cosentino

Solar and Stellar Astrophysics

Theoretical models for the Late Thermal Pulse in post-AGB stars: the case of DY Cen

Zhongyang Liu, C. Simon Jeffery, Xianfei Zhang et al. (+2 more)

We present theoretical predictions of the born-again scenario for post-asymptotic giant-branch stars. An extensive model grid for born-again objects has been constructed, particularly including models for the Very Late Thermal Pulse with and without convective overshooting, and also including models for the Late Thermal Pulse. We constructed a large parameter space to analyze the dependencies of the born-again model on core mass, hydrogen-envelope mass, and overshoot parameters, and we analyzed how changes in these parameters affect the models' evolution. We applied our grid of models to interpret observations of DY\,Cen, a star exhibiting characteristics similar to confirmed born-again stars. We compared DY\,Cen with models from multiple aspects, including heating rate, evolutionary tracks, and surface abundances. Ultimately, we concluded that none of our born-again models could match all of the observed properties of DY\,Cen, especially its surface chemistry; DY\,Cen is therefore an unlikely born-again star.
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All authors Zhongyang Liu, C. Simon Jeffery, Xianfei Zhang, Shaolan Bi, Tanda Li

Instrumentation and Methods for Astrophysics

A Large-Area Optical Time Projection Chamber for Hard X-ray Polarimetry with Directional Imaging of Low-Energy Electron Recoils

Davide Fiorina, Elisabetta Baracchini, Giorgio Dho et al. (+8 more)

We report on the development of a large-volume, wide field-of-view time projection chamber (TPC) for X-ray polarimetry, featuring a triple-GEM amplification stage and optical readout. Originally developed within the CYGNO program for directional dark matter searches, the system employs a scientific CMOS (sCMOS) camera and a photomultiplier tube (PMT) to collect secondary scintillation light produced during charge amplification. A prototype with a cylindrical active volume (radius 3.7 cm, height 5 cm) was tested at the INAF–IAPS calibration facility (Rome, Tor Vergata) to assess sensitivity to low-energy electron directionality. We fully reconstruct electrons in the 10-60 keV range, obtain angular resolutions as good as 15{\deg}, and infer modulation factors up to 0.9. These first results demonstrate robust photoelectron tracking at tens of keV with strong modulation, indicating that photoelectric-effect polarimetry can be extended to higher energies. This capability is promising for rapid transients (GRBs, solar flares) and would broaden the astrophysical reach of X-ray polarimetry.
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All authors Davide Fiorina, Elisabetta Baracchini, Giorgio Dho, Paolo Soffitta, Samuele Torelli, David J. M. Marques, Enrico Costa, Sergio Fabiani, Fabio Muleri, Giovanni Mazzitelli, Atul Prajapati

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

Testing the type-II seesaw mechanism with gravitational waves

Yonghua Wang and Wei Chao

Traditional seesaw mechanisms provide an elegant theoretical framework for explaining the small yet non-zero masses of neutrinos. Nevertheless, they face significant experimental challenges, primarily because the energy scale associated with the seesaw mechanism is too high to be directly probed in terrestrial experiments. In this paper, we explore the gravitational waves (GWs) generated via graviton bremsstrahlung during the decay of seesaw particles in the early Universe. Specifically, we compute the GW spectrum resulting from the decay of the Higgs triplet within the type-II seesaw model. Our results demonstrate that the resulting GW spectrum depends sensitively on the mass of the Higgs triplet and its couplings to the Standard Model Higgs doublet and the left-handed lepton doublet. The detection of such a high-frequency GW background could offer a unique experimental window into the seesaw mechanism and provide indirect evidence for its validity.
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Solar and Stellar Astrophysics

Stellar Parameters and Evolutionary Pathways of the Subgiant system HIP 72217

Naufa Nazar, Mashhoor A. Al-Wardat, Ahmad Abushattal et al. (+1 more)

In this study, we applied Al-Wardat's method to analyze the subgiant system HIP72217 for which we obtained accurate parameters including stellar masses, effective temperatures ($T_{\text{eff}}$) and system age.For the primary component we determined a stellar mass of $M_A = 1.14 \pm 0.15\,M_{\odot}$ and effective temperature $T_{\text{eff,A}} = 6125 \pm 50$\,K while for the secondary component we obtained the values of $M_B = 1.12 \pm 0.14\,M_{\odot}$ and $T_{\text{eff,2}} = 5950 \pm 50$\,K. The system's age was estimated to be $3.548 Gyr$, which is consistent with the predicted evolutionary period of a subgiant binary. The evolutionary timeline of HIP\,72217 becomes clearer through our study, which also demonstrates Al-Wardat's approach as an effective approach for binary star system characterization. These findings contribute to a better understanding of the physical mechanisms that control subgiant binary evolution and their broader role in stellar evolutionary processes.
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All authors Naufa Nazar, Mashhoor A. Al-Wardat, Ahmad Abushattal, Hassan B. Haboubi

Solar and Stellar Astrophysics

A new particle-based code for Lagrangian stochastic models applied to stellar turbulent convection

J. Philidet and K. Belkacem

The inclusion of convection in stellar evolution models lacks realism, especially near convective-radiative interfaces. Furthermore, the interaction of convection with oscillations prevent us from accurately predicting seismic frequencies, and therefore from fully exploiting the asteroseismic data of low-mass stars. We aim to develop a new formalism to model the one-point statistics of stellar convection, to implement it in a new numerical code, and to validate this implementation against benchmark cases. This new formalism is based on Lagrangian Probability Density Function (PDF) methods, where a Fokker-Planck equation for the PDF of particle-based turbulent properties is integrated in time. We then develop a Monte-Carlo implementation of this method, where the flow is represented by a large number of notional particles acting as realisations of the PDF. Notional particles interact with each other through the time- and space-dependent mean flow, which is estimated from the particle realisations through a scheme similar to Smoothed Particle Hydrodynamics. We establish a model for the evolution of turbulent properties along Lagrangian trajectories applicable to stellar turbulent convection, with only a minimal number of physical assumptions necessary to close the system. In particular, no closure is needed for the non-linear advection terms, which are included exactly through the Lagrangian nature of formalism. The numerical implementation of this new formalism allows us to extract time-dependent maps of the statistical properties of turbulent convection in a way which is not possible in grid-based large-eddy simulations, in particular the turbulent pressure, Reynolds stress tensor, internal energy variance and convective flux.
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High Energy Astrophysical Phenomena

Rotation Measure Analysis of Shocks and Sloshing Fronts in a Galaxy Cluster Merger Simulation

Jia-Rou Liou, Alvina Y. L. On, H. -Y. Karen Yang et al. (+1 more)

Recent observations of the Fornax cluster show depolarization signatures on megaparsec scales, which may be associated with shocks and/or sloshing motions during cluster merger and/or in-fall. To investigate the possible reasons behind the depolarization, we carry out analytical and full polarized radiative transfer (PRT) calculations of radio point sources behind a merging galaxy cluster simulated using the FLASH code. With uniform background light, we analyzed the rotation measure (RM) morphology near the shock front and the cluster center, where sloshing cold fronts appear. For the shock scenario, we find a local RM enhancement by $\sim56\%$ behind the shock front on megaparsec scales, arising from the compression of hot gas and magnetic field lines. Behind the sloshing cold front, the cluster center shows decrement in RM magnitude by $\sim23.3\%$, as a result of the cancellation effect of randomly-oriented magnetic fields induced by sloshing-driven turbulence. We find that beam depolarization increases behind shock fronts and across sloshing cold fronts, indicating enhanced magnetic field fluctuations across the plane of the sky in both scenarios. By fully accounting for all radiative transfer coefficients in the PRT calculations, the uniform background light becomes more depolarized near the cluster center, with the effect growing more pronounced as background intensity decreases. This suggests that synchrotron emission and Faraday rotation of the intracluster medium can significantly influence the polarization of background sources.
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All authors Jia-Rou Liou, Alvina Y. L. On, H. -Y. Karen Yang, J. A. ZuHone

High Energy Astrophysical Phenomena

Multi-Faceted Emission Properties of PSR J2129+4119 Observed with FAST

Habtamu Menberu Tedila, Di Li, Pei Wang et al. (+10 more)

We present a detailed single-pulse study of the long-period pulsar PSR J2129+4119 using high-sensitivity FAST observations. Despite locating well below the traditional death line, the pulsar exhibits sustained and multi-modal emission behavior, including nulls, weak pulses, regular emission, and occasional bright pulses. The nulling fraction is measured to be $8.13\% \pm 0.51\%$, with null durations typically under four pulse periods. Fluctuation spectral analysis reveals both phase-modulated subpulse drifting and intermittent beat-like modulation. At the same time, polarization profiles show high linear polarization and stable polarization position angle (PPA) swings consistent with a near-tangential sightline geometry. Quasi-periodic microstructures are detected in 11.54% of regular pulses, with a mean periodicity and width of 4.57 ms and 4.30 ms, respectively. A well-defined scintillation arc in the secondary spectrum confirms the presence of a localized scattering screen. These results indicate that PSR J2129+4119 remains magnetospherically active and coherently emitting despite its low energy loss rate, offering key insights into pulsar emission physics near the death line.
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All authors Habtamu Menberu Tedila, Di Li, Pei Wang, Rai Yuen, Ziwei Wu, Shijun Dang, Jianping Yuan, Na Wang, Marilyn Cruces, Jun Shuo Zhang, Juntao Bai, De Zhao, FAST Collaboration

High Energy Astrophysical Phenomena

Direct Numerical Simulations of Oxygen-Flame-Driven Deflagration-to-Detonation Transition in Type Ia Supernovae

Xiaoyu Zhang, Lile Wang, Yang Gao et al. (+1 more)

We present direct numerical simulations demonstrating deflagration-to-detonation transition (DDT) driven by oxygen flames in Type Ia supernova progenitors. Using the Castro hydrodynamics code coupled with the “aprox13” 13-isotope nuclear network, we simulate combustion in isolated fuel regions where oxygen flames trail carbon flames. In a fiducial one-dimensional run at $\rho_{0}=3.5\times10^{7}\ \mathrm{g\ cm^{-3}}$ we observe spontaneous DDT of the oxygen flame via the Zel'dovich gradient mechanism when the carbon-oxygen separation reaches $\sim 10\ \mathrm{km}$. The oxygen detonation then captures the carbon flame and triggers a stable carbon detonation. Systematic one-dimensional parameter scans show that successful carbon DDT requires upstream densities in the range $(3.1$–$3.6)\times10^{7}\ \mathrm{g\ cm^{-3}}$ and a minimum carbon-flame thickness of $\gtrsim 20\ \mathrm{m}$. Two-dimensional simulations confirm DDT and demonstrate that the multidimensional cellular structure of the oxygen detonation can promote carbon detonation at somewhat lower densities than in one dimension. These results provide direct numerical evidence that oxygen-flame-driven DDT is physically plausible in turbulent white-dwarf environments and underscore the importance of multidimensional effects for Type Ia supernova explosion modeling.
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All authors Xiaoyu Zhang, Lile Wang, Yang Gao, Yao Zhou

Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

KM3-230213A and IceCube Neutrino Events from Metastable Dark Matter of Primordial Black Hole Origin

Prabhav Singh, Mansi Dhuria and Nathanael Varghese Job

We investigate a scenario in which the recently observed ultra-high-energy neutrino event KM3-230213A, with a median energy of approximately 220 PeV, as well as the high-energy neutrinos detected by IceCube Observatory, originate from the decay of superheavy dark matter (DM) particles produced through primordial black hole (PBH) evaporation. To establish this connection, we derive constraints on the PBH abundance parameter $\beta$ as a function of the initial PBH mass $M_{\mathrm{BH_0}}$ and DM mass $m_{\mathrm{DM}}$, by considering the bound from the observed relic DM abundance. Using these constraints, we compute the resulting neutrino flux and show that DM masses in the PeV-EeV range can yield neutrinos of comparable energies, capable of accounting for both the KM3-230213A and IceCube events while remaining consistent with the relic abundance constraint. Interestingly, the scenario remains viable over a broad region of parameter space while satisfying existing cosmological and astrophysical bounds. Overall, our results demonstrate that PBH evaporation followed by DM decay provides a consistent and natural explanation for the observed ultra-high-energy neutrino events in the absence of accompanying multimessenger signatures.
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High Energy Astrophysical Phenomena

Evidence of cosmic-ray acceleration up to sub-PeV energies in the supernova remnant IC 443

Zhen Cao, F. Aharonian, Y.X. Bai et al. (+313 more)

Supernova remnants (SNRs) have been considered as the primary contributors to cosmic rays (CRs) in our Galaxy. However, the maximum energy of particles that can be accelerated by shocks of SNRs is uncertain, and SNRs' contribution to CRs around PeV energies is unclear. In this study, we present observations of high-energy $\gamma$-ray emission from the SNR IC 443 using the Large High Altitude Air Shower Observatory (LHAASO). The morphological analysis reveals a pointlike source whose location and spectrum are consistent with those of the Fermi-LAT-detected compact source with $\pi^0$-decay signature, and a more extended source that is associated with a newly discovered Fermi source. The spectrum of the point source can be described by a power-law function with an index of $\sim3.0$, extending beyond $\sim 30$ TeV without apparent cutoff. Assuming a hadronic origin of the $\gamma$-ray emission, the $95\%$ lower limit of accelerated protons reaches about 300 TeV. The extended source might be associated with IC 443, SNR G189.6+3.3 or the putative pulsar wind nebula CXOU J061705.3+222127, and can be explained by either a hadronic or a leptonic model with particles reaching hundreds of TeV. These LHAASO results provide compelling evidence that sub-PeV CRs can be accelerated efficiently by shocks of SNRs.
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All authors Zhen Cao, F. Aharonian, Y.X. Bai, Y.W. Bao, D. Bastieri, X.J. Bi, Y.J. Bi, W. Bian, A.V. Bukevich, C.M. Cai, W.Y. Cao, Zhe Cao, J. Chang, J.F. Chang, A.M. Chen, E.S. Chen, G.H. Chen, H.X. Chen, Liang Chen, Long Chen, M.J. Chen, M.L. Chen, Q.H. Chen, S. Chen, S.H. Chen, S.Z. Chen, T.L. Chen, X.B. Chen, X.J. Chen, Y. Chen, N. Cheng, Y.D. Cheng, M.C. Chu, M.Y. Cui, S.W. Cui, X.H. Cui, Y.D. Cui, B.Z. Dai, H.L. Dai, Z.G. Dai, Danzengluobu, Y.X. Diao, X.Q. Dong, K.K. Duan, J.H. Fan, Y.Z. Fan, J. Fang, J.H. Fang, K. Fang, C.F. Feng, H. Feng, L. Feng, S.H. Feng, X.T. Feng, Y. Feng, Y.L. Feng, S. Gabici, B. Gao, C.D. Gao, Q. Gao, W. Gao, W.K. Gao, M.M. Ge, T.T. Ge, L.S. Geng, G. Giacinti, G.H. Gong, Q.B. Gou, M.H. Gu, F.L. Guo, J. Guo, X.L. Guo, Y.Q. Guo, Y.Y. Guo, Y.A. Han, O.A. Hannuksela, M. Hasan, H.H. He, H.N. He, J.Y. He, X.Y. He, Y. He, S. Hernández-Cadena, B.W. Hou, C. Hou, X. Hou, H.B. Hu, S.C. Hu, C. Huang, D.H. Huang, J.J. Huang, T.Q. Huang, W.J. Huang, X.T. Huang, X.Y. Huang, Y. Huang, Y.Y. Huang, X.L. Ji, H.Y. Jia, K. Jia, H.B. Jiang, K. Jiang, X.W. Jiang, Z.J. Jiang, M. Jin, S. Kaci, M.M. Kang, I. Karpikov, D. Khangulyan, D. Kuleshov, K. Kurinov, B.B. Li, Cheng Li, Cong Li, D. Li, F. Li, H.B. Li, H.C. Li, Jian Li, Jie Li, K. Li, L. Li, R.L. Li, S.D. Li, T.Y. Li, W.L. Li, X.R. Li, Xin Li, Y. Li, Y.Z. Li, Zhe Li, Zhuo Li, E.W. Liang, Y.F. Liang, S.J. Lin, B. Liu, C. Liu, D. Liu, D.B. Liu, H. Liu, H.D. Liu, J. Liu, J.L. Liu, J.R. Liu, M.Y. Liu, R.Y. Liu, S.M. Liu, W. Liu, X. Liu, Y. Liu, Y. Liu, Y.N. Liu, Y.Q. Lou, Q. Luo, Y. Luo, H.K. Lv, B.Q. Ma, L.L. Ma, X.H. Ma, J.R. Mao, Z. Min, W. Mitthumsiri, G.B. Mou, H.J. Mu, A. Neronov, K.C.Y. Ng, M.Y. Ni, L. Nie, L.J. Ou, P. Pattarakijwanich, Z.Y. Pei, J.C. Qi, M.Y. Qi, J.J. Qin, A. Raza, C.Y. Ren, D. Ruffolo, A. Sáiz, D. Semikoz, L. Shao, O. Shchegolev, Y.Z. Shen, X.D. Sheng, Z.D. Shi, F.W. Shu, H.C. Song, Yu.V. Stenkin, V. Stepanov, Y. Su, D.X. Sun, H. Sun, Q.N. Sun, X.N. Sun, Z.B. Sun, N.H. Tabasam, J. Takata, P.H.T. Tam, H.B. Tan, Q.W. Tang, R. Tang, Z.B. Tang, W.W. Tian, C.N. Tong, L.H. Wan, C. Wang, G.W. Wang, H.G. Wang, J.C. Wang, K. Wang, Kai Wang, Kai Wang, L.P. Wang, L.Y. Wang, L.Y. Wang, R. Wang, W. Wang, X.G. Wang, X.J. Wang, X.Y. Wang, Y. Wang, Y.D. Wang, Z.H. Wang, Z.X. Wang, Zheng Wang, D.M. Wei, J.J. Wei, Y.J. Wei, T. Wen, S.S. Weng, C.Y. Wu, H.R. Wu, Q.W. Wu, S. Wu, X.F. Wu, Y.S. Wu, S.Q. Xi, J. Xia, J.J. Xia, G.M. Xiang, D.X. Xiao, G. Xiao, Y.L. Xin, Y. Xing, D.R. Xiong, Z. Xiong, D.L. Xu, R.F. Xu, R.X. Xu, W.L. Xu, L. Xue, D.H. Yan, T. Yan, C.W. Yang, C.Y. Yang, F.F. Yang, L.L. Yang, M.J. Yang, R.Z. Yang, W.X. Yang, Z.H. Yang, Z.G. Yao, X.A. Ye, L.Q. Yin, N. Yin, X.H. You, Z.Y. You, Q. Yuan, H. Yue, H.D. Zeng, T.X. Zeng, W. Zeng, X.T. Zeng, M. Zha, B.B. Zhang, B.T. Zhang, C. Zhang, F. Zhang, H. Zhang, H.M. Zhang, H.Y. Zhang, J.L. Zhang, Li Zhang, P.F. Zhang, P.P. Zhang, R. Zhang, S.R. Zhang, S.S. Zhang, W.Y. Zhang, X. Zhang, X.P. Zhang, Yi Zhang, Yong Zhang, Z.P. Zhang, J. Zhao, L. Zhao, L.Z. Zhao, S.P. Zhao, X.H. Zhao, Z.H. Zhao, F. Zheng, W.J. Zhong, B. Zhou, H. Zhou, J.N. Zhou, M. Zhou, P. Zhou, R. Zhou, X.X. Zhou, X.X. Zhou, B.Y. Zhu, C.G. Zhu, F.R. Zhu, H. Zhu, K.J. Zhu, Y.C. Zou, X. Zuo

High Energy Astrophysical Phenomena

Accretion rates of stellar-mass compact objects embedded in AGN discs

Cheng-Liang Jiao, Liying Zhu, Er-gang Zhao et al. (+1 more)

Stellar-mass compact objects (COs) embedded in active galactic nucleus (AGN) discs are commonly assumed to accrete via Bondi or Bondi-Hoyle-Lyttleton (BHL) prescriptions, neglecting gas angular momentum. We show that differential rotation in AGN discs can impart non-negligible angular momentum, in which case accretion proceeds through a viscous disc rather than Bondi/BHL flow. Our model provides a new framework estimating the CO accretion rate as $\dot{M}_\mathrm{CO} = \min\{\dot{M}_\mathrm{vis}, \dot{M}_\mathrm{BHL}\}$, where the viscous rate $\dot{M}_\mathrm{vis}$ accounts for gas–CO relative motion decomposed into a local gradient term (due to differential rotation) and bulk motion (from differing orbital parameters). This rate can be expressed as $\dot{M}_\mathrm{vis} = \alpha \xi (r_\mathrm{H}/r_\mathrm{BHL})^3\dot{M}_\mathrm{BHL}$, where $\xi$ is a coefficient of order unity. It can also be approximately scaled to the global AGN accretion rate as $\dot{M}_\mathrm{vis} \propto \dot{M}_1$, with the scaling coefficients in both forms determined by the specific dynamical configuration. The accretion is viscosity-limited when $q > [\alpha \xi(1+\mathcal{M}^2)^{3}/3]^{1/2} h^3$, where $q$ is the mass ratio between the CO and the supermassive black hole, $\alpha$ the viscosity parameter, $\mathcal{M}$ the Mach number of the bulk relative motion, and $h$ the aspect ratio of the AGN disc. In thin AGN discs this condition is satisfied for most stellar-mass or more massive COs. Our framework also naturally allows for the inclusion of established outflow corrections, thereby enabling a more realistic treatment of super-Eddington flows. Our formulation thus improves upon Bondi/BHL prescriptions and offers a more physically motivated basis for studying CO evolution in AGN environments.
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All authors Cheng-Liang Jiao, Liying Zhu, Er-gang Zhao, Jia Zhang

Earth and Planetary Astrophysics

Planet Across Space and Time (PAST). VII. The origin and tidal evolution of hot Jupiters constrained by a broken age-frequency relation

Di-Chang Chen, Ji-Wei Xie, Ji-Lin Zhou et al. (+4 more)

The discovery of hot Jupiters has challenged the classical planet formation theory. Although various formation mechanisms have been proposed, the dominant channel and relative contributions remain unclear. Furthermore, hot Jupiters offer a unique opportunity to test tidal theory and measure the fundamental tidal quality factor, which is yet to be well-constrained. In this work, based on a hot Jupiter sample around single Sun-like stars with kinematic properties, {we find that the declining trend of their frequency is broken with a ridge at about 2 Gyr, providing direct evidence that hot Jupiters are formed with multiple origins of different timescales. By fitting with the theoretical expectations, we provide a constraint of tidal factor for Sun-like stars, which aligns well with the detected number of hot Jupiters with orbital decay. Moreover, we simultaneously constrain the relative importance of different channels: although the majority of hot Jupiters are formed early, within several tenths of Gyr via 'Early' models (e.g., in-situ formation, disk migration, planet-planet scattering and Kozai-Lidov interaction), a significant portion (about 40%) should be formed late on a relatively long timescale extending up to several Gyr mainly via the secular chaos mechanism, further supported by the obliquity distribution of 'late-arrived' hot Jupiters. Our findings provide a unified framework that reconciles hot Jupiter demographics and long-term evolution with multichannel formation.
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All authors Di-Chang Chen, Ji-Wei Xie, Ji-Lin Zhou, Fei Dai, Bo Ma, Songhu Wang, Chao Liu

Astrophysics of Galaxies

Ram pressure shaping high-velocity cloud droplets — FAST HI observations of HVC AC-III and theoretical interpretation

Xunchuan Liu

FAST HI observations reveal unprecedented internal structures within the high-velocity cloud (HVC) AC-III, which consists of several coherent subclumps (D1–D6). These subclumps exhibit nearly constant line widths of $\sim 20$ km/s across a velocity ($v_{\rm LSR}$) range of $-220$ to $-180$ km/s. They display a parabolic morphology consistent with ram-pressure-confined droplets whose heads are oriented toward the Galactic plane. A steady-state analytic model can reproduce both the morphologies and the exponential density profiles of these droplets. The density at the droplet heads reaches $\sim 2$ cm$^{-3}$, implying an ambient gas density of $\sim 10^{-3}$ cm$^{-3}$, which matches the conditions of the Galactic warm ionized medium (WIM). Deviations from axial symmetry, alongside rich dynamic patterns (ridges, rings, and cavities), indicate complex internal dynamics within AC-III. These patterns may be caused by fluid loops induced by interactions between neighboring droplets. Additionally, we find an intermediate-velocity component ($-150$ to $-100$ km s$^{-1}$) showing a distinct shell-like morphology that envelops the head of AC-III, suggesting a non-contact interaction possibly mediated by the WIM. Overall, we suggest that AC-III is actively entering the Galactic WIM layer and being sculpted by external drag into a droplet-like morphology, providing an ideal laboratory for investigating the evolution of halo gas infalling toward the Galactic plane.
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High Energy Astrophysical Phenomena

Gravitational-Wave Constraints on Neutron-Star Pressure Anisotropy via Universal Relations

Victor Guedes, Siddarth Ajith, Shu Yan Lau et al. (+1 more)

Neutron stars may exhibit pressure anisotropy arising from various physical mechanisms, such as elasticity, magnetic fields, viscosity, and superfluidity. We compute the tidal deformability and the $f$-mode oscillation frequency of anisotropic neutron stars using a phenomenological quasi-local model characterized by a single dimensionless anisotropy parameter. We find that while the relation between the tidal deformability and the $f$-mode frequency depends on the degree of anisotropy, it remains largely insensitive to variations in the equation of state (the relation between radial pressure and energy density) for a fixed anisotropy parameter, similar to the isotropic case. Leveraging this anisotropy-dependent universal relation within a statistical framework, we place constraints on the anisotropy parameter using both the gravitational wave observation of GW170817 and simulated data for a GW170817-like event observed by a future network of detectors. We find that the anisotropy parameter can be constrained to order unity with current data, and the bounds remain comparable with future detector sensitivities. Importantly, these constraints are only weakly affected by uncertainties in the neutron-star equation of state.
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All authors Victor Guedes, Siddarth Ajith, Shu Yan Lau, Kent Yagi

Wednesday 29 October

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

Beyond GRMHD: A Robust Numerical Scheme for Extended, Non-Ideal General Relativistic Multifluid Simulations

Jonathan Gorard, James Juno and Ammar Hakim

The equations of general relativistic magnetohydrodynamics (GRMHD) have become the standard mathematical framework for modeling high-energy plasmas in curved spacetimes. However, the fragility of the primitive variable reconstruction operation in GRMHD, as well as the difficulties in maintaining strong hyperbolicity of the equations, sharply limit the applicability of the GRMHD model in scenarios involving large Lorentz factors and high magnetizations, such as around neutron stars. Non-ideal effects, such as electron inertia and Hall terms, are also neglected, and the absence of an explicitly evolved electric field precludes the self-consistent modeling of the strong poloidal fields found around spinning black holes, which are known to be crucial for jet formation. Here, we present a general relativistic multifluid model which strictly generalizes the GRMHD equations, consisting of an arbitrary number of relativistic fluid species interacting with a shared electromagnetic field via an explicit coupling of their source terms, thus allowing for the incorporation of non-ideal effects. We sketch how our model may be derived from general relativistic kinetics (via moments of the relativistic Boltzmann-Vlasov equation), as well as how GRMHD may be recovered in the single-fluid limit as the mobility of charge carriers goes to infinity. We present a numerical scheme for solving the general relativistic multifluid equations, and validate it against the analogous scheme for the GRMHD equations. Since the primitive variable reconstruction operation for our multifluid model is purely hydrodynamic, and therefore independent of the magnetic field, the resulting solver is highly robust, and able to simulate significantly larger Lorentz factors and higher magnetizations (across both black hole and neutron star spacetimes) than GRMHD without loss of either accuracy or stability.
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