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Physics Astrophysics

astro-ph Watch Astrophysics

Tuesday 29 September

High Energy Astrophysical Phenomena

NEXUS: Discovery of a Luminous Young Transient at $z=5$

Ming-Yang Zhuang, Yue Shen, Lei Hu et al. (+8 more)

We report the discovery of AT 2026abck, a young transient in a star-forming galaxy at $z=5.017\pm0.001$, identified from Year-2 observations of the NEXUS JWST treasury program. AT 2026abck was detected on 2026 Aug 7 in NIRCam F200W imaging, with contemporaneous detection from parallel NIRCam F150W+F277W imaging. It brightened by $>1.4$ mag in F200W over $\lesssim 12$ rest-frame days, as inferred from the last non-detection. AT 2026abck has nearly identical magnitudes of $\sim27.4$ mag in F150W, F200W, and F277W, and is undetected in F444W ($>27.9$ mag, 3$\sigma$). It is the first robustly detected $z\gtrsim 5$ transient in a non-lensing field. A blackbody fit yields a hot photosphere ($\sim1.7\times10^4\, \mathrm{K}$) with a radius of $\sim7\times10^{14}$ cm ($\sim46$ au) and a bolometric luminosity of $\sim3\times10^{43}\, {\rm erg\, s^{-1}}$. Its off-nucleus location and hot, UV-bright SED favor a core-collapse supernova origin, while its temperature and luminosity exceed those of normal SNe II at comparable early phases. We estimate that AT 2026abck was detected $\sim4$-10 rest-frame days after explosion. The host is an intermediate-mass ($M_*\approx10^{9.8}\,M_\odot$), star-forming (${\rm SFR}\approx20\,M_\odot\,{\rm yr^{-1}}$) galaxy on the $z\approx5$ main sequence, $\sim3.3$ dex more massive than the ultra-faint host of the lensed SN Eos at $z=5.13$. Together, the two events show that CCSNe at $z>5$ occur across a wide range of host environments, and AT 2026abck demonstrates that high-cadence blank-field JWST surveys can capture high-redshift SNe during their early, UV-bright phase.
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All authors Ming-Yang Zhuang, Yue Shen, Lei Hu, Justin D. R. Pierel, Zhiwei Pan, Zachary Stone, Adam J. Burgasser, Xiaohui Fan, Jenny E. Greene, Junyao Li, Feige Wang

Astrophysics of Galaxies

A New Hope for AGN SED Fitting: X-Ray Spectral Analysis of 12MGS AGN with the C2PO-Torus Model

Carys J. E. Gilbert, Lucia Marchetti, Luigi Barchiesi et al. (+8 more)

In this paper, we present the results of comprehensive, broadband X-ray spectral analysis of a subsample of 43 mid-Infrared (IR) selected star-forming active galactic nuclei (AGN). We analysed archival NuSTAR, Chandra, and XMM-Newton data, and present the first pointed X-ray observation of IRASF03450+0055. We introduce a new physically-based X-ray spectral model, C2PO-Torus, based on a two-phase clumpy torus geometry. It is designed to provide constraints on physical parameters of the torus that can be directly linked to the parameters of the AGN model SKIRTOR. This model will allow us to exploit the synergies between the X-ray and IR regimes, place stronger constraints on the torus properties, and improve spectral energy distribution (SED) fitting by providing priors for the SKIRTOR parameters. We compare the results from the C2PO-Torus fitting to those obtained from traditional models, finding that it is an extremely effective tool for fitting a wide variety of X-ray spectra. We observe that our sample closely follows the established L_[OIII] - L_X AGN relation, while presenting an IR and Radio excess with respect to the L_X - L_12micron and L_X - L_1.4GHz AGN relations at low luminosities. We believe that this excess is due to contamination from star formation related emission. We anticipate that the C2PO-Torus model will aid substantially in completing full-spectrum SED fitting of this sample, allowing us to properly disentangle the different emission components of these objects, which are local analogues of AGN at Cosmic Noon.
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All authors Carys J. E. Gilbert, Lucia Marchetti, Luigi Barchiesi, Mattia Vaccari, Cristian Vignali, Stefano Marchesi, Francesca Pozzi, Francesco Salvestrini, Anna Feltre, Malebo Moloko, Carlotta Gruppioni

High Energy Astrophysical Phenomena

VLBI Astrometric Procedure and Performance with CHIME/FRB Outriggers

Shion Andrew, Mattias Lazda, Calvin Leung et al. (+15 more)

Precise localizations of fast radio bursts (FRBs) to specific environments within their host galaxies with very-long-baseline interferometry (VLBI) have provided valuable insights into the nature of their progenitors. To date, such localizations have relied primarily on traditional VLBI facilities, whose observational capabilities limit them to targeted follow-up on a small subset of repeating FRBs that are observable at frequencies above $\sim$1 GHz. CHIME/FRB Outriggers is a low-frequency (400–800 MHz), wide-field VLBI network designed to overcome these limitations, aiming to localize a substantial fraction of CHIME-detected FRBs to $\sim$50 mas precision. In this work, we detail the analysis procedure used to localize FRBs with the CHIME/FRB Outriggers VLBI array. With over a thousand test localizations consisting of continuum calibrators and pulsars with known VLBI positions, we provide a comprehensive demonstration of the VLBI array's ability to robustly meet, and under some conditions exceed, the nominal astrometric specification required for the central science goals of CHIME/FRB Outriggers. By demonstrating that a wide-field survey instrument can deliver routine $\sim$50 mas astrometry on one-off FRBs, this work lays the foundation for the upcoming generation of wide-field VLBI FRB surveys and the population-scale FRB samples they will enable.
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All authors Shion Andrew, Mattias Lazda, Calvin Leung, Kiyoshi W. Masui, Danté M. Hewitt, Jacob Willis, Alyssa Atkinson, Kevin Bandura, Shami Chatterjee, Nina V. Gusinskaia, Adam E. Lanman, Juan Mena-Parra, Kenzie Nimmo, Aaron B. Pearlman, Ziggy Pleunis, Alexander W. Pollak, Gurman Sachdeva, Vishwangi Shah

High Energy Astrophysical Phenomena

HESS J1507-622: A Plausible Young Galactic Kilonova Remnant

Prantik Sarmah, Xilu Wang, Myles Carley et al. (+3 more)

HESS J1507$-$622 is an unusual TeV gamma-ray source in the H.E.S.S. Galactic Plane Survey, distinguished by its off-plane location ($b \simeq -3.5^{\circ}$) and compact angular extent ($\sim 0.3^{\circ}$). Its location and morphology challenge conventional pulsar wind nebula and supernova remnant interpretations, and the absence of a detected shell or associated pulsar leaves its origin unresolved. Here, we propose that HESS J1507$-$622 is a young Galactic kilonova remnant with gamma-ray emission of leptonic origin. In this scenario, the source's off-plane location is naturally explained by the natal kick imparted to the progenitor binary neutron star system. We demonstrate that a leptonic kilonova remnant model can successfully reproduce the observed GeV-TeV flux via inverse-Compton scattering from shock-accelerated electrons, while the corresponding synchrotron emission remains below current X-ray and radio upper limits. Based on the source's energetics and angular size, we infer a distance of 3.8-14.3 kpc and an age of 0.2-3.0 kyr. Importantly, our analysis favors a kilonova remnant younger than 1 kyr. The lack of a historical naked-eye record can be explained by the transient's rapid fading, far-southern declination, and potential line-of-sight extinction. We further assess the detectability of unique multi-wavelength signatures that could confirm this hypothesis. We find promising prospects for detecting the unique MeV gamma rays from the decay of $r$-process radioisotopes using future observatories, alongside complementary low-energy signatures from thermal dust emission and a possible optical light echo. These distinct signals offer definitive tests of the kilonova remnant interpretation and motivate targeted observations with current and future facilities.
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All authors Prantik Sarmah, Xilu Wang, Myles Carley, Shu-Xu Yi, Ming-Yu Ge, Rebecca Surman

High Energy Astrophysical Phenomena

Cosmic-neighbor-associated distances to VHE and candidate VHE BL Lacs via SDSS photometric redshifts

Karri I. I. Koljonen and Elina Lindfors

BL Lac objects have distances that are difficult to estimate because their optical spectra rarely show identifiable emission or absorption lines. We present a method to constrain BL Lac distances by associating them with their cosmic neighborhoods (local overdensities in the cosmic web) identified in optical fields. Distances to galaxies in these neighborhoods are estimated using the Sloan Digital Sky Survey (SDSS) photometric redshifts. Using cosmic-neighborhood associations previously established from multi-object spectroscopy and BL Lacs with spectroscopically measured redshifts as control samples, we find that the redshifts of the associated neighborhoods can be recovered with an accuracy of $\Delta z \sim 0.02-0.04$. When multiple candidate neighborhoods are present in the field, we rank the candidate associations using a central-location criterion. In 50% of cases, the BL Lac is associated with the correct neighborhood. We provide distance estimates for BL Lac objects without secure redshift measurements, focusing on sources detected at very high energies (VHE; $E>100$,GeV) and on candidate VHE emitters. Combining intervening absorption systems, host-galaxy imaging, $\gamma$-ray constraints, and our SDSS group-association method, we propose likely redshifts for TXS 0141$+$268 ($z\sim0.93-0.95$), PKS 0735$+$178 ($z=0.64$), 3FHL J0905.5$+$1357 ($z=0.644$), B2 0912$+$29 ($z = 0.53$), 3FHL J1120.8$+$4212 ($z=0.363$), BZB J1243$+$3627 ($z=0.485$), 3FHL J1253.1$+$5300 ($z=0.664$), and RX J2156.0$+$1818 ($z=0.635$). Finally, we discuss how forthcoming narrow-band photometric-redshift surveys can improve blazar distance estimates via group-association techniques.
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Astrophysics of Galaxies

Learning Gaia: A Generative Model Trained on the Gaia Data Release 3 Source Catalog

John Soltis

I train a conditional flow matching model on the positions, photometry, and astrometry of $\sim$ 579 million sources from the Gaia Data Release 3 Source Catalog. Using a probabilistic masking scheme during training, I develop a model capable of generating mock Gaia catalog sources with and without partial measurements of real Gaia sources. The model can produce coarse-grained mock Gaia Source Catalogs that reproduce the Milky Way disk and Magellanic Clouds, and can impute photometric and kinematic information. The model fails to reproduce small, distinct subpopulations like $\omega$ Centauri. This model is a necessary step towards a foundation model trained on the entirety of Gaia data products, and provides a blueprint for future work with the upcoming Gaia Data Release 4.
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High Energy Astrophysical Phenomena

Revealing the accretion disc structure in Changing State AGN: NGC1566

Huimei Wang, Chris Done, Riccardo Middei et al. (+1 more)

NGC1566 is a low mass ($\sim 6\times 10^6M_\odot$) Active Galactic Nuclei (AGN) with unusual large amplitude variabilty in its accretion flow. In the optical it cycles between recurring short bright episodes with strong blue/UV continuum and clear broad line emission, and a much weaker persistent level without strong broad lines, making it a repeating changing-state (CS) AGN. We re-analyse the intensive optical/UV and X-ray monitoring during the 2018 bright flare, showing for the first time the corresponding evolution of the (host galaxy subtracted) optical/UV AGN spectrum as well as the X-ray emission. As seen in other CS-AGN, the strongest spectral evolution is in a UV/soft X-ray component which likely disappears below $0.01L_{\rm Edd}$ while the hot corona and a redder optical/UV component show weaker variability. We show that the red optical emission is likely a composite of the intrinsic and reprocessed emission from the outer disc, together with diffuse continuum from the BLR, and discuss the different signatures predicted by this for continuum reverberation across the changing-state event. The historical lightcurves show repeated changing-state events tied to unusually large amplitude discrete flares in the lightcurve. While these do not appear to be periodic, the behaviour does support previous suggestions that the accretion flow is repeatedly perturbed, perhaps by an intermediate mass black hole companion.
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All authors Huimei Wang, Chris Done, Riccardo Middei, Xue-Bing Wu

Instrumentation and Methods for Astrophysics

An X-ray Extremely wide-field Spectroscopic Surveyor (XESS, or "chess")

W. P. Maksym, M. Ulmer, S. Friedrich et al. (+20 more)

We propose an X-ray observatory, the X-ray Extremely wide-field Spectroscopic Surveyor (XESS, pronounced "Chess") to bring high energy resolution X-ray spectroscopy into the era of synoptic surveys and of time domain and multi-messenger astrophysics (TDAMM). Such an observatory is optimally suited to addressing key elements of the 2020 Decadal Survey on Astronomy and Astrophysics.
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All authors W. P. Maksym, M. Ulmer, S. Friedrich, F. Tombesi, R. Arcodia, S. P. Singam, S. Bongiorno, N. Thomas, M. Elvis, Ming Sun, John ZuHone, M. Lynne Saade, Philip Kaaret, Doug Swartz, Erin Hicks, Ming-Yi Lin, Pallavi Patil, Jimmy Irwin, Shrabani Kumar, Kirtan Dixit, Sudip Chakraborty, Andrea Gnarini, Steven Ehlert

Solar and Stellar Astrophysics

Surveying for Close Binaries among Lithium-rich Giants

Jesús Torres, Sebastián Vilaza-Dallago, Julio Chanamé et al. (+1 more)

One of the possible scenarios usually considered to explain the existence of low-mass red giants with anomalously high lithium (Li) abundances requires the interaction with a close binary companion. If dominant, this enrichment channel predicts a significantly enhanced binary fraction among Li-rich giants relative to Li-normal ones. We aim to constrain the binary interaction hypothesis by using multi-epoch radial velocities (RVs) to measure the close binary fraction of a statistically robust sample of Li-rich giants. We are surveying both red clump (RC) and first ascent red giant (RGB) targets, which allows us to examine potential differences in the binary fraction as a function of evolutionary stage. We are monitoring 70 targets (40 RC and 30 RGB, all Li-rich) using ESPRESSO at the VLT, 57 of which (32 RC + 25 RGB) already have 4 or more visits over a $\sim$7 month baseline; the analysis in this paper is based on the latter. For the targets with RV variation larger than that expected for intrinsic jitter, we use a Monte Carlo analysis to estimate the probability that the observed RV variation is due to a stellar ($P_{SB}$) or sub-stellar ($P_{SS}$) companion. Discounting the 38 targets consistent with stellar jitter, we are left with 11 RC and 8 RGB stars (34% and 32% of the respective sub-samples) with RV variability consistent with the presence of a close companion with an orbital period shorter than 3000 days. Of these 19, 3 can be confirmed as stellar-mass companions. The distributions of $P_{SB}$ and $P_{SS}$ are statistically indistinguishable between the RC and RGB samples. We thus find no evidence so far in our current data for a binary fraction for Li-rich giants (in either evolutionary stage) larger than that expected for a Li-normal population. Extending the RV baseline to probe longer orbital periods will be important to keep exploring the plausibility of this scenario.
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All authors Jesús Torres, Sebastián Vilaza-Dallago, Julio Chanamé, Rafael Brahm

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

Constraints on Complex Electroweak WIMPs from Solar Capture

Juan P. Luengas, Maxim Pospelov and Harikrishnan Ramani

One of the most convincing realizations of particle dark matter is provided by models based on electroweak multiplets. The lightest neutral state of such a multiplet can be a stable Majorana fermion or a real scalar, and its phenomenology is governed by two parameters: the dark matter mass and the mass splitting $\{m_\chi,\delta\}$. The sub-MeV range for $\delta$ is particularly important as it allows for a large inelastic scattering cross section. In this paper, we analyze solar capture, energy loss, and annihilation of scalar and fermionic electroweak multiplet dark matter, generalizing earlier results for the Higgsino to other representations. We find that, while the case of a doublet is somewhat special, higher representations behave in a self-similar way. For the higher multiplets, IceCube results constrain the splitting to $\delta \gtrsim 350$ keV for all dark matter masses up to $100$ TeV. Assuming thermal freeze-out and a Standard Halo Model velocity distribution, this rules out most models of electroweak dark matter as a possible origin of the single high-recoil event reported by the LZ collaboration. We also show that a high-velocity tail in the dark matter distribution, such as that induced by the Large Magellanic Cloud, raises the splitting preferred by the LZ event above the solar bounds for many of the multiplets.
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Earth and Planetary Astrophysics

An Infant Sub-Saturn in Scorpius-Centaurus

Gurmeher Kathuria, Luke G. Bouma, Andrew W. Boyle et al. (+25 more)

We report the discovery and validation of TIC 88297141 Ab, a transiting planet orbiting a pre-main-sequence M dwarf (0.31 solar masses, 0.74 solar radii) in the Scorpius-Centaurus OB association. Group membership implies an age of $15.6 \pm 1.6$ Myr, supported by lithium and isochrone age-dating. We detected the TESS transits using a custom pipeline designed for variable stars and used ground-based photometry (LCO/Swope and LCOGT/Sinistro) to confirm the on-target nature of the signal. Our global fit of the transits and stellar activity gives an orbital period of 4.64 days and a planet radius of $7.21 \pm 0.32$ Earth radii (0.64 Jupiter radii). The star has a comoving M-dwarf companion (0.4 solar radii) at a projected separation of $260$ au; speckle imaging (Gemini-S/Zorro) reveals no additional stellar companions, and radial velocities from Magellan/PFS are consistent with stellar activity, giving $M_p \sin i < 4.7$ Jupiter masses at $2\sigma$. A statistical validation analysis yields a false positive probability of $10^{-5}$, validating the planet. A mass measurement will determine whether TIC 88297141 Ab is a low-density super-puff, like other comparably sized planets at these ages, or an unusually massive planet for its radius.
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All authors Gurmeher Kathuria, Luke G. Bouma, Andrew W. Boyle, Roberto Tejada Arevalo, Andrew W. Mann, Nidia Morrell, Karen A. Collins, R. Paul Butler, Stephen A. Shectman, Johanna Teske, Jeffrey D. Crane, Shreyas Vissapragada, Madeleine Mckenzie, Khalid Barkaoui, Norio Narita, Akihiko Fukui, Jerome P. De Leon, Toshi Suganuma, Madyson G. Barber, Francis P. Wilkin, Victoria Zaledonis, Sophia Garcia-Gallet, Glauk Hizmo, Steve B. Howell, David R. Ciardi, Colin Littlefield, Sarah Deveny, Douglas A. Caldwell

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

How collective modes terminate in the dispersion relation of neutrino plasmas

Damiano F. G. Fiorillo

Neutrino plasmas support flavor waves, collective oscillations of flavor, that neutrinos can resonantly emit or absorb when they fulfill the Cherenkov condition. For massless neutrinos, this condition leads to singular features in the dispersion relation: Landau-damped modes, for example, disappear abruptly as their phase velocity approaches the speed of light. We investigate how these singularities change when nonzero neutrino masses introduce a spread in velocities. We classify singular behavior of the frequency as a function of wavevector into i) degeneracy points, where two modes merge, and ii) abrupt mode termination, when the frequency approaches a branch cut of the flavor dielectric function. For each class, we derive a local expansion and determine where such behavior can occur. Degeneracy points, frequently encountered in axially symmetric dispersion relations in the fast limit, are not generic. Nonzero neutrino masses lift the degeneracy along the symmetry axis; in three-dimensional wavevector space, there might still be degeneracy lines, not necessarily intersecting the symmetry axis. Abrupt mode termination, instead, persists generically, except for special energy distributions which are entire functions. As a byproduct of this study, we show that a collisionless plasma with a Lorentzian neutrino-frequency distribution is exactly equivalent to a monochromatic one with an effective collision rate. Approximating a general frequency distribution by a sum of Lorentzians therefore substantially simplifies the dispersion relation.
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Instrumentation and Methods for Astrophysics

Gestalt: a meta-foundation model for astronomy

Michael J. Smith and Shashwat Sourav

The Platonic Representation Hypothesis predicts that sufficiently scaled foundation models converge on a shared representation of the world. As each non-converged model gives a noisy view of a common structure when passed the same input, we ask whether we can combine models into a representation that outperforms its individual components. We test this on galaxies: we embed images via a basket of 22 frozen foundation models from eight families, whiten each view, and take a randomised SVD of the embedding concatenation. The resulting 1024-dimensional embedding outperforms every basket member on 19/21 of our tested metrics for physical property and galaxy morphology estimation for HSC, JWST, and DESI Legacy Survey imagery. We find that performance rises with basket size and basket architectural diversity, and that the meta-foundation model's performance transfers across astronomical surveys. We conclude that a useful astronomical foundation model can be assembled from existing generalist models with no training required beyond a single unsupervised projection. By leveraging the community's already-spent work, we save a lot of compute: a fresh pre-train of a comparable single-domain model would cost $\mathcal{O}(10^{4}$–$10^{5})$ A100 GPU hours (emitting several tonnes of CO$_2$eq.), whereas assembling Gestalt requires minutes on a single machine.
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Cosmology and Nongalactic Astrophysics

Testing Offsets Between Cluster-Scale Halos and BCGs in Strong Lensing Models Using the Jackknife Method

Sangjun Cha, Marceau Limousin and M. James Jee

Offsets between cluster-scale dark matter halos and brightest cluster galaxies (BCGs) inferred from strong lensing (SL) models have been used to investigate cluster dynamics and dark matter physics. However, it is necessary to test whether these offsets are required by SL data themselves. We investigate whether allowing these offsets improves predictive accuracy and examine the effects on magnification, time-delay predictions, and critical curve positions. We evaluate lens models of Abell 370, RX J1347.5-1145, and MACS J0416.1-2403 using the jackknife method, which assesses predictions for source systems excluded from the lens reconstruction. Models are constructed with MrMARTIAN, which combines analytic mass profiles with a regularized grid that represents mass structure not captured by the profiles. We compare fixed and free halo position models across regularization weights. The root mean square values averaged over excluded systems agree within bootstrap uncertainties, while the median paired differences remain close to zero across all three clusters and tested regularization weights. These results indicate comparable predictive accuracy between the fixed and free models. The total mass distributions remain broadly similar, although the magnitudes of fitted halo offsets vary with regularization, showing that the mass decomposition is not unique. For these three clusters, offsets between BCGs and cluster-scale halos in the lens models are not required by the SL data to improve predictive accuracy within the MrMARTIAN framework. The mass decomposition between the regularized grid and cluster-scale halos is not uniquely determined. These findings limit physical interpretations based on fitted halo positions alone. The model dependence of magnification and time-delay estimation can also affect the inferred intrinsic properties of high-redshift sources or the Hubble constant.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Searching for BSM Experimental Signatures with Large Lagrangian Models

Ibrahim Elsharkawy, Victoria Knapp-Perez, Wahid Bhimji et al. (+1 more)

The search for physics Beyond the Standard Model (BSM) is generally limited not by the supply of theory descriptions but by the lack of discriminating experimental observations. A case in point is dark matter, where the overwhelming gravitational evidence only goes so far in distinguishing between models within a vast theory space. Exploring the space of testable model signatures may help identify overlooked experimental observables and indicate the utility of future experiments. A challenge is designing a search through model signatures outside what is found in the literature. Our primary contribution is hAIthem, a framework that combines the self-guided exploration of reinforcement learning (RL) with the broad literature-derived knowledge of LLMs. We build an RL agent that learns to find which portions of a theory's high-dimensional parameter space are not excluded under some subset of constraints by playing a Battleship-style "game" against a suite of phenomenology tools. The agent is built as a Large Lagrangian Model (LLaM), an autoregressive transformer that reads a tokenized Lagrangian, is pretrained at scale (here on ~1 billion tokens from ~10,000 Lagrangians), and is fine-tuned in a live environment. The framework then constructs a decision tree that separates RL-found regions using observables computed with established tools, and passes the remaining degenerate regions to a set of LLM agents that compete to produce realistic signatures. In this proof of concept, RL-search outperforms an evolutionary-algorithm baseline, finding more viable regions with greater physical diversity. In a restricted space of single dark scalar multiplet models, we find that hAIthem proposes interesting combinations of previously studied observables, such as the application of a halo-independent kinematic ratio to paleo-detectors.
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All authors Ibrahim Elsharkawy, Victoria Knapp-Perez, Wahid Bhimji, Aishik Ghosh

Cosmology and Nongalactic Astrophysics

Close Pulsar Pairs See Dark Matter Substructure through the Gravitational-Wave Background

Abhiram Cherukupalli, Vincent S. H. Lee, Kim V. Berghaus et al. (+1 more)

Pulsar timing arrays detect signals from dark matter substructure, but a nanohertz gravitational-wave background (GWB) severely degrades their sensitivity. We show that timing pulsar pairs separated by sub-parsec distances avoids this degradation. The GWB imprints nearly the same perturbation on both pulsars and is suppressed in the difference of their residuals, whereas a nearby dark matter subhalo perturbs the two differently and survives. We find that an array with pulsar pairs separated by $0.1\,\mathrm{pc}$ improves the sensitivity to subhalos of order $\sim10^{-6}$ solar masses by more than an order of magnitude, relative to an otherwise identical array of widely separated pulsars. Dense globular clusters naturally host many such pairs, though the baryonic environment must be separated from a dark matter substructure signal.
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All authors Abhiram Cherukupalli, Vincent S. H. Lee, Kim V. Berghaus, Kathryn M. Zurek

High Energy Astrophysical Phenomena

Nearby Supernova Science in the Era of High Cadence, Deep Time Domain Surveys

David J. Sand, Bhagya Subrayan, Conor L. Ransome et al. (+41 more)

We discuss science opportunities for nearby supernovae (SNe) in the era of the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), emphasizing the youngest phases of SN evolution and the value of contemporaneous high-cadence shadowing surveys. LSST's Wide Fast Deep survey will discover enormous numbers of SNe, but its $\sim 3$ day cadence will miss signatures that evolve on timescales of hours to days, requiring cadence augmentation by other programs. We present simple depth and cadence metrics for early SN signatures and use empirical nearby SN discovery rates to estimate the resulting science opportunities. LSST will detect or set limits on precursor emission from hundreds of core-collapse SNe, including $\gtrsim 50$-100 Type IIn and normal Type II SNe annually, as well as $\gtrsim$5 Type Ibn. Within $\sim 200$ Mpc, roughly 250 Type Ia SNe are classified annually, providing the potential to identify hundreds of early light-curve excesses, and a robust measurement of the incidence of extreme high-velocity features ($>$25,000 km s$^{-1}$). A shadowing survey with a depth of $\sim 22.5$ mag could search for early light-curve features, akin to the light-curve excess seen in SN2023ixf, across dozens of nearby core-collapse SNe every year. Rapid spectroscopy of $\gtrsim$150 Type II SNe per year within $\sim$160 Mpc could constrain the incidence of short-lived flash features to $\lesssim$10% within a few years, while also tying this to the rate of precursor emission in normal Type II SNe. We also discuss the challenges posed by early classification ambiguity and faint nonterminal transients, and we request that the 80 hour embargo on LSST images be waived for nearby galaxy fields. Finally, we advocate for a Nearby Galaxy Transient Broker that integrates transient streams, archival data, forced photometry, and follow-up triggering in a single platform.
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All authors David J. Sand, Bhagya Subrayan, Conor L. Ransome, Griffin Hosseinzadeh, K. Azalee Bostroem, Jennifer E. Andrews, Jeniveve Pearson, Yize Dong, Stefano Valenti, Saurabh W. Jha, Brian Hsu, Aravind P. Ravi, Manisha Shrestha, Charles D. Kilpatrick, Noah Franz, Mojgan Aghakhanloo, Igor Andreoni, Moira Andrews, Katie Auchettl, Federica B. Bianco, Régis Cartier, Siddharth Chaini, Diva Chaudhary, Analía V. Smith Castelli, Shar Daniels, Alexei V. Filippenko, Melissa L. Graham, Xander J. Hall, Daichi Hiramatsu, D. Andrew Howell, Lindsey A. Kwok, Amanda R. Lopes, Thomas Matheson, Curtis McCully, Maryam Modjaz, Phillip Noel, Gregory S. H. Paek, Antonella Palmese, Anthony L. Piro, Monika Soraisam, Sergiy Vasylyev, József Vinkó, J. Craig Wheeler, Samuel D. Wyatt

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

A Strong Cosmologically Coupled Mass Growth Solution in Linearized General Relativity

Kevin S. Croker, Joshua Cohon, Viktor T. Toth et al. (+13 more)

DESI Data Release 2 has heightened interest in cosmologically coupled black holes (CCBHs) as an explanation for accelerating late-time expansion, but known models for CCBHs grow too slowly to drive it. We construct at linear order a solution to General Relativity whose Hernandez-Misner-Sharp mass passively grows $\propto a^\kappa$ as the universe expands. The constant $\kappa$ enters all eigenvalues of the Weyl and stress-tensor operators, so growth is unambiguously physical in both free-field and material sectors. The solution is causal over spacetime relevant for stellar collapse compact objects. A cloud of energized vacuum emerges at $\kappa = 3$, complementing the $a^3$ scaling predicted elsewhere for the masses of energized vacuum black holes. This first step toward $\kappa \sim 3$ CCBH exact solutions enables observational tests with pulsar binaries.
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All authors Kevin S. Croker, Joshua Cohon, Viktor T. Toth, Massimiliano Rinaldi, Nicolas Fernandez, Joel L. Weiner, Duncan Farrah, Gregory Tarlé, Gustavo Niz, Christopher Cain, James W. Rohlf, Alejandro Aviles, Jake Summers, Leander Thiele, Rogier A. Windhorst, Joshua Grumski-Flores

High Energy Astrophysical Phenomena

Not So Heavy Metal I: Low Metallicity Enhances the Rate of high-redshift Superluminous Supernovae

Ben Margalit, Allison L. Strom, Cristina Andrade et al. (+4 more)

Type-I superluminous supernovae (SLSNe) are extremely luminous stellar explosions originating from the core collapse of massive stars. Unlike normal core-collapse supernovae, however, SLSNe occur almost exclusively in metal-poor environments. As a result, their volumetric rate should not simply trace the cosmic star-formation history, but instead evolve with the metallicity distribution of galaxies across cosmic time. In this work, we introduce a new theoretical framework for modeling the volumetric rate of astrophysical sources as a function of redshift and metallicity. Our formalism combines metallicity scaling relations, stellar-mass distribution functions, and metallicity-dependent production efficiencies. Importantly, it also distinguishes between metallicity measures based on O/H and Fe/H abundances. Applying our new formalism to SLSNe, we predict volumetric rates that are nearly an order of magnitude higher at $z \gtrsim 2$ compared to models in which the rate traces the cosmic star-formation rate alone. This has direct implications on the detection prospects of high-$z$ SLSNe with the Vera C. Rubin Observatory and other time-domain surveys.
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All authors Ben Margalit, Allison L. Strom, Cristina Andrade, Michael W. Coughlin, Adam A. Miller, Steve Schulze, Ved G. Shah

Astrophysics of Galaxies

The orbital dynamics of the LMC and SMC about the Milky Way

Gurtina Besla and Nitya Kallivayalil

This review summarizes the changing observational and theoretical landscape that has led to rapid developments in our understanding of the orbital history of the Milky Way's most massive satellite galaxies, the LMC and SMC. The determination of high precision center-of-mass proper motions for the Clouds and their environs, coupled with new insights into the structure of dark matter halos from cosmological simulations, indicates that the Clouds are recent additions to the Milky Way's satellite system, where the LMC was sufficiently massive at infall to induce distortions in the Milky Way's dark matter halo, and bring in satellites of its own. To gain insights into the orbital history of the Clouds it is, therefore, necessary to integrate orbits in a time evolving Milky Way and LMC potential. The proper motion measurements further indicate that the LMC and SMC have had a recent strong encounter with each other. This leaves open the impact of this encounter on their internal structures, and the degree of correspondence between their stellar, HI and dark matter centers. The choice of kinematic center in turn affects the center of mass proper motions; this is the biggest observational unknown, even in the high precision Gaia era. Given the expected mass loss of the SMC, the distortions it induces in the LMC halo, and the recent LMC-SMC close encounter, understanding the orbits of objects about the LMC (including the SMC) and interpreting observational data of the Clouds requires full N-body simulations that are constrained by their present-day structure and kinematics.
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Astrophysics of Galaxies

LYRA: The formation and growth of nuclear star clusters in dwarf galaxies

Joaquin Sureda, Azadeh Fattahi, Sownak Bose et al. (+5 more)

Nuclear star clusters (NSCs) are dense massive star clusters ubiquitous in galactic centres across various mass regimes, present in up to $\sim30$ per cent of dwarf galaxies with $M_\star<10^{7}\, \mathrm{M}_\odot$. Although their main growth pathways are well studied, there is still ongoing debate regarding their formation. We use the LYRA cosmological hydrodynamical simulations to study the formation and growth of NSCs in a sample of six dwarf galaxies, where we identify NSCs with masses $M_\mathrm{NSC} \sim 10^4 - 10^6 \, \mathrm{M}_\odot$ in all galaxies in our sample. The NSCs in our sample emerge at early times as a compact component, with the host galaxy growing around them. This is consistent with a scenario in which NSCs are among the earliest structures to form in a galaxy. We characterise the origin of the stars that constitute the NSCs and explore the properties of the stellar populations in each of these channels. We find that most of the NSC mass originates from in-situ star formation within the NSCs themselves. While this might appear at odds with current observations, the bulk of this star formation occurs at early times, resulting in a significant population of old and metal-poor stars, in agreement with an early NSC formation. We also identify galaxy-galaxy mergers as a significant contributor to the NSC mass, with usually one merger dominating this contribution. Our results highlight the complexity in the growth history of NSCs in dwarf galaxies and highlight the importance of considering additional growth channels for NSCs in dwarfs galaxies.
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All authors Joaquin Sureda, Azadeh Fattahi, Sownak Bose, Mariya Lyubenova, Jessica E. Doppel, Thales Gutcke, Katja Fahrion, Rüdiger Pakmor

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

Exact bispectra in strongly mixed multifield inflation

Lucas Pinol

Using the effective field theory of inflationary fluctuations extended by a massive isocurvature scalar, at unit sound speed, we compute the primordial bispectra generated by every cubic interaction of dimension four or less in the unitary gauge, with the curvature–isocurvature quadratic mixing resummed to all orders in the dimensionless mixing strength $\lambda=\rho/H$. Placing the mixing in the free Hamiltonian [...], every scale-invariant tree-level bispectrum reduces to a single Schwinger-parameter integral over independent leg kernels, and what perturbation theory calls single, double and triple exchange are contact diagrams of the resummed theory. At strong mixing we unveil genuinely new, exact multifield bispectrum shapes with no correlation to the standard shapes used in data analyses. The squeezed limit carries a cosmological collider signal with a universal frequency set by the dressed mass parameter $\mu_{\rm eff}$ at every exchange order. At weak mixing the clock coefficients are obtained in closed form and reduce [...] to the known perturbative single- and double-exchange results, exactly; the triple-exchange leading coefficient is obtained in closed form at every mass, in terms of hypergeometric functions, for the first time. At strong mixing the collider amplitude scales as $e^{\pi(\lambda-2\mu_{\rm eff})/2}$ over the whole $(\lambda,\mu_{\rm eff})$ plane. This encompasses all previously studied regimes as particular cases [...]. A first confrontation of the six exact shapes with the Planck PR4 binned bispectrum shows a preference for strong mixing, where the cosmological collider oscillations invade mildly squeezed configurations [...]. The shapes of all interactions over the whole plane [...] are released with the code that produced them, so that a template at any mixing and mass can be read off in milliseconds.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-th

CMB Injection Bounds on Moduli Fields

Naman Agarwal, Andrew R. Frey, Ratul Mahanta et al. (+3 more)

Light scalar fields with gravitational strength interactions (moduli) are ubiquitous in string, supergravity, and extra-dimensional models. If produced in the early universe with lifetimes exceeding the epoch of recombination, slowly decaying moduli inject energetic particles into the Standard Model plasma, leaving distinct imprints on the cosmic microwave background (CMB). In this paper we systematically derive CMB injection bounds on light moduli ($10^3\ {\rm eV}\leq m_{\rm{mod}}\leq 10^8\ {\rm eV}$) across the primary cosmological production channels: vacuum misalignment, heavy scalar/inflaton decay, thermal emission from the primordial plasma including production from a Hagedorn phase of hot strings. We show that these injection bounds impose stringent constraints that are complementary to both the standard cosmological moduli problem and fifth force bounds.
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All authors Naman Agarwal, Andrew R. Frey, Ratul Mahanta, Anshuman Maharana, Fernando Quevedo, Gonzalo Villa

Earth and Planetary Astrophysics

Atmospheric dynamics and variability of periodically irradiated tidally locked planets

Deepayan Banik, Vasilii Pustovoit, Thaddeus D. Komacek et al. (+1 more)

Time-variable stellar heating may create new climate states, such as permanently hot or cold hemispheres, on asynchronous planets. Planets on eccentric orbits, circumbinary planets and planets on polar orbits around gravity-darkened stars all receive variable irradiation, yet variably irradiated planets (VIPs) have not been studied from a fundamental dynamical perspective. We present the first generalized study of the atmospheric response of variably irradiated tidally locked planets, using a 1.5-layer shallow-water model forced towards a time-dependent radiative equilibrium. We derive linearized analytic expressions for the mean, amplitude and phase lag of the response, which is that of a forced damped harmonic oscillator, and recover the established trends of tidally locked planets in the limit of steady forcing. When the instellation variation and wave timescales match and the damping is weak, a resonance occurs because waves propagate and interfere freely. The resonance produces a global standing wave mode and consequently a temporarily warmer nightside. Linear and nonlinear simulations reproduce these results and show that the hotspot oscillates longitudinally at the forcing frequency and reaches the antistellar point at resonance. The oscillation is eastward or westward of the substellar longitude depending on the rotation rate and the nonlinearity of the system. Synthetic phase curves carry signatures of this variability and invert in shape at resonance because of nightside warming. Among observed systems, we find 11 planets with at least 10% phase curve modulation from variable irradiation. Eccentric, pseudo-synchronous giants are the most promising targets, with HAT-P-2b having the largest predicted modulation.
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All authors Deepayan Banik, Vasilii Pustovoit, Thaddeus D. Komacek, Marta L. Bryan

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

Fast and Flow-rious: Gravitational Wave Background Bayesian Model Comparison using Normalizing Flows and Nested Sampling

David C. Wright, Aaron D. Johnson, Jeffrey S. Hazboun et al. (+1 more)

In 2023, pulsar timing array (PTA) collaborations around the world announced evidence for a nanohertz gravitational-wave background (GWB). Beyond increasing the detection significance, the next major goal is to characterize the GWB and identify its source(s). The typical model-comparison metric used in PTA analyses is the Bayes factor (BF). However, BF computation methods used in the PTA literature, such as product-space sampling and thermodynamic integration, are computationally expensive or inefficient if the models compared are dissimilar or non-nested. In this work, we introduce a PTA model-comparison method using a combination of normalizing flows, nested sampling, and a PTA likelihood that is marginalized over all non-GWB parameters. The flows are trained using a newly-developed Python package coppuccino, which learns the data's copula: the dependence structure after transforming to uniform marginals. We validate the method on both analytic targets and simulated PTA data, recovering the analytic evidence within error bars and achieving an area-under-the-curve (AUC) score of 0.996 when treating the BFs as binary classifiers of the injected models. The result is a method to perform Bayesian model comparison of arbitrary GWB spectral models in minutes on consumer hardware.
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All authors David C. Wright, Aaron D. Johnson, Jeffrey S. Hazboun, William G. Lamb

Cosmology and Nongalactic Astrophysics

Gravitational-wave dark sirens as astrophysical probes: inferring galaxy-merger relations and identifying individual hosts

Alessandro Agapito and Michele Mancarella

Gravitational-wave (GW) “dark siren” techniques, namely the statistical association of GW events with a galaxy catalog, have so far been used as a cosmological tool. Yet this association carries distinctive astrophysical information that remains unused—specifically, how compact binaries populate galaxies and the identities of individual hosts. In this work, we turn the dark-siren program into an astrophysical probe by inferring both simultaneously within a single hierarchical framework. The method reduces the many potential hosts to a compact, probabilistically ranked target list, providing a direct follow-up decision tool. Applied blindly to GW170817, it recovers NGC~4993 as the highest-ranked host and constrains the galaxy luminosity weighting, mildly favoring non-uniform host weights. Fixing the galaxy weighting can instead degrade host identification and even favor the wrong host. This framework recasts dark sirens as a potentially unique probe of the astrophysics of electromagnetically dark compact-binary mergers.
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High Energy Astrophysical Phenomena

Very High-Energy GRB Afterglow Fits with Neural-Network Emulation and Bayesian inference

Edilberto Aguilar-Ruiz, Jessica Saldívar Talavera, Ramandeep Gill et al. (+1 more)

The very high-energy ($E > 100$ GeV) afterglow emission in gamma-ray bursts (GRBs) can be explained by synchrotron self-Compton (SSC). Modeling this emission accurately requires fully numerical codes that solve the time-dependent coupled particle–photon kinetic equations while accounting for Klein–Nishina suppression. However, their substantial computational cost makes them impractical for traditional parameter inference techniques such as Markov Chain Monte Carlo (MCMC) sampling. In this work, we present a highly efficient surrogate model based on artificial neural networks (ANNs) that emulates a one-zone kinetic SSC radiation code. To train our ANN, we use a multi-dimensional model parameter grid that constitutes a sample of 100,800 spectra produced by an infinitely thin spherical blast wave propagating inside a constant density interstellar medium (ISM). The ANN reproduces the numerical spectra well, with the median relative error generally below 2 per cent across the full frequency domain. Using this surrogate framework, we perform Bayesian parameter posterior estimation via MCMC and fit the broadband GRB afterglow observations of GRB 190114C from X-rays to TeV $\gamma$-rays at different epochs. We find an energetic jet with isotropic-equivalent kinetic energy of $E_{\rm k,iso}\simeq7.2\times10^{54}$ erg moving with a coasting bulk Lorentz factor of $\Gamma\gtrsim500$ into an ISM with density $n\simeq0.1\,{\rm cm^{-3}}$. The deviation of our model light curve from observations on long timescales suggests that an ISM environment is inconsistent and a radially stratified external medium, as pointed out by \citet{Aguilar-Ruiz+26}, is preferred instead.
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All authors Edilberto Aguilar-Ruiz, Jessica Saldívar Talavera, Ramandeep Gill, Sundar Srinivasan

Astrophysics of Galaxies

Discovery of the Lyman-$\alpha$ Forest in a Star-Forming Galaxy at $z \approx 7.57$: Implications for the Timing and Topology of Cosmic Reionization

Jakob M. Helton, Joris Witstok, Marta Laska et al. (+15 more)

The escape of ionizing photons from galaxies during the Epoch of Reionization is often inferred indirectly because the intervening neutral hydrogen in the intergalactic medium (IGM) is expected to absorb all flux blueward of Lyman-$\alpha$ at these redshifts. However, in this work, we report the significant detection ($\approx7\sigma$) of flux blueward of Ly$\alpha$ in a massive ($M_{\ast}\approx10^{9.1}\ M_{\odot}$) star-forming galaxy at $z\approx7.57$, JADES-GS-z7-LAF, using JWST/NIRSpec PRISM spectroscopy. We independently confirm this detection of the Ly$\alpha$ forest (LAF) using JWST/NIRCam F090W imaging ($\approx10\sigma$). The blueward flux in F090W is spatially coincident with the galaxy and emerges $\approx0.4$ pkpc from its UV centroid at $\lambda_{\mathrm{rest}}\approx1500\ \mathrm{\mathring{A}}$. We find no evidence for a foreground interloper producing the observed flux, although we cannot formally reject this interpretation with the existing data. The transmitted flux implies an extraordinarily transparent sightline ($x_{\mathrm{HI}}\ll0.1\%$) through the IGM at $z \approx 5.5-7.5$, a sightline that is spatially and kinematically coincident with numerous high-redshift galaxy overdensities, which possibly explains the emergence of this flux. We observe a smooth and gradual turnover around the Ly$\alpha$ break that is either consistent with an extreme damping-wing ($N_{\mathrm{HI}}\approx10^{23.3}\ \mathrm{cm}^{-2}$) or a strong nebular continuum (contributing $\approx60-70\%$ of the flux at $\lambda_{\mathrm{rest}}=1500\ \mathrm{\mathring{A}}$). JADES-GS-z7-LAF is far more luminous ($M_{\mathrm{UV}}\approx-21.5$, similar to GN-z11) than the faint sources usually invoked as the drivers of cosmic reionization. If such systems are common in the early Universe, massive galaxies may contribute more to the ionizing photon budget than suggested by other indirect constraints.
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All authors Jakob M. Helton, Joris Witstok, Marta Laska, Kevin N. Hainline, Nikko J. Cleri, Francesco D'Eugenio, Ignas Juodžbalis, Nimisha Kumari, Joel Leja, Sandro Tacchella, Rachana Bhatawdekar, Andrew J. Bunker, Daniel J. Eisenstein, Zhiyuan Ji, Benjamin D. Johnson, Pierluigi Rinaldi, Brant E. Robertson, Christopher N. A. Willmer

High Energy Astrophysical Phenomena · Cross-listed from nucl-th

Bayesian Constraints on Bosonic Dark Matter in Neutron Stars within a Covariant Density-Functional Equation-of-State Ensemble

Davood Rafiei Karkevandi, Alexander Ayriyan and Armen Sedrakian

We investigate neutron stars admixed with repulsively self-interacting bosonic dark matter (DM) using current multi-messenger observations. The dark component is modeled as a complex scalar field coupled to baryonic matter only through gravity, allowing dark-core and dark-halo configurations. To quantify baryonic equation-of-state (EOS) uncertainties, we combine a three-dimensional scan over $(m_\chi,\lambda,F_\chi)$ with 27 DDME2-based covariant density-functional EOSs spanning $L_{\rm sym}=30,50,70$ MeV and $-600\le Q_{\rm sat}\le 1000$ MeV. The models are confronted in a Bayesian framework with NICER mass–radius measurements from the Amsterdam and Maryland/Illinois analyses and the GW170817 tidal-deformability constraint. After Bayesian evidence weighting over the EOS ensemble, the EOS-relevant dark-sector scale $\mu_\chi\equiv m_\chi/\lambda^{1/4}$ is localized around 250 MeV for both NICER data sets. In the $(m_\chi,\lambda)$ parameterization, the marginalized boson-mass posterior peaks at $m_\chi\simeq300$–$330$ MeV, while $\lambda$ remains broadly distributed. Since the bosonic EOS depends on $m_\chi$ and $\lambda$ only through $\mu_\chi$, their separate marginals are conditional on the adopted independent priors. The inferred DM fraction is more data-set dependent, with posterior modes near 4% for Maryland/Illinois and 12% for Amsterdam. Bosonic DM also shifts the inferred $Q_{\rm sat}$ from negative toward intermediate and positive values, whereas the response of $L_{\rm sym}$ is weaker. This indicates a model-dependent trade-off between DM-induced compactification and the high-density stiffness of baryonic matter. These constraints remain conditional on the selected EOS family and the assumption of a common effective $F_\chi$ for all sources.
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Astrophysics of Galaxies

Detecting HI Self-Absorption using Neural Networks

Eric G. M. Muller, Naomi M. McClure-Griffiths, Hiep Nguyen et al. (+5 more)

Cold atomic hydrogen plays a crucial role in the life cycle of interstellar gas. It serves as the intermediary phase in the condensation and cooling processes that bridge the warm diffuse gas in and around galaxies, and the cold molecular gas that drives star formation. HI self-absorption in the 21-cm emission line is the most direct observational tracer of cold HI gas that does not require a continuum background source, yet its systematic extraction remains a long-standing challenge. Existing methods of self-absorption identification rely on subjective by-eye inspection or modelling of the underlying emission, making repeatable, thorough, and large-scale applications difficult. We present a lightweight convolutional neural network designed to detect self-absorption features and infer their velocities via a post-hoc process, without assumptions about the underlying emission or the use of ancillary data. Trained on synthetic emission spectra, the neural network achieves 96.5 per cent accuracy, 96.0 per cent precision, and 97.0 per cent recall on held-out synthetic data. Applied to observed 21-cm emission data from the Riegel–Crutcher cloud and giant molecular filament regions towards the Galactic Plane, the network recovers the spatial distributions and velocities of known self-absorption structures when compared to previous analyses and ancillary 13CO emission data. Crucially, the neural network is computationally efficient, processing detections for ~15,000 spectra per second on a single consumer laptop GPU, enabling real-time cold HI detection at the data rates anticipated by next-generation facilities such as the Square Kilometre Array.
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All authors Eric G. M. Muller, Naomi M. McClure-Griffiths, Hiep Nguyen, Matthew J. Alger, Frances Buckland-Willis, J. R. Dawson, Min-Young Lee, Antoine Marchal