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Physics Astrophysics

astro-ph Watch Astrophysics

Tuesday 20 January

High Energy Astrophysical Phenomena

Exotic optical variability in the black hole X-ray binary IGR J17091-3624

Payaswini Saikia, David M. Russell, D. M. Bramich et al. (+9 more)

IGR J17091-3624 is a distinctive black hole X-ray binary exhibiting exotic variability, including complex "heartbeat" oscillations in its X-ray light curves, similar to those observed in GRS 1915+105, a system renowned for its structured, rapid X-ray variability but heavily obscured at optical wavelengths. In contrast, IGR J17091-3624 is less obscured, making it a more accessible target for optical investigations. Due to its weak radio emission, optical and infrared data are essential to probe the jet and outer disc behavior of IGR J17091-3624. This study presents the first long-term optical monitoring of IGR J17091-3624, using data from the Las Cumbres Observatory (LCO) over its 2011, 2016, and 2022 outbursts. We combine these observations with quasi-simultaneous X-ray data from Swift/XRT, RXTE, and NICER, employing light curve and variability analysis, spectral energy distributions, color-magnitude diagrams, and optical/X-ray correlations to investigate optical emission mechanisms. We find that the optical and X-ray fluxes are significantly correlated, following a power-law relation with the index 0.40\pm0.04, suggesting that the optical emission in IGR J17091-3624 is dominated by an X-ray-irradiated accretion disk. Based on optical spectral slope constraints, we estimate the extinction toward IGR J17091-3624 A_V = 4.3 to 6.6 mag. The global optical/X-ray correlation suggests a distance estimate of 8-17 kpc, in line with previous findings. High-cadence optical observations show tentative evidence of optical oscillations that may arise from reprocessed X-ray modulations, although confirming this will require higher time-resolution optical data.
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All authors Payaswini Saikia, David M. Russell, D. M. Bramich, Kevin Alabarta, Sandeep Rout, Federico Vincentelli, Mariano Méndez, Diego Altamirano, Federico García, M. C. Baglio, Fraser Lewis, Yi-Jung Yang

Solar and Stellar Astrophysics

Spectral Variability of the Star HD 141569A

H. N. Adigozalzade, N. Z. Ismailov, U. Z. Bashirova et al. (+1 more)

The paper presents the results of long-term homogeneous spectroscopic studies of the Herbig Ae/Be star HD 141569A obtained in the optical range. We detect spectral variability on timescales of months to years, including changes in the spectrophotometric parameters of the Halpha emission line, as well as other hydrogen absorption lines. Based on the derived physical parameters and age, the star appears to be in a transitional evolutionary stage from a protoplanetary disk to a debris disk. We determine that the star axial rotation period is approximately 12 hours, and its equatorial velocity is significantly below the critical rotation velocity. The observed spectral variability is likely caused by partial obscuration of the central star radiation by extended fragments of the disrupted disk.
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All authors H. N. Adigozalzade, N. Z. Ismailov, U. Z. Bashirova, S. A. Alishov

Instrumentation and Methods for Astrophysics

Control algorithms for dual-wavefront sensor single-conjugate adaptive optics

Aditya R. Sengupta, Lisa A. Poyneer, Benjamin L. Gerard et al. (+2 more)

High-contrast imaging systems using active control with adaptive optics (AO) are often limited by non-common path (NCP) aberrations that are seen only at the final science image. AO systems employing focal-plane wavefront sensors (FP-WFSs) are able to simultaneously correct NCP aberrations and measure science images, but they typically require a second stage of control that adds system cost and complexity. We present control algorithms to augment AO systems with FP-WFSs within their existing control setup. We demonstrate inter-arm NCP aberration transfer can be mitigated through temporal filtering, present frequency- and time-domain validation of controller stability and performance, and discuss the optimality of the chosen controllers. This work will enable the development, testing, and installation of FP-WFS technologies for direct imaging of exoplanets.
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All authors Aditya R. Sengupta, Lisa A. Poyneer, Benjamin L. Gerard, Rebecca Jensen-Clem, Aaron J. Lemmer

Solar and Stellar Astrophysics

Extra modes in helium-core-burning stars probing an infra core cavity

B. Mosser, M. Takata, C. Pinçon et al. (+5 more)

Dipole mixed modes observed in the oscillation pattern of red giant stars probe the radiative regions in the stellar core. Oscillation spectra of helium-core-burning stars sometimes show extra peaks that remain unexplained by the dipole mixed-mode pattern expected from the coupling of a radiative cavity in the stellar core and a pressure cavity in the stellar envelope. We use the asymptotic expansion developed for a multi-cavity star in order to characterize these extra peaks. The analytical resonance condition of the multi-cavity gravito-acoustic modes, with two inner gravity cavities and an outer pressure cavity, helps us explain that the apparent extra peaks are dipole mixed modes that follow the 3-cavity oscillation pattern. The derivation of the two asymptotic period spacings associated with the two distinct regions in the radiative core provides an estimate of the full radiative cavity. Our results provide new constraints for analysing the overshoot or mixing in the core of helium-core-burning stars. An important structure discontinuity inside the radiative core may explain the larger than expected observed period spacings.
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All authors B. Mosser, M. Takata, C. Pinçon, M.S. Cunha, M. Vrard, K. Belkacem, S. Deheuvels, M. Matteuzzi

High Energy Astrophysical Phenomena

Searching for Isolated Black Hole Candidates within 15 pc of the Solar System in Gaia DR3

Abdurakhmon Nosirov, Cosimo Bambi, Leda Gao et al. (+4 more)

Theoretical models predict that the Galaxy hosts $10^8$-$10^9$ black holes formed from the complete gravitational collapse of heavy stars and that most of these black holes are isolated, without any companion. Within 15 pc of the Solar System ($\sim 50$ ly), there may be a few black holes. If located inside one of the Local Interstellar Clouds - which occupy 5-20% of this local volume - an isolated black hole could produce detectable electromagnetic emission via accretion from the interstellar medium, given the capabilities of current or near-future observatories. However, precise predictions remain challenging due to large uncertainties in the expected accretion spectra. Outside these clouds, the accretion rate would be too low; according to our models, the resulting electromagnetic flux is well below the detection thresholds of current and near-future observational facilities. While astrometric detection via gravitational perturbation of nearby stars is conceivable, the local stellar density is too low for this method to be realistically successful. We have searched the Gaia DR3 catalog for candidate isolated black holes accreting from the interstellar medium and identified five sources. All candidates lie close to the Galactic plane, making them likely spurious astrometric solutions, for instance caused by unmodelled background sources (crowding) and/or unmodelled binarity. Our search for infrared and radio emission from these sources further suggests that they are unlikely to be black holes accreting from the interstellar medium.
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All authors Abdurakhmon Nosirov, Cosimo Bambi, Leda Gao, Jos de Bruijne, Jiachen Jiang, Andrea Santangelo, Fu-Guo Xie

High Energy Astrophysical Phenomena

The failed failed-supernova scenario of M31-2014-DS1

Noam Soker

I examine a recently proposed failed-supernova scenario for the fading of the yellow supergiant event M31-2014-DS1, and find that it requires unlikely fine-tuned parameters to work, if at all. In the failed-supernova scenario, most of the yellow supergiant collapsed to form a black hole. Due to the energy carried by neutrinos from the cooling, collapsing core, gravity decreases, leading to the ejection of a small fraction of the outer envelope, some of which remains bound. The fallback accreted gas possesses large angular-momentum fluctuations due to the pre-collapse envelope convection. The fallback material forms intermittent accretion disks around the black hole that launch jets (or disk wind), which unbind most of the bound material. The failed-supernova scenario for M31-2014-DS1 requires that only <1% of the bound material be accreted by the black hole, but the jets do not shut down the backflow for over 10 years. I find this fine-tuned requirement unlikely. I also find that, due to the rapid radiative cooling of the outflow interaction zone with the outer gas, the expected radiation is about an order of magnitude or more above the observed value. These, as well as earlier challenges raised against the failed-supernova scenario, make the alternative type II intermediate-luminosity optical transient scenario, in which fading is due to dust ejection in a violent binary interaction, more likely. The fading event M31-2014-DS1 does not support the failed-supernova scenario predicted by the neutrino-driven explosion mechanism of core-collapse supernovae.
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Cosmology and Nongalactic Astrophysics

Dark Energy Survey Year 6 Results: MagLim++ Lens Sample Selection and Measurements of Galaxy Clustering

N. Weaverdyck, M. Rodríguez-Monroy, J. Elvin-Poole et al. (+85 more)

Galaxy clustering is a sensitive probe of the expansion history and growth of structure of the universe, and key degeneracies can be broken by combining these data with measurements of cosmic shear and galaxy-galaxy lensing (a so-called 3$\times$2pt analysis). The largest and least biased statistical samples of galaxies for use in clustering analyses can be collected photometrically through large imaging surveys. However, selecting clean photometric subsamples for cosmology are crucial for avoiding contamination that can bias cosmological constraints. Here we present the MagLim++ galaxy sample, selected to optimize for cosmological constraining power and incorporating an array of novel quality cuts to identify and remove residual contamination. This sample comes from the full six years of observations from the Dark Energy Survey. We present measurements of the two-point angular clustering ($w(\theta)$) of 9,186,205 galaxies distributed over 4031 sq. degrees and in six tomographic redshift bins centered at $\bar{z}\approx$ [0.31, 0.44, 0.62, 0.78, 0.90, 1.01]. These measurements are used as part of the 3$\times$2pt and other DES Y6 legacy cosmological analyses in companion works. We describe the battery of null tests and mitigation schemes implemented to address observational, astrophysical, and methodological systematics in the analysis. The resulting $w(\theta)$ measurements have a S/N = 149 (90.2 for linear scales only), which we use to place galaxy-clustering-only constraints on the matter density of the Universe, $\Omega_m=0.311^{+0.023}_{-0.035}$, and amplitude of galaxy clustering in each redshift bin, $b_i\sigma_8=[1.16^{+0.04}_{-0.06},\ 1.40^{+0.04}_{-0.06},\ 1.57^{+0.04}_{-0.06},\ 1.59^{+0.04}_{-0.05},\ 1.50^{+0.04}_{-0.05},\ 1.74^{+0.06}_{-0.08}]$.
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All authors N. Weaverdyck, M. Rodríguez-Monroy, J. Elvin-Poole, I. Sevilla-Noarbe, A. Porredon, S. Avila, S. Lee, W. Riquelme, M. Tabbutt, D. Huterer, J. Prat, J. De Vicente, J. Mena-Fernández, M. Crocce, C. Sánchez, G. M. Bernstein, E. Henning, R. Cawthon, A. J. Ross, T. M. C. Abbott, M. Aguena, S. S. Allam, O. Alves, F. Andrade-Oliveira, D. Bacon, K. Bechtol, E. Bertin, J. Blazek, S. Bocquet, D. Brooks, R. Camilleri, A. Carnero Rosell, J. Carretero, F. J. Castander, A. Choi, L. N. da Costa, M. E. da Silva Pereira, T. M. Davis, H. T. Diehl, C. Doux, A. Drlica-Wagner, T. Eifler, S. Everett, A. Evrard, B. Flaugher, J. García-Bellido, M. Gatti, E. Gaztañaga, G. Giannini, D. Gruen, G. Gutierrez, S. R. Hinton, D. L. Hollowood, K. Honscheid, B. Jain, T. Kacprzak, K. Kuehn, O. Lahav, J. L. Marshall, F. Menanteau, R. Miquel, J. J. Mohr, J. Muir, J. Myles, R. Nichol, R. L. C. Ogando, A. Palmese, M. Paterno, W. J. Percival, A. A. Plazas Malagón, R. Rosenfeld, E. Rykoff, S. Samuroff, E. Sanchez, D. Sanchez Cid, E. Sheldon, N. Sherman, M. Smith, M. Soares-Santos, E. Suchyta, M. E. C. Swanson, T. Gregory, D. Thomas, C. To, D. L. Tucker, V. Vikram, M. Yamamoto, B. Yanny

High Energy Astrophysical Phenomena

Supermassive black hole mass inference with the optical flares of tidal disruption events

Andrew Mummery

Tidal disruption events (TDEs) represent a truly unique, and potentially very powerful, probe of the quiescent supermassive black hole (SMBH) population. Given current observational survey capabilities the vast majority of the TDEs discovered in the next decade will be observed only across optical-UV wavelengths. A set of questions of broad scientific interest relating to SMBH demographics and SMBH-galaxy correlations could in principal be answered by using TDE emission as an efficient means to constrain SMBH masses. In this paper we argue for using well-understood elements of TDE emission (the thermal X-ray continuum and late-time UV plateau) to derive empirical relationships between the more poorly understood early optical/UV flare and the black hole mass, before using these empirical relationships to measure TDE black hole masses simply and rapidly. We provide a publicly available code TDEFLARE which does this, showing (i) it produces results consistent with disk codes containing far more physics, (ii) it reproduces galactic scaling relationships at high ($>5\sigma$) significance, (iii) it produces reliable mass estimates for both partial and full disruptions, and (iv) it does not require late time data to derive mass constraints. We provide 89 TDE black hole mass constraints, derive the intrinsic black hole mass function implied by the current TDE population, and discuss the Malmquist-Hills bias, an important confounding factor in TDE science.
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High Energy Astrophysical Phenomena

Prospects of Prompt Gamma-Ray Burst Polarimetry with POLAR-2

Ramandeep Gill, Jiang He, Jonathan Granot et al. (+6 more)

The dominant radiation mechanism that powers the prompt $\gamma$-ray emission in gamma-ray bursts (GRBs) remains poorly understood. High quality, time- and energy-resolved linear polarization measurements of prompt $\gamma$-ray photons can distinguish between synchrotron and inverse-Compton processes and provide crucial constraints on the outflow properties. This will be achieved by POLAR-2 that is proposed as a dedicated GRB polarimeter and successor to POLAR. The High-energy Polarimetry Detector (HPD) is one of the three instruments of POLAR-2 that features significantly improved sensitivity in the $(40-1000)$\,keV energy range and a detection area four times larger than that of POLAR. Here we demonstrate the capabilities of the HPD to constrain key physical model parameters by creating and fitting to synthetic sources using a time-resolved spectro-polarimetric theoretical model of prompt GRB emission. The time-resolved spectral and polarization fits are performed using a novel technique featuring maximum likelihood over an unbinned (in time and energy) list of detected events. The constrained model parameters directly relate to the underlying source physics that would reveal an accelerating, coasting or decelerating emission region. For a pulse fluence of $\mathcal{F}=10^{-5}\mathcal{F}_{-5}\,{\rm erg\,cm^{-2}}$ and higher we can constrain the time-integrated polarization degree to an absolute accuracy ($1\,\sigma$) of about $2.2\mathcal{F}_{-5}^{\,-1/2}$ per cent, as long as source photons dominate over the background. In bright GRBs, such unprecedented accuracy at these energies will allow to distinguish between different models for the prompt GRB emission mechanism and constrain the magnetic field geometry, jet angular structure and outflow composition.
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All authors Ramandeep Gill, Jiang He, Jonathan Granot, Jian-Chao Sun, Shuang-Nan Zhang, Yuan-Hao Wang, Johannes Hulsman, Nicolas Produit, Shao-Lin Xiong

Solar and Stellar Astrophysics

Astrometric Radial Velocities from the Hipparcos-Gaia Catalog of Accelerations and Implications for Astrometric Acceleration Measurements

Timothy D Brandt

Astrometry from the Gaia satellite and from the long-term combination of Hipparcos and Gaia are now sensitive to sky-plane accelerations as low as $\approx$1 m/s/yr. This paper quantifies and explores an important caveat: apparent nonlinear motion due to a star's nonzero radial velocity can be indistinguishable from real astrometric acceleration. This nonlinear motion is parallel to the proper motion, so it can be both quantified and avoided by projecting apparent astrometric accelerations into components parallel and perpendicular to the proper motion. We illustrate this distinction for a sample of very nearby, fast-moving stars from the Hipparcos-Gaia Catalog of Accelerations (HGCA). We then generalize the effect of stellar radial velocity and projections of the astrometric acceleration to binary stars in which we observe the acceleration of both components. Finally, we demonstrate that the proper motion differences in the HGCA are statistically well-behaved even for the nearest and fastest-moving stars, at least in the component perpendicular to the proper motion. This distinction – between astrometric acceleration parallel and perpendicular to proper motion – could have important consequences for future missions reaching extreme astrometric sensitivities around nearby stars.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Neutrino production mechanisms in strongly magnetized quark matter: Current status and open questions

Igor A. Shovkovy and Ritesh Ghosh

We review the main neutrino emission mechanisms operating in dense quark matter under strong magnetic fields, with particular emphasis on conditions expected in the interiors of compact stars. We discuss the direct Urca and neutrino synchrotron processes in unpaired quark matter, incorporating the effects of Landau-level quantization. For the direct Urca process, the quantization of the electron energy spectrum plays a critical role, whereas quark quantization can often be neglected at sufficiently high baryon densities. The resulting field-dependent neutrino emissivity is anisotropic and exhibits an oscillatory behavior as a function of magnetic-field strength. We explore the implications of these effects for magnetar cooling and for possible anisotropic neutrino emission that could contribute to pulsar kicks. In addition, we review the $\nu\bar{\nu}$ synchrotron emission process, which, although subdominant, provides valuable insights into the interplay between magnetic fields and weak interactions in dense quark matter. Overall, our analysis highlights the nontrivial influence of strong magnetic fields on neutrino production in magnetized quark cores, with potential consequences for the thermal and dynamical evolution of compact stars.
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Instrumentation and Methods for Astrophysics

PyISH: Python Integral Field Spectroscopy Simulation for HWO

Grace Sweetak, Breann Sitarski, Kevin France et al. (+2 more)

The Habitable Worlds Observatory (HWO) will be a large ultraviolet/optical/near-infrared space telescope operating at the Sun-Earth Lagrange point L2. HWO was highly recommended by the National Academies` 2020 decadal survey and will be the first telescope designed specifically to search for life on planets orbiting other stars. HWO will also be able to perform a host of other transformational astrophysics, including cosmology, galaxy evolution, solar system science, and beyond. The development of the telescope and instrument suite is an iterative process. Example observatory architectures, called exploratory analytic cases (EACs) by the HWO Technology Maturation Project Office (TMPO), are modeled end-to-end to explore the engineering and science trade space. Recently, an ultraviolet Integral Field Spectrograph (UV IFS) was added to HWO's instrument suite for the EACs 4 and 5. To explore the science and engineering trade space for this specific instrument, we developed a high-fidelity UV IFS simulation tool, PyISH. The UV IFS simulation tool is designed to be used by scientists to model specific science cases as seen by a UV IFS on HWO, as well as engineers to explore the trade space when developing potential instrument architectures. The modular components, deliverables, and an example of the tool simulating a specific science case proposed for HWO are described in this paper.
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All authors Grace Sweetak, Breann Sitarski, Kevin France, Randall McEntaffer, Richard Cartwright

Earth and Planetary Astrophysics

Inferring Meteoroid Properties with Dynamic Nested Sampling: A Case Study of Orionid and Capricornid Shower Meteors

Maximilian Vovk, Peter G. Brown, Denis Vida et al. (+2 more)

Accurate estimation of meteoroid bulk density is crucial for assessing spacecraft impact hazards from sub-millimeter to millimeter-sized meteoroids. Previous studies often used manual tuning or optimization methods to fit ablation and fragmentation models to optical meteor data, but subjective choices made physical properties and uncertainties difficult to compare. We develop a global, statistically robust method that uses Dynamic Nested Sampling to fit an erosion-fragmentation model to meteor light curves and deceleration measured by the Canadian Automated Meteor Observatory (CAMO) mirror tracking system and Electron-Multiplied CCD (EMCCD) cameras. Applied to 15 shower meteors, the method returns posterior distributions and Bayesian evidences for single- and double-fragmentation scenarios. Tests on four synthetic cases recover the known inputs, with best-guess solutions matching the true parameters. For 9 Orionids and 6 Alpha Capricornids with masses 1e-6 to 1e-5 kg, the median bulk density is 159 (+558/-57) kg/m3 for Orionids and 333 (+1089/-114) kg/m3 for Alpha Capricornids. Orionids are consistent with low-density cometary material, while Alpha Capricornids are systematically denser and show a second density cluster near 1300 kg/m3, consistent with higher-density asteroidal material. This framework enables automated, statistically rigorous characterization of meteoroid properties and will be extended to larger samples of shower and sporadic meteors across orbital classes.
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All authors Maximilian Vovk, Peter G. Brown, Denis Vida, Daeyoung Lee, Emma G. Harmos

Cosmology and Nongalactic Astrophysics

Deep Chandra Observations of the z = 1.16 Relaxed, Cool-core Galaxy Cluster SPT-CL J2215-3537

Haley R. Stueber, Adam B. Mantz, Steven W. Allen et al. (+10 more)

Galaxy clusters serve as a unique and valuable laboratory for probing cosmological models and understanding astrophysics at the high-mass limit of structure formation. Clusters that are dynamically relaxed are especially useful targets of study because of their morphological and dynamical simplicity. However, at redshifts z > 1, very few such clusters have been identified. We present results from new Chandra observations of the cluster SPT-CL J2215-3537 (hereafter SPT J2215), at z = 1.16, the second most distant relaxed, cool-core cluster identified to date. We place constraints on the cluster's total mass profile and investigate its thermodynamic profiles, scaling relations (gas mass, average temperature, and X-ray luminosity), and metal enrichment, resolving the cool core and providing essential context for the massive starburst seen in its central galaxy. We contextualize the thermodynamic and cosmological properties of the cluster within a sample of well-studied, lower-redshift relaxed systems. In this way, SPT J2215 serves as a powerful high-redshift benchmark for understanding the formation of cool cores and the evolution of massive clusters of galaxies.
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All authors Haley R. Stueber, Adam B. Mantz, Steven W. Allen, Anthony M. Flores, R. Glenn Morris, Abigail Y. Pan, Taweewat Somboonpanyakul, Lindsey E. Bleem, Michael Calzadilla, Benjamin Floyd, Julie Hlavacek-Larrondo, Michael McDonald, Arnab Sarkar

Astrophysics of Galaxies

The Carousel Lens I: A Spectroscopic Survey of the Carousel Lens Field

Jackson H. O'Donnell, Demetrius Y. Williams, Tesla E. Jeltema et al. (+18 more)

We present a spectroscopic survey of field galaxies and lensed sources in the vicinity of the strong lensing galaxy cluster known as the Carousel lens at z=0.49. Using both Gemini/GMOS slitmask spectra and deep VLT/MUSE observations, we bring the total number of lensed sources up to 12, including three which were not previously known from imaging observations but are apparent in the MUSE data as emission-line sources. Of these sources, 10 have confident redshifts, and an additional 2 have tentative redshifts from likely Ly$\alpha$ emission (including seven new redshifts determined here adding to those presented previously in \cite{Sheu.Cikota.ea2024}). In total, we identify 42 images of these 12 sources. This lens system is remarkably symmetric and well-modeled by a simpler lens model than typical cluster lenses, and the large number of sources and their large range of redshifts make this cluster ideal for constraining cosmological parameters such as $w$ and $\Omega_m$ as well as the cluster density profile. Additionally, we present a catalog of 57 unlensed field galaxies with confident redshifts, of which 49 are associated with the cluster. We measure a cluster velocity dispersion of about 1100 km s$^{-1}$ from which we estimate a halo mass $M_{200c} \sim 1.2 \times 10^{15} M_\odot$.
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All authors Jackson H. O'Donnell, Demetrius Y. Williams, Tesla E. Jeltema, William Sheu, Felipe Urcelay, Xiaosheng Huang, Tucker Jones, Karl Glazebrook, Tania M. Barone, Aleksandar Cikota, Fuyan Bian, Christopher J. Storfer, Daniel J. Ballard, Gabriel Caminha, Glenn G. Kacprzak, Themiya Nanayakkara, Nandini Sahu, Hannah Skobe, Anowar J. Shajib, Kim-Vy Tran, Keerthi Vasan G. C.

Astrophysics of Galaxies

The Role of Inner Halo Angular Momentum (Spin) in Shaping Dark Matter Bars in Milky Way Analogs

Shouvik Ghosh and Sandeep Kumar Kataria

Studies of galactic bars have primarily focused on stellar bars, since they can be directly observed through ultraviolet to infrared wavebands. Cosmological as well as idealised simulations reveal that the dark matter (DM) haloes interact with baryonic matter, primarily the stellar bars, dynamically by means of the exchange of angular momentum. In these simulations, the spherical DM halo dynamically responds to interaction with the stellar bar by reshaping its orbital structure in the proximity of the stellar bar, forming a bar-like configuration, called as the Dark Matter (DM) bar. Using N-body simulations of Milky Way analogs we discuss the role of inner halo angular momentum, measured as halo spin parameter {\lambda} of the dark matter halo, on formation and evolutionary characteristics of the DM bars. Our systematic study involves haloes with initial spin configurations ranging from {\lambda} = 0 to 0.1. The result conveys that DM bar formation and its characteristics are extensively dependent on the initial spin parameter {\lambda} of the DM halo. We demonstrate that the strength of the dark matter bar gradually increases with an increase in halo spin in long-term evolution, with a significant impact of stellar bar buckling on dark matter bar strength. The evolutionary characteristics of the DM bar are strongly influenced by the initial spin of the host halo.
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Astrophysics of Galaxies

Stirring Things Up: Bar-induced substructures in the stellar halo of a cosmological Milky Way analogue

Thomas Tomlinson, Francesca Fragkoudi, Andreia Carrillo et al. (+8 more)

The stellar halo of the Milky Way contains the remnants of past accretion events, which could be detectable as substructures in the classical integrals of motion space, such as energy and angular momentum (E-Lz). However, our galaxy also contains a non-axisymmetric stellar bar, which traps stars in resonant orbits, leading to substructures in phase-space. Using a high-resolution magneto-hydrodynamic cosmological zoom-in simulation of a Milky Way analogue, we explore the connection between the bar and the accreted stellar halo. We find that the bar induces prominent substructures, or "ridges", in E-Lz, caused by the resonances. The most pronounced of these is caused by the corotation and the retrograde 1:1 resonances, with weaker ridges visible due to the prograde 1:1 and outer Lindblad resonance. The ridges are present across much of the stellar halo, with variations in radius due to the morphology of different orbital families. We explore the scattering of orbits at the resonances, finding that stars trapped at the 1:1 retrograde resonance become more circularised and have more negative angular momentum. Additionally, stars can move between the corotation and retrograde 1:1 families, thus alternating between prograde and retrograde motion. Due to these scatterings and the pre-existing metallicity gradients in the accreted population, the bar-induced substructures have distinct metallicities compared to stars in the surrounding phase-space. Our results suggest the need for caution when searching the Milky Way stellar halo for accreted substructures in both integral of motions and chemical spaces, since these can be induced by internal perturbations.
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All authors Thomas Tomlinson, Francesca Fragkoudi, Andreia Carrillo, Azadeh Fattahi, Paula Gherghinescu, Alis Deason, Rüdiger Pakmor, Robert J. J. Grand, Facundo A. Gómez, Freeke van de Voort, Rebekka Bieri

High Energy Astrophysical Phenomena

Erythrohenosis – The crimson chronicles of two giants

Pau Amaro Seoane

We investigate erythrohenosis – the collision and merger of two red giants – establishing an end-to-end model for this fundamental evolutionary channel in dense stellar environments. Combining three-dimensional SPH simulations of a binary with analytical modeling, we characterize the event from initial encounter to terminal explosion. We demonstrate that grazing encounters induce tidal capture and rapid orbital decay, accompanied by large-amplitude, nonlinear stellar oscillations. The subsequent inspiral spins up the common envelope into a stable, non-spherical equilibrium, powering a luminous precursor with quasi-periodic bursts. The terminal explosion, modeled with angular momentum conservation, produces an intrinsically flattened remnant that preserves a geometric memory, or morphomnesia, of its binary origin. The associated gravitational wave signal features a rapid, drag-dominated frequency evolution, identifiable by a unique time-varying apparent chirp mass. These results define a distinctive multi-stage observational fingerprint – linking transient optical precursors, asymmetric nebulae, and anomalous gravitational wave chirps – to guide identification in current and future multi-messenger surveys.
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Solar and Stellar Astrophysics

Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656

Johanna Müller-Horn, Varsha Ramachandran, Kareem El-Badry et al. (+11 more)

The Galactic Be star binary MWC 656 was long considered the only known Be star+black hole (BH) system, making it a critical benchmark for models of massive binary evolution and for the expected X-ray emission of Be+BH binaries. However, recent dynamical measurements cast doubt on the presence of a BH companion. We present new multi-epoch ultraviolet spectroscopy from the Hubble Space Telescope, combined with high-resolution optical spectra, to reassess the nature of the companion. The far-ultraviolet spectra reveal high-ionisation features – including prominent N v and He ii lines – which are absent in the spectra of normal Be stars and are indicative of a hot, luminous companion. Spectral modelling shows that these features cannot originate from the Be star or from an accretion disc around a compact object. Instead, we find that the data are best explained by a hot ($T_\mathrm{eff} \approx 85$ kK), compact, hydrogen-deficient star with strong wind signatures, consistent with an intermediate-mass stripped star. Our revised orbital solution and composite spectroscopic modelling yield a companion mass of $M_2 = 1.48^{+0.55}_{-0.46}\,\mathrm{M}_\odot$, definitively ruling out a BH and disfavouring a white dwarf. MWC 656 thus joins the growing class of Be+stripped star binaries. The system's unusual properties – including a high companion temperature and wind strength – extend the known parameter space of such binaries. The continued absence of confirmed OBe+BH binaries in the Galaxy highlights a growing tension with population synthesis models.
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All authors Johanna Müller-Horn, Varsha Ramachandran, Kareem El-Badry, Andreas A. C. Sander, Julia Bodensteiner, Douglas R. Gies, Ylva Götberg, Thomas Rivinius, Tomer Shenar, Elisa C. Schösser, Luqian Wang, Allyson Bieryla, Lars A. Buchhave, David W. Latham

Astrophysics of Galaxies

Growth of Light Seed Black Holes in the Early Universe

Daxal H. Mehta, John A. Regan and Lewis Prole

Observations from the James Webb Space Telescope (JWST) have uncovered supermassive black holes (SMBHs) with masses exceeding $10^6 \mathrm{M}_{\odot}$ at redshifts $z > 8$, posing significant challenges to existing models of early black hole formation and growth. Here we show, in a fully cosmological setting, that light seed black holes (LSBHs), remnants of Population III stars, can grow rapidly to $\sim10^4 \mathrm{M}_{\odot}$ in the early Universe. This growth is enabled by our novel black hole seeding prescription and the unprecedented resolution of our zoom-in cosmological simulations, which resolve the dense environments necessary for efficient accretion. Our results provide robust evidence that LSBHs can attain the masses required to serve as the dominant progenitors of the SMBH population observed at later cosmic epochs. These findings have far-reaching implications for the interpretation of JWST observations and future gravitational wave detections with LISA.
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High Energy Astrophysical Phenomena

MeV absorption in gamma-ray bursts as a probe of their progenitor environments

Gor Oganesyan, Om Sharan Salafia, Emanuele Sobacchi et al. (+8 more)

A small fraction of X-ray photons from $\gamma$-ray bursts (GRBs), after escaping the relativistic jet, are scattered by electrons in the circumburst medium. Subsequent photon-photon absorption between the incoming MeV $\gamma$-rays and the back-scattered X-rays generate electron-positron pairs, enriching the surrounding medium with leptons. We investigate how these back-scattered photons modify the prompt GRB spectrum through $\gamma-\gamma$ absorption. In a dense and pair-loaded environment, the emerging spectra exhibit a broad absorption feature, whose profile is sensitive to the low-energy spectral index $\alpha$. In particular, spectra with $\alpha > -1$ develop a pronounced, saddle-shaped absorption between 1 and 100 MeV (rest frame). Such external MeV absorption could account for the spectral curvature seen in some bright GRBs, and may point to a dense circum-stellar medium (CSM) around their progenitor stars - consistent with early observations of core-collapse supernovae. In this scenario, the blastwave caused by the GRB is expected to start off with a relatively low Lorentz factor, and undergo an acceleration phase when traversing the large density drop at the interface between the dense CSM and the surrounding medium. The impact of these non-trivial dynamics on the afterglow emission is yet to be explored.
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All authors Gor Oganesyan, Om Sharan Salafia, Emanuele Sobacchi, Samanta Macera, Giancarlo Ghirlanda, Lara Nava, Annarita Ierardi, Biswajit Banerjee, Alessio Mei, Stefano Ascenzi, Marica Branchesi

Cosmology and Nongalactic Astrophysics

An azimuthally resolved study of sloshing cold fronts in three nearby galaxy clusters

I-Hsuan Li, Shutaro Ueda, I-Non Chiu et al. (+1 more)

We present a detailed analysis of sloshing cold fronts in a sample of three nearby galaxy clusters (Abell 496, Abell 2029, and Abell 1644) observed with the Chandra X-ray Observatory. Cold fronts manifest as sharp edges in the X-ray surface brightness of the intracluster medium (ICM) in galaxy clusters. In the residual X-ray surface brightness maps, where the global ICM distribution has been subtracted, cold fronts generated by gas sloshing are observed at the boundaries of the spiral excesses. We perform a systematic and comprehensive study of the surface brightness edges along the spiral excesses. We find the deficit of the thermal pressure radially inward of the brightness edges, in contrast to stripping cold fronts that typically exhibit higher thermal pressure in brightness edges. Assuming that the sharp edges in the X-ray surface brightness distributions are sustained entirely by the gas bulk motions, we estimate the velocity gradients across the edges that are required to compensate for the deficit of the thermal pressure. We do not find statistically significant velocity gradients along the azimuthal direction. Our results suggest that alternative mechanisms such as magnetic fields and viscosity are necessary to maintain the sharpness of sloshing cold fronts.
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All authors I-Hsuan Li, Shutaro Ueda, I-Non Chiu, Keiichi Umetsu

Astrophysics of Galaxies

Cold gas formation triggered by active galactic nuclei jet feedback in galaxy cluster cores

Stefano Sotira, Martin A. Bourne, Debora Sijacki et al. (+2 more)

Extended warm and cold gas nebulae, with complex morphologies and kinematics, have been observed in the centres of cool-core galaxy clusters. Their origin within the hot intracluster medium (ICM) is still puzzling, and among many mechanisms, positive feedback from the central active galactic nucleus (AGN) has been proposed. In this work, we performed a suite of very high-resolution hydrodynamic simulations of a Perseus-like cool-core galaxy cluster subject to self-regulated AGN jet feedback, which leads to realistic ICM properties. By explicitly following warm ionized, neutral, and molecular gas phases, we studied the complex interplay between AGN activity and the multi-phase ICM. While AGN feedback globally heats the ICM, we find that during the individual AGN jet bursts, hot material is also injected laterally to the jet axis, within the turbulent mixing layer. This material, as it expands, compresses the surrounding hot ICM, reducing the local cooling time, and leads to the formation of cold clumps on a characteristic timescale of $\sim 30$ Myr. By employing tracers, we explicitly track cooling within the affected regions, finding that very hot gas identified in high-compression, low-vorticity zones condenses in situ to form cold clumps. A statistical analysis reveals that the condensation of cold gas is highly promoted once the local turbulent Mach number, $\sigma_{hot}/c_{s,hot}$, in the hot gas component ($T \geq 10^7$ K) takes values around ~0.3. The presented process is a further important step in understanding the physical mechanisms that lead to the formation of cold gas in the cluster core. Our measured values of the characteristic turbulent Mach number, together with detailed multi-phase gas kinematics predictions, provide important theoretical tools to interpret future X-ray spectroscopy and deep radio data, ultimately to constrain the origin of cool-core cluster nebulae.
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All authors Stefano Sotira, Martin A. Bourne, Debora Sijacki, Franco Vazza, Fabrizio Brighenti

High Energy Astrophysical Phenomena

Neutron star cooling implications and magnetic field of the Vela Junior central compact object from all XMM-Newton and Chandra spectra

Wynn C. G. Ho, Esther Simkhayeva and Alexander Y. Potekhin

The central compact object (CCO) in the Vela Junior supernova remnant is a young neutron star whose relatively low X-ray flux and small distance suggest it has a mass high enough to activate fast neutrino cooling processes. Here we analyse all XMM-Newton MOS and pn and Chandra ACIS-S spectra of the Vela Junior CCO, with observations taking place over the 9 years from 2001 to 2010. We find that the best-fit flux and spectral model parameters do not vary significantly when treating each observation independently, and therefore we fit all the spectra simultaneously using various spectral models to characterize the predominantly thermal emission from the neutron star surface. Our results indicate the Vela Junior CCO has an atmosphere composed of hydrogen, a hot spot temperature (unredshifted) of 3.5x10^6 K, and a colder surface temperature of (6.6-8.8)x10^5 K. Possible absorption lines at ~0.6 keV and 0.9 keV provide evidence for the first-time of an average surface magnetic field B~3x10^10 G for this CCO, which is similar to the magnetic field of other CCOs. At the accurate new Vela Junior distance of 1.4 kpc, the observed luminosity that is dominated by the hot spot is ~5x10^32 erg s^-1. The luminosity from the rest of the colder surface is (1.3-4.0)x10^32 erg s^-1. The cool luminosity and temperature imply the Vela Junior CCO is indeed colder than many other young neutron stars and probably has a high mass that triggered fast neutrino cooling.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Emergent Large Lepton Mixing from Neutrino Refraction in Dark Matter

Susobhan Chattopadhyay, Yuber F. Perez-Gonzalez and Manibrata Sen

We propose a novel origin for the disparity between quark and lepton flavor mixing based on the refractive nature of neutrino masses. We postulate that the fundamental mixing in both the quark and lepton sectors is CKM-like, together with tiny vacuum neutrino masses, while the observed PMNS mixing matrix emerges dynamically from coherent forward scattering of neutrinos on an ultralight dark matter background. The resulting in-medium Hamiltonian rotates CKM mixing angles into large effective lepton mixings, naturally realizing quark–lepton complementarity without invoking new flavor symmetries. This framework links neutrino mass generation, flavor mixing, and dark matter, and predicts environment-dependent neutrino oscillation effects testable in current and future experiments.
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Astrophysics of Galaxies

Mirror images of lensed star clusters with mismatched spectral energy distributions: A possible signature of top-heavy stellar initial mass functions and extreme stars in high-redshift star clusters

Erik Zackrisson, Jose M. Diego, Jose M. Palencia et al. (+4 more)

Strongly lensed star clusters have recently been detected up to redshift $z\approx 10$ in galaxy cluster fields using the James Webb Space Telescope (JWST). When pairs of mirror images of such star clusters appear across the lensing critical curve, it is usually assumed that both images will display identical spectral energy distributions (SEDs). However, this assumption may be invalidated in the presence of gravitational microlensing from stars or other compact objects in the lens, since microlensing will affect the SED contribution from bright stars within the star cluster independently in the two mirror images. Here, we explore under what circumstances mismatched mirror-image SEDs are likely to be observable, and argue that SED differences detectable in JWST observations of lensing-cluster fields will be limited to star clusters of mass $< 10^5\ M_\odot$ and ages $\lesssim 5$ Myr. The probability of severely mismatched mirror-image SEDs increases if the stellar initial mass function is very top-heavy and extends to stellar masses $\gg 100\ M_\odot$, as has been suggested to be the case for Population III stars. The prevalence of lensed star clusters with highly discrepant mirror-image SEDs could therefore serve as a probe of very massive stars and extreme stellar populations in the early Universe.
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All authors Erik Zackrisson, Jose M. Diego, Jose M. Palencia, Francesco Gabrielli, Armin Nabizadeh, Angela Adamo, Guglielmo Costa

High Energy Astrophysical Phenomena

The Self-Limiting Nature of Jet-Modulated Thermal Conduction in Cool Core Clusters

Jennifer Stafford, Sebastian Heinz, Mateusz Ruszkowski et al. (+2 more)

Conduction as a mechanism for explaining the disrupted cooling-flow in galaxy clusters has been mostly discounted, as the process is inefficient at transporting heat all the way from the cluster into the core. However, thermal conduction can be strongly enhanced when materials of significantly different temperature are brought into proximity, and thus into close thermal contact. Jets of active galactic nuclei may act as heat pumps by bringing low-entropy gas from the cluster core into thermal contact with the hot outer atmosphere of the cluster, significantly increasing the feedback efficiency of active galactic nuclei. We test this hypothesis by running a suite of 3D magnetohydrodynamic simulations of active galactic nuclei jets in a Perseus-like cluster, including anisotropic conduction. We find that the heat pump efficiency $\eta$ can reach up to 50% of the maximum possible efficiency $\eta_{\rm max}$ if conduction operates near the Spitzer-Braginskii limit, while $\eta\approx f_{\rm sp}\eta_{\rm max}$ if conduction along the field lines is substantially suppressed below the Spitzer-Braginskii value by a factor $f_{\rm sp}$ by kinetic effects, as recently suggested. We further find that jet-induced thermal conduction is self-limiting: Magnetic draping during the uplift results in a magnetic field orientation close to perpendicular to the induced temperature gradients, significantly reducing conduction along the ideal conductive pathways. Thus, for conservative assumptions about thermal conduction suppression by $f_{\rm sp} \lesssim 0.1$, the heat pump effect leads to only marginal heat transfer and, correspondingly, to immaterial changes in the overall thermal evolution of cool core clusters beyond the isolated effects of conduction and jet-induced heating alone.
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All authors Jennifer Stafford, Sebastian Heinz, Mateusz Ruszkowski, Torsten Enßlin, Yi-Hao Chen

Solar and Stellar Astrophysics

A Not-So-Compact Companion: Massive, Oversize White Dwarf in a Post-Common Envelope Eclipsing Binary

Erin M. Motherway, Evan Linck, Robert D. Mathieu et al. (+5 more)

We provide a detailed characterization of 2M07515777+1807352, a post-common envelope eclipsing binary system with a 10.3 d, nearly, but not quite, circular orbit (e = 0.02). This system consists of a massive white dwarf (WD) ($1.08$ M$_{\odot}$) and a 4400 K main-sequence companion (0.66 M$_{\odot}$). This WD is among the most massive known within post-common envelope binary systems. We also find, through both spectral energy distribution and $\it{TESS}$ light curve analyses, that the WD has a radius of $1.54\pm 0.07 R_{\oplus}$, roughly $12\sigma$ larger than the expected value from WD mass-radius relationships. Both the Lomb-Scargle analysis and the $v \sin{i}$ of the system indicate the main-sequence companion to be super-synchronously rotating at a period of $\sim$6 days, which may suggest accretion occurred during the evolution of the system. This binary also shares similar physical characteristics with six other post-common envelope systems hosting massive WDs, which may point to a shared formation pathway. We model the history of this system with COSMIC and find that it likely formed through an episode of common envelope evolution following the onset of mass transfer when the progenitor primary was on either the early or the thermally pulsing stages of the asymptotic giant branch. As a result of its properties, the study of 2M07515777+1807352 can provide new insights regarding many key outstanding questions in our understanding of common envelope evolution.
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All authors Erin M. Motherway, Evan Linck, Robert D. Mathieu, Don Dixon, Keivan G. Stassun, Katelyn Breivik, Steven R. Majewski, Onno Pols

Cosmology and Nongalactic Astrophysics

Cosmo-FOLD: Fast generation and upscaling of field-level cosmological maps with overlap latent diffusion

Satvik Mishra, Roberto Trotta and Matteo Viel

We demonstrate the capabilities of probabilistic diffusion models to reduce dramatically the computational cost of expensive hydrodynamical simulations to study the relationship between observable baryonic cosmological probes and dark matter at field level and well into the non-linear regime. We introduce a novel technique, Cosmo-FOLD (Cosmological Fields via Overlap Latent Diffusion) to rapidly generate accurate and arbitrarily large cosmological and astrophysical 3-dimensional fields, conditioned on a given input field. We are able to generate TNG300-2 dark matter density and gas temperature fields from a model trained only on ~1% of the volume (a process we refer to as `upscaling'), reproducing both large scale coherent dark matter filaments and power spectra to within 10% for wavenumbers k <= 5 h Mpc^-1. These results are obtained within a small fraction of the original simulation cost and produced on a single GPU. Beyond one and two points statistics, the bispectrum is also faithfully reproduced through the inclusion of positional encodings. Finally, we demonstrate Cosmo-FOLD's generalisation capabilities by upscaling a CAMELS volume of 25 (Mpc h^-1)^3 to a full TNG300-2 volume of 205 (Mpc h^-1)^3$ with no fine-tuning. Cosmo-FOLD opens the door to full field-level simulation-based inference on cosmological scale.
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High Energy Astrophysical Phenomena

Fast X-ray transients in NuSTAR data

Murray Brightman, Joahan Castañeda Jaimes, Daniel Stern et al. (+1 more)

Fast X-ray transients (FXTs) are flashes of X-rays that last for a few hundreds of seconds to a few hours. An enigmatic population of these transients that did not have a clear origin has been known for several decades, mostly found serendipitously in soft X-ray imaging observations. Recent progress in this field by Einstein Probe has found that many FXTs are associated with gamma-ray bursts and the collapse of massive stars. Motivated by this, we searched the NuSTAR archive in the harder 3–79 keV band for $\sim1000$ s duration transients. From 204 Ms of exposure we present five candidate FXTs, four of which are spectrally hard, with power-law indices $-3<\Gamma<0$, standing them apart from FXTs discovered in the soft band. Three have potential associations with galaxies at $z=0.1-2$, implying 3–79 keV luminosities of $10^{43}$ to $10^{48}$ erg s$^{-1}$ and volumetric event rates of 125–2900 Gpc$^{-3}$ yr$^{-1}$. The properties of these NuSTAR FXTs most resemble low-luminosity gamma-ray bursts, and would be much more common than their higher-luminosity counterparts in this redshift range.
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All authors Murray Brightman, Joahan Castañeda Jaimes, Daniel Stern, Brian Grefenstette