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Physics Astrophysics

astro-ph Watch Astrophysics

Tuesday 27 January

Astrophysics of Galaxies

CH3CCH as a thermometer in warm molecular gas

Yuqiang Li, Junzhi Wang, Juan Li et al. (+9 more)

Kinetic temperature is a fundamental parameter in molecular clouds. Symmetric top molecules, such as NH$_3$ and CH$_3$CCH, are often used as thermometers. However, at high temperatures, NH$_3$(2,2) can be collisionally excited to NH$_3$(2,1) and rapidly decay to NH$_3$(1,1), which can lead to an underestimation of the kinetic temperature when using rotation temperatures derived from NH$_3$(1,1) and NH$_3$(2,2). In contrast, CH$_3$CCH is a symmetric top molecule with lower critical densities of its rotational levels than those of NH$_3$, which can be thermalized close to the kinetic temperature at relatively low densities of about 10$^{4}$ cm$^{-3}$. To compare the rotation temperatures derived from NH$_3$(1,1)$\&$(2,2) and CH$_3$CCH rotational levels in warm molecular gas, we used observational data toward 55 massive star-forming regions obtained with Yebes 40m and TMRT 65m. Our results show that rotation temperatures derived from NH$_3$(1,1)$\&$(2,2) are systematically lower than those from CH$_3$CCH 5-4. This suggests that CH$_3$CCH rotational lines with the same $J$+1$\rightarrow$$J$ quantum number may be a more reliable thermometer than NH$_3$(1,1)$\&$(2,2) in warm molecular gas located in the surroundings of massive young stellar objects or, more generally, in massive star-forming regions.
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All authors Yuqiang Li, Junzhi Wang, Juan Li, Xing Lu, Siqi Zheng, Chao Ou, Qian Huang, Miguel Santander-García, José Jairo Díaz Luis, Seokho Lee, Tie Liu, Zhiqiang Shen

High Energy Astrophysical Phenomena

STONKS first results: Long-term transients in the XMM-Newton Galactic plane survey

Robbie Webbe, E. Quintin, N.A. Webb et al. (+5 more)

The study of astronomical transients at high energies provides insights into some of the most extreme physical events in the universe; however, carrying out their detection and fast follow-up studies are limited by instrumental constraints. Search for Transient Object in New observations using Known Sources (STONKS) is a near-real-time transient detection system for XMM-Newton offering the capability to detect transients in XMM-Newton observations at fainter fluxes than can be achieved with wide survey instruments. We present the transients detected with the STONKS pipeline found in an XMM-Newton multi-year heritage survey of the Galactic plane to identify and classify highly variable X-ray sources that have recently been reported in this region. We examined the alerts created by the STONKS pipeline from over 200 XMM observations of the Galactic plane, screening for instrumental effects. The 78 alerts associated with 70 real astrophysical sources were then subjected to further temporal and spectral analysis. From the 70 sources we identified, we were able to classify 32 with a high degree of confidence, including 7 X-ray binaries, 1 $\gamma$-Cas analogue, and 1 magnetar candidate. Of the 70 sources, 23 were detected for the first time in X-rays. This systematic analysis of publicly available data has shown the value and potential of STONKS in the application to XMM-Newton observations. It will enable the community to detect transient and highly variable sources at fainter fluxes than with any other X-ray transient detection systems.
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All authors Robbie Webbe, E. Quintin, N.A. Webb, Gabriele Ponti, Tong Bao, Chandreyee Maitra, Shifra Mandel, Samaresh Mondal

Solar and Stellar Astrophysics · Cross-listed from physics.ao-ph

Continued activity of the 25th cycle: largest in 20 years. Ground-level enhancement and Forbush decrease

B.Sargsyan and A.Chilingarian

After a very calm 24th solar activity cycle, the 25th cycle has already seen several interesting events. A Ground Level Enhancement GLE77 was observed on 11 November 2025 following an X5.1 class solar flare. A strong Forbush decrease occurred on 19 and 20 January 2026 during one of the most intense geomagnetic storms of Solar Cycle 25. Events were recorded coherently by the global neutron monitor network and by SEVAN detectors at multiple altitudes. Using spectrometric capabilities, we reconstruct energy spectra of missing neutrons and muons during the FD and compare them with corresponding spectra measured during GLE77. The analysis demonstrates that FD and GLE signatures are intrinsically asymmetric. FDs selectively suppress the preexisting galactic cosmic ray population, whereas GLEs introduce an additional, harder particle component. Neutron and muon channels exhibit markedly different spectral behavior, particularly at higher deposited energies, reflecting their sensitivity to different primary energy ranges. These results show that combined NM and SEVAN observations provide robust, complementary diagnostics of rigidity dependent cosmic ray modulation during extreme heliospheric disturbances.
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Cosmology and Nongalactic Astrophysics

Local environmental dependence on weak-lensing shear statistics

Sonia Akter Ema, Md Rasel Hossen, Krzysztof Bolejko et al. (+1 more)

Despite the assumption that an ideal FLRW observer is not dependent on the local environment, observations are biased by the positions of the observers due to the matter correlations in the large-scale structure (LSS) of the universe. The variation of the mass distribution of the LSS of the universe implies that observers residing in different locations may suffer bias in their measurements when they look at the images of distant galaxies. Here, we assess the influence of the local environment on weak gravitational lensing (WL) shear statistics in the context of relativistic $N$-body code, gevolution. We derive numerical constraints on the cosmological parameters from the WL shear angular power spectrum and comment on the local environment's influence on WL shear. We find tighter constraints on the parameter $\Omega_\mathrm{m}$ above redshift $z$ = 0.2, which implies over this redshift the local environment's impact is minor. We also investigate the bispectrum and conclude that on average the impact of the local environment on $f_{\rm NL}$ (a measure of non-Gaussianities) is minimal and consistent with zero effect. However, we find that within the assembly of all possible observers/locations, there will also be a few that could infer the parameter $f_{\rm NL}$ of the order 10. These results could thus be used to estimate the uncertainty in the inference of cosmological parameters such as $f_{\rm NL}$ based on WL shear bispectrum and thus may have implications for future surveys requiring precision at the percent level.
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All authors Sonia Akter Ema, Md Rasel Hossen, Krzysztof Bolejko, Geraint F. Lewis

Instrumentation and Methods for Astrophysics · Cross-listed from hep-ex

Cosmic Rays as an Interdisciplinary Earth Observation Tool: From Particle Physics and Atmospheric Processes to Geosciences and Urban Science

Bugra Bilin and Nuhcan Akçit

The exploration of cosmic rays, which are high-energy particles originating from space and the atmosphere, has historically been associated with particle physics and astrophysics. In the last 20 years, these particles have evolved into valuable tools for observing Earth's systems. This review compiles the use of cosmic rays in three primary areas: (1) particle physics and atmospheric processes, which include cosmic- ray-induced cascades, ionization, and their impact on atmospheric chemistry and radiation; (2) geosciences, where cosmogenic radionuclides assist in the dating of geological materials and cosmic-ray neutrons are used for large-scale monitoring of soil moisture and snow water equivalents; and (3) urban science, where cosmic- ray muons are employed for non-invasive subsurface imaging and, when paired with distributed sensors, serve as the basis for smart city monitoring. This review places particular emphasis on integrating these methods with remote sensing and geographic information systems (GIS), which helps close the persistent scale gap between point measurements and satellite observations, thereby enabling three-dimensional digital representations of subsurfaces. This review concludes by discussing data standards, their integration into operational Earth observation workflows, and future research directions.
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High Energy Astrophysical Phenomena

Constraining FRB Microstructure with Polarised Shot Noise

J. C. F. Balzan, A. Bera, C. W. James et al. (+1 more)

We present FIRES, a polarised shot-noise (PSN) framework that models fast radio burst (FRB) dynamic spectra as the incoherent superposition of Gaussian microshots with varying polarisation angles (PAs). Applied to the CRAFT bursts FRB 20191001A and FRB 20240318A, FIRES can reproduce key spectro-polarimetric behaviours seen in these data: scattering suppresses PA variability on the trailing edge, while the leading edge preferentially retains intrinsic structure when sufficient signal-to-noise is present. We quantify this behaviour using the PA variance ratio $\mathcal{R}_\psi$ and explore the joint plane of measured linear polarisation fraction $\Pi_L$ versus PA variance to identify allowed regions of microshot number $N$, intrinsic PA dispersion $\sigma_\psi$, and intrinsic linear fraction $\Pi_{L,0}$ at fixed signal-to-noise. This restricts these combinations permitted within the adopted shot-noise framework. For FRB 20191001A, the data are consistent with an extended parameter space, reflecting degeneracies between intrinsic PA structure, microshot superposition, scattering, finite sampling, noise, and the assumed microshot-property distributions. FRB 20240318A occupies a more restricted region, favouring fewer microshots and larger intrinsic PA dispersion. By combining an emission-mechanism-independent forward-modeling framework with minimal assumptions and observational constraints, FIRES facilitates qualitative and quantitative exploration of how microshot superposition, scattering, finite sampling, and noise can shape observed FRB polarimetry under the PSN model.
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All authors J. C. F. Balzan, A. Bera, C. W. James, B. Meyers

Instrumentation and Methods for Astrophysics

Development of Electroformed X-ray Optics Bridging Synchrotron Technology and Space Astronomy

Ryuto Fujii, Koki Sakuta, Kazuki Ampuku et al. (+21 more)

We have developed X-ray telescope mirrors using an original electroforming replication technique established through the fabrication of millimeter-aperture, ultra-short-focal-length nanofocusing mirrors for synchrotron X-ray microscopy. This paper presents detailed results of X-ray illumination tests of a 60-mm-diameter, full-circumference, double-reflection monolithic electroformed nickel mirror and its Mirror Module Assembly (MMA). The experiments were conducted at the 1-km beamline BL29XUL at SPring-8. To simulate a parallel X-ray beam from celestial sources, we constructed a dedicated evaluation system, the High-Brilliance X-ray Kilometer-long Large-Area Expanded-beam Evaluation System (HBX-KLAEES). Owing to the high photon flux and the quasi-point-like source with a small divergence provided by HBX-KLAEES, the imaging performance was evaluated with high fidelity, resolving both the sharp core and large-angle components of the Point Spread Function (PSF). The results show an extremely sharp core with a Full Width at Half Maximum (FWHM) of 0.7 arcsec and a Half Power Diameter (HPD) of 14 arcsec, even after integration into the MMA. In addition, a positive correlation was found between angular resolution and axial figure error in both the primary and secondary mirror sections, indicating that axial figure errors contribute to image degradation. Based on these results, the MMA was selected as one of the hard X-ray optics for the FOXSI-4 sounding rocket experiment, which performs high-resolution soft and hard X-ray imaging spectroscopy of solar flares and was successfully launched. These results demonstrate the potential for further improvements in angular resolution and the development of high-resolution, ultra-short focal length X-ray optics for small satellites, including CubeSats.
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All authors Ryuto Fujii, Koki Sakuta, Kazuki Ampuku, Yusuke Yoshida, Makoto Yoshihara, Ayumu Takigawa, Keitoku Yoshihira, Tetsuo Kano, Naoki Ishida, Noriyuki Narukage, Keisuke Tamura, Kikuko Miyata, Gota Yamaguchi, Hidekazu Takano, Yoshiki Kohmura, Shutaro Mohri, Takehiro Kume, Yusuke Matsuzawa, Yoichi Imamura, Takahiro Saito, Kentaro Hiraguri, Hirokazu Hashizume, Hidekazu Mimura, Ikuyuki Mitsuishi

Cosmology and Nongalactic Astrophysics

Q-balls from thermal balls during a first-order phase transition: a numerical study

Yuan-Jie Li, Jing Liu and Zong-Kuan Guo

We numerically study the Q-ball formation triggered by a cosmological first-order phase transition within the Friedberg-Lee-Sirlin model. By performing lattice simulations, we track the nonequilibrium dynamics throughout the transition, providing a precise description of the Q-ball formation mechanism and the resulting mass spectrum. Collapsing false-vacuum regions first form thermal balls, which subsequently cool via dissipative interactions and stabilize into long-lived Q-balls with nonzero spin. We observe a large population of low-mass Q-balls, as well as rare, massive Q-balls that are several times larger than the analytical prediction. The final Q-ball population exhibits a broad mass spectrum spanning over two orders of magnitude, characterized by an exponential tail of number density at large masses. The simulations suggest that the Q-ball abundance is approximately $50\%$ higher than predicted by analytical estimates, adjusting the result in the context of Q-balls as dark matter candidates.
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Astrophysics of Galaxies

ERGO-ML: The assembly histories of HSC galaxy images via invertible neural networks, contrastive learning, and cosmological simulations

Lukas Eisert, Connor Bottrell, Annalisa Pillepich et al. (+4 more)

In this paper of ERGO-ML (Extracting Reality from Galaxy Observables with Machine Learning), we develop a model that infers the merger/assembly histories of galaxies directly from optical images. We apply the self-supervised contrastive learning framework NNCLR (Nearest-Neighbor Contrastive Learning of visual Representations) on realistic HSC mock images (g,r,i - bands) produced from galaxies simulated within the TNG50 and TNG100 flagship runs of the IllustrisTNG project. The resulting representation is then used as conditional input for a cINN (conditional Invertible Neural Network) to gain posteriors for merger/assembly statistics, particularly the lookback time and stellar mass of the last major merger and the fraction of ex-situ stars. Through validation against the ground truth available for simulated galaxies, we assess the performance of our model, achieving good accuracy in inferring the stellar ex-situ fraction ($\le \pm 10$ per cent for 80 per cent of the test sample) and the mass of the last major merger (within $\pm 0.5 \log \MSUN$ for stellar masses $>10^{9.5} \MSUN$ ). We successfully apply the TNG-trained model to simulated mocks from the EAGLE simulation, demonstrating that our model is applicable outside of the TNG domain. We use our simulation-based model to infer aspects of the history of observed galaxies, in particular for HSC images that are close to the domain of TNG ones. We recover the trend of increasing ex-situ stellar fraction with stellar mass and more spherical morphology, but we also identify a discrepancy between TNG and HSC: on average, observed galaxies generally exhibit lower ex-situ fractions. Despite challenges such as information loss (e.g. projection effects and surface brightness limits) and domain shifts (from simulations to observations), our results demonstrate the feasibility of extracting the merger past of galaxies from their optical images.
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All authors Lukas Eisert, Connor Bottrell, Annalisa Pillepich, Dylan Nelson, Rhythm Shimakawa, Marc Huertas-Company, Ralf S. Klessen

High Energy Astrophysical Phenomena

Latent characterisation of the complete BATSE gamma ray bursts catalogue using Gaussian mixture of factor analysers and model-estimated overlap-based syncytial clustering

Fan Dai and Ranjan Maitra

Characterising and distinguishing gamma-ray bursts (GRBs) has interested astronomers for many decades. While some authors have found two or three groups of GRBs by analyzing only a few parameters, recent work identified five ellipsoidally-shaped groups upon considering nine parameters $T_{50}, T_{90}, F_1, F_2, F_3, F_4, P_{64}, P_{256}, P_{1024}$. Yet others suggest sub-classes within the two or three groups found earlier. Using a mixture model of Gaussian factor analysers, we analysed 1150 GRBs, that had nine parameters observed, from the current Burst and Transient Source Experiment (BATSE) catalogue, and again established five ellipsoidal-shaped groups to describe the GRBs. These five groups are characterised in terms of their average duration, fluence and spectrum as shorter-faint-hard, long-intermediate-soft, long-intermediate-intermediate, long-bright-intermediate and short-faint-hard. The use of factor analysers in describing individual group densities allows for a more thorough group-wise characterisation of the parameters in terms of a few latent features. However, given the discrepancy with many other existing studies that advocated for two or three groups, we also performed model-estimated overlap-based syncytial clustering (MOBSynC) that successively merges poorer-separated groups. The five ellipsoidal groups merge into three and then into two groups, one with GRBs of low durations and the other having longer duration GRBs. These groups are also characterised in terms of a few latent factors made up of the nine parameters. Our analysis provides context for all three sets of results, and in doing so, details a multi-layered characterisation of the BATSE GRBs, while also explaining the structure in their variability.
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High Energy Astrophysical Phenomena

Maximum Energy of Particles Accelerated in Gamma-Ray Burst Afterglow Shocks

Zhao-Feng Wu, Sofía Guevara-Montoya, Paz Beniamini et al. (+3 more)

Particle acceleration in relativistic collisionless shocks remains an open problem in high-energy astrophysics. Particle-in-cell (PIC) simulations predict that electron acceleration in weakly magnetized shocks proceeds via small-angle scattering, leading to a maximum electron energy significantly below the Bohm limit. This upper bound on electron energy manifests observationally as a characteristic synchrotron cutoff, providing a direct probe of the underlying acceleration physics. Gamma-ray burst (GRB) afterglows offer an exceptional laboratory for testing these predictions. Here, we model the spectral evolution of GRB afterglows during the relativistic deceleration phase, incorporating PIC-motivated acceleration prescriptions and self-consistently computing synchrotron and synchrotron self-Compton emission. We find that low-energy bursts in low-density environments, typical of short GRBs, exhibit a pronounced synchrotron cutoff in the GeV band within minutes to hours after the trigger. Applying our framework to GRB 190114C and GRB 130427A, we find that current observations are insufficient to discriminate between PIC-motivated acceleration and the Bohm limit, primarily due to poor photon statistics in the Fermi-LAT band. Nevertheless, future MeV-TeV afterglow observations can break model degeneracies and place substantially tighter constraints on the mechanisms responsible for particle acceleration in relativistic shocks. To this end, we simulate a fiducial nearby short GRB as a promising probe of the cutoff location, for which the two acceleration scenarios are cleanly distinguishable and the detection of such an event in the near future remains feasible.
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All authors Zhao-Feng Wu, Sofía Guevara-Montoya, Paz Beniamini, Dimitrios Giannios, Daniel Grošelj, Lorenzo Sironi

Earth and Planetary Astrophysics · Cross-listed from gr-qc

Spiral Density Waves and Torque Balance in the Kerr Geometry

Conor Dyson and Daniel J. D'Orazio

Extreme mass-ratio inspirals (EMRIs) in relativistic accretion discs are a key science target for the upcoming LISA mission. Existing models of disc-EMRI interactions typically rely on crude dynamical friction or Newtonian planetary migration prescriptions, which fail to capture the relativistic fluid response induced by the binary potential. In this work we address this gap by providing the relativistic calculation. We apply standard methods from self-force theory, black hole perturbation theory, and relativistic stellar perturbation theory to perform the full fluid calculation of the relativistic analogue of planetary migration for the first time. We calculate the response of a fluid in the perturbing potential of an EMRI consistently incorporating pressure effects. Using a master enthalpy-like variable and linearised fluid theory, we reconstruct the fluid perturbations and relativistic spiral arm structure for a range of spin values in the Kerr geometry. We conclude by deriving a relativistic torque-balance equation that enables computation and comparison of local torques with advected angular momentum through the disc. This opens a promising route towards establishing torque-balance relations between integrated disc torques arising from fluid perturbations and the forces acting on EMRIs embedded in matter.
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Cosmology and Nongalactic Astrophysics

Non-linear evolution in $f(R)$ gravity: perturbative modelling of the Chameleon mechanism

Sharvari Nadkarni-Ghosh and Tanush Reddy Vaka

We investigate the non-linear evolution of matter perturbations in $f(R)$ models with the Chameleon screening mechanism. The novel feature of our investigation is an iterative solution for the non-linear equation for the scalar field $\chi = \Phi - \Psi$, where $\Phi$ and $\Psi$ are the potentials that characterise scalar perturbations of the metric. We demonstrate the scheme on spherical perturbations - smooth, compensated top-hats of varying length scales. We find that the effect of the Chameleon mechanism is seen most prominently on scales where the size of the top-hat is comparable to the Compton scale of the background. There is a density enhancement near the outer edge of the top-hat and the top-hat does not retain its shape. We explain this well-known observation in the context of the spatio-temporal evolution of the Compton scale. Additionally, we find a slight enhancement of the density near the origin, a feature not reported previously in the literature. On scales much smaller or much larger than the background Compton length, including the Chameleon screening has no appreciable effect on the perturbations. In the former, the growth is enhanced as compared to GR and is almost the same as GR in the latter. Finally, we examine the non-linear density velocity divergence (DVDR) relation and find that for evolution affected by Chameleon screening, the DVDR is no longer one-to-one even for a single profile. The relation between density and velocity depends on the location within the perturbation.
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Monday 26 January

High Energy Astrophysical Phenomena

A multiwavelength view of the nearby Calcium-Strong Transient SN 2025coe in the X-Ray, Near-Infrared, and Radio Wavebands

Sahana Kumar, Raphael Baer-Way, Aravind P. Ravi et al. (+18 more)

Calcium-strong transients (CaSTs) are a subclass of faint and rapidly evolving supernovae (SNe) that exhibit strong calcium features and notably weak oxygen features. The small but growing population of CaSTs exhibits some aspects similar to thermonuclear supernovae but others similar to massive star core-collapse events, leading to intriguing questions on their physical origins. SN~2025coe is one of the nearest CaSTs discovered to date, and our coordinated multi-wavelength observations obtained days to weeks post-explosion reveal new insights on these enigmatic transients. With the most robust NIR spectroscopic time-series of a CaST collected to date, SN 2025coe shows spectral signatures characteristic of Type Ib SNe (i.e. He-rich stripped-envelope SNe). SN~2025coe is the third X-ray detected CaST and our analysis of Swift X-ray data suggest interaction with 0.12 $\pm\,0.11\ M_{\odot}$ of circumstellar material (CSM) extending to at least $2 \times 10^{15} $cm ($\sim 30,000\ R_{\odot}$, while our analysis of the 1–240 GHz radio non-detections gives an outer radius of that CSM of at most $\sim 5\times 10^{15}$ cm. This inferred nearby high-density CSM extending out to $(3.5 \pm 1.5) \times10^{15}$ cm is similar to that seen in the other two X-ray detected CaSTs, and its presence suggests that either intensive mass-loss from a massive star or some exotic pre-supernova mass ejection may be a common feature of this subclass. Our work also expands upon recent studies on the optical properties of SN 2025coe and explores our current understanding of different progenitor systems that could possibly produce CaSTs.
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All authors Sahana Kumar, Raphael Baer-Way, Aravind P. Ravi, Maryam Modjaz, Poonam Chandra, Stefano Valenti, Lindsey A. Kwok, Samaporn Tinyanont, Ryan J. Foley, D. Andrew Howell, Daichi Hiramatsu, Jennifer E. Andrews, K. Azalee Bostroem, Collin Christy, Noah Franz, Brian Hsu, Jeniveve Pearson, David J. Sand, Manisha Shrestha, Nathan Smith, Bhagya Subrayan

Astrophysics of Galaxies · Cross-listed from hep-ph

Low Energy Neutrinos in Milky Way and Cloud-9

Eduardo Flores, Elise Cantu, Ian Marano et al. (+3 more)

We study low-energy galactic neutrinos in the Milky Way under two descriptions of gravity and show that they can prove gravity underlying nature. If gravity is quantum, its long-range interaction produces an atom-like bound neutrino structure. However, we find that withing 292 kpc, this structure contributes only 10 to the -29 of the galaxy dark matter, ruling it out as a deark-matter candidate. Its detection would nevertheless provide direct evidence for quantum gravity mediated by gravitons. If gravity instead arises from spacetime curvature, neutrinos interact only through the short-range weak force and behave as free, collisionless classical particles orbiting the galaxy. In this regime, they do not experience Fermi pressure and can form a sufficiently compact population to reproduce the Milky Way rotation curve. We further model Cloud-9 dark-matter compoent as free, collisionless neutrinos. Because neutrino-antinetrino annihilation cross section is extremely small, neutrinos and antineutrinos may remai near equilibrium over cosmological timescales, potentially relating this framework to the observed matter-antimatter assymmetry.
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All authors Eduardo Flores, Elise Cantu, Ian Marano, Osvan Vivar-Garcia, Shabhaz Khalandar, Patrick Walker

Astrophysics of Galaxies

High resolution observations of 'dark' neutral hydrogen clouds in the Virgo cluster with the Very Large Array

Robert Minchin, Rhys Taylor, Emmanuel Momjian et al. (+2 more)

We have observed six `dark' neutral hydrogen (HI) clouds discovered in the Virgo cluster by the Arecibo Galaxy Environment Survey (AGES) with the Karl G. Jansky Very Large Array (VLA), giving higher angular and velocity resolution than the original AGES observations. We detected compact HI emission in two of the sources, AGESVC1 231 and AGESVC1 274, allowing us to firmly associate them with faint ($m_g > 18.5$), blue ($g-i < 0.1$) optical counterparts with high $M_{HI}/L_g$ ratios. In a further two sources, we detected low column-density extended HI emission, consistent with these being dispersing clouds from ram-pressure stripping or tidal interactions. The final two sources were not detected with the VLA, allowing us to set low column-density limits on the HI detected by AGES that are consistent with these clouds also being formed from HI that is dispersing into the intra-cluster medium. The four HI sources not associated with optical counterparts thus appear likely to be relatively short-lived objects. No evidence was found for either pressure-supported turbulent spheres or stable dark galaxies.
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All authors Robert Minchin, Rhys Taylor, Emmanuel Momjian, Boris Deshev, Vojtěch Partík

High Energy Astrophysical Phenomena · Cross-listed from physics.plasm-ph

Energy partition in collisionless counterstreaming plasmas

Alexis Marret and Frederico Fiuza

Fast, counter-streaming plasma outflows drive magnetic field amplification, plasma heating, and particle acceleration in numerous astrophysical environments, from supernova remnant shocks to active galactic nuclei jets. Understanding how, in the absence of Coulomb collisions, energy is redistributed between the different plasma species remains a fundamental open question. We use 3D fully-kinetic simulations to investigate energy partition in weakly magnetized counter-propagating plasmas. Our results reveal a complex interplay between different processes, where at early times the Weibel instability drives a first stage of magnetic field amplification and at late times the kinking of current filaments drives a second amplification stage via a dynamo-type mechanism. Electrons are heated primarily during the latter phase through magnetic pumping. By the time the flows thermalize, we observe that the final temperature ratio $T_e/T_i$ and energy partition depend on the ion-to-electron mass ratio. For electron-proton flows, the electron thermal energy only reaches up to a few percent of the initial ion kinetic energy.
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High Energy Astrophysical Phenomena

Progenitor of the recoiling super-massive black hole RBH-1 identified using HST/JWST imaging

Tousif Islam, Tejaswi Venumadhav and Digvijay Wadekar

Using a combination of Hubble Space Telescope and James Webb Space Telescope imaging, a runaway supermassive black hole (RBH-1) was recently identified with an inferred velocity of $954^{+110}_{-126}\,\mathrm{km\,s^{-1}}$, likely ejected from a compact star-forming galaxy (denoted as GX) at $z \approx 0.96$. Assuming the runaway black hole was the outcome of the gravitational-wave-driven merger of two black holes, we use its measured runaway velocity together with gravitational-wave recoil predictions from numerical relativity and black hole perturbation theory to constrain the mass ratio and spin configuration of the progenitor SMBHs that overcame the final-parsec problem and merged $\sim 70$~Myr ago. We find that the progenitor binary must have been precessing, with a mass ratio $m_1/m_2\lesssim 6$, and that the more massive SMBH must have possessed a high spin (dimensionless spin magnitude $\sim 0.75$) in order to generate a recoil of this magnitude. This has important astrophysical implications as similar SMBH mergers can be an interesting source population for the upcoming LISA mission with signal-to-noise ratios $\gtrsim$ 1000. Furthermore, the progenitor SMBH properties imply that GX was likely formed through a major, gas-rich (“wet”) merger between two galaxies of comparable mass, with a mass ratio $\lesssim 4$.
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Solar and Stellar Astrophysics

The MIRI Excess around Degenerates (MEAD) Survey II: A Probable Planet detected via IR Excess around WD 0644+025

Sabrina Poulsen, John Debes, Ashley Messier et al. (+14 more)

The MIRI Excess Around Degenerates (MEAD) Survey is a cycle 2 JWST program designed to image nearby white dwarfs with MIRI at 10 and 15 microns. This survey targeted 56 white dwarfs within 25 pc to search for mid-infrared excesses, flux deficits from collision-induced absorption, and resolved substellar companions. In this paper we present our analysis of WD 0644+025, an unusually massive white dwarf (0.95 Msun) and the MEAD target exhibiting the most significant mid-infrared excess. The observed JWST MIRI photometry shows a 7.3 sigma excess at 15 microns and a 3.6 sigma excess at 10 microns, which may be associated with either a planetary companion or a circumstellar dust disk. This excess corresponds to a companion mass of 6.8 Mjup (Teff=261 +/- 9 K) with orbital distance <11.8 au, although substantially lower masses are possible if we consider a closely orbiting insolated companion. No spatially resolved sources are detected within 200 au, with contrast curve analysis excluding planets more massive than 2 Mjup beyond ~12 au. Metal pollution is confirmed in both archival Keck HIRES spectra from 1999 and new observations from 2025, with no evidence suggesting the accretion rate has substantially changed over the decades. We explore possible dust disk morphologies to describe the observed IR excess, and find that traditional debris disks struggle to fit our data. WD 0644+025 thus represents a compelling case study in the growing population of white dwarfs with cold infrared excesses, and highlights JWST's ability to probe planetary system remnants inaccessible to prior infrared observatories.
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All authors Sabrina Poulsen, John Debes, Ashley Messier, Erika Le Bourdais, Carl Melis, Misty Cracraft, Samuel Boucher, Mukremin Kilic, Scott Kenyon, Mark C. Wyatt, Seth Redfield, Patrick Dufour, Loic Albert, Susan E. Mullally, William T. Reach, Fergal Mullally, David A. Golimowski

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

Reconstructing inflation in Einstein-Gauss-Bonnet gravity in light of ACT data

Ramón Herrera and Carlos Ríos

During the inflationary epoch, we investigate the reconstruction of the background variables within the framework of Einstein-Gauss-Bonnet gravity, considering the scalar spectral index $n_s(N)$ and the tensor-to-scalar ratio $r(N)$, where $N$ denotes the number of $e-$folds. Under a general formalism, we determine the effective potential and the coupling function associated with the Gauss-Bonnet term as functions of the cosmological parameters $n_s(N)$ and $r(N)$, respectively. To implement the reconstruction methodology for the background variables, we study an example in which the attractors for the index $n_s$ and the ratio $r$ are in agreement with Atacama Cosmology Telescope (ACT) data. In this context, explicit expressions for the effective potential $V(\phi)$ and the coupling parameter $\xi(\phi)$ are reconstructed. Moreover, the reconstruction based on observational parameters shows that $V(\phi)\not\propto 1/\xi(\phi)$, in contrast to the assumption adopted in the literature for the study of the evolution of the universe in Einstein-Gauss-Bonnet gravity.
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Astrophysics of Galaxies

Statistical Predictions of the Accreted Stellar Halos around Milky Way-Like Galaxies

J. Sebastian Monzon, Frank C. van den Bosch and Martin P. Rey

In the $\Lambda$CDM paradigm, stellar halos form through the accretion and disruption of satellite galaxies. We introduce new semi-analytic modeling within the SatGen framework to track the ex-situ stellar components of Milky Way–like galaxies across large ensembles of merger trees, enabling a statistical study of the stochastic nature of galaxy assembly. We find that accreted stellar halos are typically built by only a few progenitors and are highly sensitive to the fate of the most massive satellite, producing order-of-magnitude variations in accreted stellar halo mass even at fixed host halo mass. Different stellar components trace distinct phases of host halo growth: central and accreted stellar mass correlate most strongly with early assembly, while surviving satellites trace more recent accretion. Finally, using Random Forest Regression, we quantify how well observable galaxy properties can recover halo assembly histories, providing a framework for interpreting upcoming low-surface-brightness observations of stellar halos.
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High Energy Astrophysical Phenomena

Multiwavelength Analysis of Six Luminous Fast Blue Optical Transients

Cassie Sevilla, Anna Y. Q. Ho, Nayana A.J. et al. (+43 more)

We present multiwavelength observations and analysis of six luminous fast blue optical transients (LFBOTs) discovered in Zwicky Transient Facility (ZTF) survey data. We identified these LFBOTs from their fast light-curve evolution ($t_{1/2}\leq 12 $d), blue colors at peak brightness ($g-r\leq-0.5 $mag), a visible host galaxy, high optical luminosity ($M_g<-20$), and an X-ray or radio detection. With the exception of AT2024aehp (ZTF24abygbss), these transients exhibit peaks in their $10\,$GHz radio light curves at $t_{\text{rest}} \approx 50-100$ d, with peak radio luminosities ranging from $10^{38}-10^{40}$ erg s$^{-1}$. Modeling the radio emission as synchrotron radiation indicates a fast ($v=0.1-0.3c$) shock in a dense ($n_e\approx10^{3}-10^{4}$ cm$^{-3}$) medium. The X-ray emission varies by $\approx2$ orders of magnitude in luminosity ($10^{42}-10^{44}$ erg s$^{-1}$) at $t_{\text{rest}}\sim20 $d. Analysis of the host-galaxy photometry and spectroscopy for each transient shows that they are predominantly nonnuclear (a few kpc offset) with star-forming host galaxies of stellar masses $10^{9}-10^{11} ,M_\odot$. Unlike all other LFBOTs to date, AT2024aehp exhibited a luminous ($M<-19 $mag) plateau in the optical light curve; spectra during this plateau phase showed a featureless blue continuum. The $6-15$ GHz radio emission of AT2024aehp brightened by over an order of magnitude from $t_{\text{rest}} \approx70 $d to $t_{\mathrm{rest}} \approx130 $d. The mostly consistent radio behavior between optically selected LFBOTs implies a similar circumburst medium, leading us to prefer a progenitor scenario in which mass is lost in a consistent way shortly prior to the terminal event, such as a massive star merging with a compact object.
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All authors Cassie Sevilla, Anna Y. Q. Ho, Nayana A.J., Steve Schulze, Daniel A. Perley, Michael Bremer, Igor Andreoni, Ivan Altunin, Thomas G. Brink, Michael Camilo, Poonam Chandra, Ping Chen, Ashley A. Chrimes, Michael W. Coughlin, Kaustav K. Das, Andrew Drake, Alexei V. Filippenko, Christoffer Fremling, James Freeburn, Avishay Gal Yam, Mary Gerhart, Matthew J. Graham, George Helou, K-Ryan Hinds, Natalya Johnson, Mansi M. Kasliwal, Harsh Kumar, Russ R. Laher, Natalie LeBaron, Maggie L. Li, Chang Liu, Ben Margalit, Gokul P Srinivasaragavan, Yu-Jing Qin, Nabeel Rehemtulla, Sophia Risin, Sam Rose, Rupak Roy, Ben Rusholme, Genevieve Schroeder, Jesper Sollerman, Kailai Wang, Jacob L. Wise, Yi Yang, Yuhan Yao, WeiKang Zheng

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

Reconsidering the consistent use of precessing, higher order multipole models for gravitational wave analyses

Charlie Hoy

The growing number of gravitational-wave (GW) observations allows for constraints to be placed on the underlying population of black holes; current estimates show that black hole spins are small, with binaries more likely to have comparable component masses. Since general relativistic effects, such as spin-induced orbital precession and higher order multipole moments, are more likely to be observed for asymmetric binary systems, a direct measurement remains unlikely. Nevertheless, we continue to consistently probe these effects by performing Bayesian inference with our most accurate and computationally expensive models. As the number of GW detections increases, it may soon become infeasible to consistently use these models for analyses. In this paper, we provide a selection criterion that determines when less accurate and computationally cheaper models can be used without giving biased estimates for the population properties of black holes in the Universe. We show that when using our selection criterion, comparable estimates can be obtained for the underlying mass and spin distribution of black holes for a simulated “worst-case” scenario population, while reducing the overall cost of performing Bayesian inference on our population by $\sim 20\%$. We anticipate a reduction of up to $78\%$ in the overall cost for an astrophysically motivated population, since there are fewer events with observable spin-precession and higher order multipole power.
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Instrumentation and Methods for Astrophysics

Uncertainties in Low-Count STIS Spectra

Joshua D. Lothringer, Leonardo dos Santos, Joleen Carlberg et al. (+2 more)

We evaluate uncertainty calculations in the calstis pipeline for data in the low-count regime. Due to the low dark rate and read-noise free nature of MAMA detectors, observations of UV-dim sources can result in exposures with 0 or 1 counts in some pixels. In this regime, the "root-N" approximation widely used to calculate uncertainties breaks down, and one must compute Poisson confidence intervals for more accurate uncertainty calculations. The CalCOS pipeline was updated in 2020 to account for these low-count uncertainties. Here, we assess how STIS observations are currently affected by this phenomenon, describe a new Jupyter notebook exploring the issue, and introduce a new utility, stistools.poisson_err, to manually calculate Poisson confidence intervals for 1D STIS spectra. Additionally, we describe a related software bug in the stistools$.$inttag utility, which splits TIME-TAG data into sub-exposures. This newly fixed bug serves as a useful case-study for the proper use of Poisson confidence intervals.
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All authors Joshua D. Lothringer, Leonardo dos Santos, Joleen Carlberg, Sean Lockwood, Jacqueline Brown

Solar and Stellar Astrophysics

The Solar Neighborhood LIV: 54 Orbits of M Dwarf Multiples within 30 Parsecs with Speckle Interferometry at SOAR

Eliot Halley Vrijmoet, Andrei Tokovinin, Todd J. Henry et al. (+3 more)

We present 1066 speckle measurements of M dwarf multiples observed over 2021-2024, all taken with HRCam on the Southern Astrophysical Research 4.1 m telescope. Among these, 900 observations resolve companions in 212 pairs, with separations spanning 17 milliarcseconds to 3.4 arcsec and brightness differences ranging from 0 to 4.9 magnitudes in the I filter. We have characterized the orbits of 54 of these companions, spanning periods of 0.67-30 yr, by combining our data with literature astrometry, radial velocities, and, in four cases, Hipparcos-Gaia accelerations. Among the orbits presented here are 28 that are the first-ever such characterizations for their systems, and 26 that revise previously-published orbits, thus providing a significant update to the observed dynamics of M dwarfs in the solar neighborhood. From these orbits, we provide new and updated dynamical total masses for these systems, precise to 0.7-7% in nearly all cases. Future mass derivations for components in these systems will contribute to efforts in refining the mass-luminosity relation for the smallest stars, and will enhance investigations of age, magnetism, and metallicity effects on luminosities at a given mass.
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All authors Eliot Halley Vrijmoet, Andrei Tokovinin, Todd J. Henry, Jennifer G. Winters, Wei-Chun Jao, Elliott Horch

Astrophysics of Galaxies

Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger

Davide Massari, Chiara Zerbinati, Cristiano Fanelli et al. (+15 more)

The merger history of the Galaxy has been traced back firmly to redshift 2 (10 Billion years ago). While evidence for the existence of at least one more significant merger before this time has been presented, its interpretation is yet to be fully established. Here we show that the population of globular clusters around the Galaxy depicts three distinct age-metallicity sequences, one associated with the progenitor of the Milky Way, one with the merger with Gaia-Enceladus 10 billion years ago, and a third intermediate sequence associated to at least one merger which we estimate took place about 1.8 billion years before Gaia-Enceladus. This discovery has been possible thanks to exquisite Hubble Space Telescope data and sophisticated analysis that enables very precise relative age determination of globular clusters. The newly identified sequence reveals that this merger took place with an object of stellar mass similar to that of Gaia-Enceladus $(\simeq5\times10^8 M_{\odot})$, and which deposited most of its mass in the inner 6 kpc of the Milky Way. The identification of a third merger event in the inner Galaxy puts to rest earlier debates, and honoring previous works we name the progenitor system Low-energy-Kraken-Heracles, or LKH for short.
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All authors Davide Massari, Chiara Zerbinati, Cristiano Fanelli, Amina Helmi, Edoardo Ceccarelli, Fernando Aguado-Agelet, Santi Cassisi, Ewoud Wempe, Matteo Monelli, Andrea Bellini, Thomas Callingham, Hanneke C. Woudenberg, Roger Cohen, Carme Gallart, Elena Pancino, Sara Saracino, Maurizio Salaris, Alessio Mucciarelli

Astrophysics of Galaxies

Hubble Study of the Proper Motion of HST-1 in the Jet of M87

Rameshan Thimmappa, Joey Neilsen, Daryl Haggard et al. (+2 more)

The radio galaxy M87 is well known for its jet, which features a series of bright knots observable from radio to X-ray wavelengths. The most famous of these, HST-1, exhibits superluminal motion, and our analysis of Chandra data \citep{Thimmappa24} reveals a correlation between the X-ray flux of HST-1 and its separation from the core. This correlation likely arises from moving shocks in the jet, allowing measurement of the internal structure of HST-1 in the X-ray band. To follow up on these results, we use observations from the Hubble Space Telescope Advanced Camera for Surveys HRC/WFC/SBC channel and the Wide Field Camera 3 (WFC3)'s UVIS to analyze the image and flux variability of HST-1. Our analysis includes 245 ACS and 120 WFC3 observations from 2002-2022, with a total exposure time of $\sim345$ ks. We study the brightness profile of the optical jet and measure the relative separation between the core and HST-1 for comparison to the X-ray. We find that the X-ray and the UV/optical emission could arise from physically distinct regions. The measured proper motion of the knot HST-1 is 1.04$\pm$0.04 c from 2002-2005 and around 2.1$\pm$0.05 c from 2005-2022. We discuss the acceleration of the jet and the flaring synchrotron emission from HST-1 from optical to X-rays.
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All authors Rameshan Thimmappa, Joey Neilsen, Daryl Haggard, Michael A. Nowak, Łukasz Stawarz

Astrophysics of Galaxies

Testing the inference of kinematics from mock JWST NIRSpec/MSA observations of TNG50 galaxies at $z\sim2-6$

Ravishankar Anirudh, Anna de Graaff, Florian Lacroix et al. (+2 more)

We use the TNG50 galaxy formation simulation to generate mock JWST NIRCam and NIRSpec microshutter array (MSA) observations of H$\alpha$-emitting gas in $M_*=10^8-10^{11.5}\,M_\odot$ star-forming galaxies at $z=2-6$. We measure morphological properties from the mock imaging through Sersic profile fitting, and gas rotational velocities ($v$) and velocity dispersions ($\sigma$) by fitting the mock spectra as thin, rotating discs. To test the efficacy of such simple parametric models in describing complex ionised gas kinematics, we compare the best-fit quantities to intrinsic simulation measurements. At $z=3$, we find that $v$ and $\sigma$ for aligned and resolved sources generally agree well with intrinsic measurements, within a factor of $\sim$2 and $\sim$1.5, respectively. The recovery of kinematics is robust for smooth, disc-like systems, but $v$ and $\sigma$ can be over- or underestimated by more than a factor of 2, respectively, for intrinsically elongated systems. The scatter in the recovery accuracy is larger at higher redshift, as TNG50 galaxies at $z>3$ deviate more strongly from the thin rotating disc assumption. Despite uncertain measurements for individual galaxies, we find that key population trends, such as the weak redshift evolution of $\sigma$ and $v/\sigma$ as well as the dependence of $\sigma$ on the global star formation rate, are broadly recovered by our kinematic modelling. Our work provides the end-to-end framework needed to compare NIRSpec MSA observations to cosmological simulations and to quantify observational biases in measuring ionised gas kinematics, highlighting the need for the development of dedicated models for high-redshift galaxies.
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All authors Ravishankar Anirudh, Anna de Graaff, Florian Lacroix, Sedona H. Price, Annalisa Pillepich

High Energy Astrophysical Phenomena

AT2018cow Powered by a Shock in Aspherical Circumstellar Media

Taya Govreen-Segal, Ehud Nakar, Kenta Hotokezaka et al. (+2 more)

We present a quantitative model for the luminous fast blue optical transient AT2018cow in which a shock propagating through an aspherical circumstellar medium (CSM) produces the X-ray and UV/optical/NIR emission. X-rays are emitted from hot post-shock electrons, and soft X-ray photons are reprocessed into optical/UV emission in the cool downstream. This naturally explains two previously puzzling features: (i) the coordinated evolution of the optical and soft X-ray after day 20, (ii) the hard X-ray hump above 10 keV that disappears around day 15 as the Thomson optical depth transitions from $\tau_T \gg1$ to $\tau_T \sim 1$. Our model is over-constrained, and it quantitatively reproduces the bolometric luminosity evolution, soft X-ray spectrum, and time-dependent soft/hard X-ray and soft X-ray/optical luminosity ratios. It also explains additional puzzles: X-ray fluctuations with $\sim4-10$ day timescales arise from a global radiative shock instability, while the NIR excess and the apparent receding blackbody radius result from reprocessed X-rays in matter far from thermodynamic equilibrium. The radio is naturally explained as originating from a shock driven by the same ejecta in the more dilute CSM. The light curve steepening after $\sim 40$ days likely indicates the shock reaches the edge of the dense CSM at $\sim {\rm few} \times 10^{15}$ cm. We infer explosion energy $\sim 1-5 \times 10^{50}$ erg, carried by an ejecta at $\sim 0.1c$ and a mass of $0.01-0.05 M_\odot$, in a dense asymmetric CSM with $\sim 0.3 M_\odot$, embedded in a more dilute CSM.
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All authors Taya Govreen-Segal, Ehud Nakar, Kenta Hotokezaka, Christopher M Irwin, Eliot Quataert

Solar and Stellar Astrophysics

Homogeneous abundance ratios of hydrostatic and explosive alpha-elements in globular clusters from high resolution optical spectroscopy

Eugenio Carretta

Galactic globular clusters (GCs) were born shortly after the Big Bang. For such old stellar systems the initial mass function (IMF) at the high mass regime can never be observed directly, because stars more massive than about 1 Mo have evolved since longtime. However, the hydrostatic to explosive alpha-element ratio (HEx ratio) offers a way to bypass the lack of observable high mass stars through the yields that massive stars released when exploding as supernovae, incorporated in the stars we presently observe in GCs. The HEx ratio measures the percentage of high mass stars over the total number of stars exploding as supernovae and it is an efficient probe of the ephemeral first phases of the GC evolution. We exploited a recently completed survey to assemble a dataset of very homogeneous abundances of alpha-elements in 27 GCs from [Fe/H]~ -2.4 to ~ -0.3 dex. In agreement with previous results from APOGEE, we confirm that the HEx ratio is indistinguishable for GCs formed in situ and accreted in the Galaxy, and that this ratio decreases with increasing metallicity. However, we posit that this trend is better explained by a metallicity-dependent IMF deficient in the highest mass stars at high metallicity, as corroborated by the declining [O/Mg] ratio as a function of the [Mg/H] ratio. At odds with the previous analysis based on APOGEE data, we detect an anti-correlation of HEx ratio with both present day and initial GC masses. Finally, we hypothesise that in that analysis, the stars of the GC M 54 were probably confused with stars in the core of the Sagittarius dwarf galaxy, where the cluster is presently immersed.
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