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Physics Astrophysics

astro-ph Watch Astrophysics

Wednesday 8 October

Cosmology and Nongalactic Astrophysics

Primordial Black Holes and their Mass Spectra: The Effects of Mergers and Accretion within Stasis Cosmologies

Keith R. Dienes, Lucien Heurtier, Fei Huang et al. (+2 more)

A variety of processes in the very early universe can give rise to a population of primordial black holes (PBHs) with an extended mass spectrum. For certain mass spectra of this sort, it has been shown that the evaporation of these PBHs into radiation can drive the universe toward an epoch of cosmological stasis which can persist for a significant number of $e$-folds of cosmological expansion. However, in general, the initial mass spectrum which characterizes a population of PBHs at the time of production can subsequently be distorted by processes such as mergers and accretion. In this paper, we examine the effects that these processes have on the spectra that lead to a PBH-induced stasis. Within such stasis models, we find that mergers have only a negligible effect on these spectra within the regime of interest for stasis. We likewise find that the effect of accretion is negligible in many cases of interest. However, we find that the effect of accretion on the PBH mass spectrum is non-negligible in situations in which this spectrum is particularly broad. In such situations, the stasis epoch is abridged or, in extreme cases, does not occur at all. Thus accretion plays a non-trivial role in constraining the emergence of stasis within scenarios which lead to extended PBH mass spectra.
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All authors Keith R. Dienes, Lucien Heurtier, Fei Huang, Tim M.P. Tait, Brooks Thomas

Tuesday 7 October

Instrumentation and Methods for Astrophysics · Cross-listed from physics.pop-ph

Propellantless space exploration

Roman Ya. Kezerashvili

Propellantless propulsion refers to methods of space travel that do not require onboard propellant, instead relying on natural forces or external energy sources. In this paper, I review different approaches that have been explored and discuss the pros and cons of each method for interstellar space exploration. Gravitational assist uses planetary gravity to change a spacecraft's speed and direction without fuel. It is effective but limited to specific alignments. Solar sails harness radiation pressure from sunlight for continuous, fuel-free acceleration. While effective over time, they require large, reflective materials that degrade in space. Speed can be enhanced by thermal desorption triggered by solar radiation. Magnetic sails generate thrust by interacting with the solar wind through superconducting loops that produce a magnetic field. They provide lower acceleration compared to solar sails, and their performance depends on the available power and the variability of solar wind conditions. Electric sails utilize charged tethers to repel solar wind protons, producing gradual acceleration. Their effectiveness depends on the successful deployment of very long, lightweight conductive wires. They can achieve higher acceleration than solar sails, and their performance is influenced by available power and solar wind conditions. Lastly, quantum effects, such as the Casimir force, offer a speculative but intriguing route to propellantless propulsion based on the vacuum energy of space.
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Instrumentation and Methods for Astrophysics

Probing the Origin of Water in Planets within Habitable Zones by HWO

Yasuhiro Hasegawa, Courtney Dressing and Ludmila Carone

How do habitable environments arise and evolve within the context of their planetary systems? This is one fundamental question, and it can be addressed partly by identifying how planets in habitable zones obtain water. Historically, astronomers considered that water was delivered to the Earth via dynamical shake-up by Jupiter, which took place during the formation and post-formation eras (e.g., $\lesssim 100$ Myr). This hypothesis has recently been challenged by a more dynamic view of planet formation; planet-forming materials move in protoplanetary disks via various physical processes such as pebble drift and planetary migration. Habitable Worlds Observatory (HWO) will open a new window to address this important, but difficult question by discovering and characterizing Earth-like exoplanets around G-type stars. In this article, we consider two possible working hypotheses: (1) the abundance of water on planets in habitable zones has any correlation with the presence of outer planets; and (2) the abundance of water on planets in habitable zones has no correlation with the presence of outer planets. We discuss what physical parameters need to be measured to differentiate these two hypotheses and what observational capabilities are desired for HWO to reliably constrain these physical parameters.
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Instrumentation and Methods for Astrophysics

An Introduction to Mars Terraforming, 2025 Workshop Summary

Devon Stork and Erika DeBenedictis

Terraforming Mars is an age old science fiction concept now worth revisiting through the lens of modern science and technology. This document serves as a summary of contemporary ideas about Mars terraforming, prepared for attendees of the 2025 Green Mars Workshop. It presents one illustrative story of how Mars might be transformed into a habitable world. The story is told in reverse, beginning with possible planetary endpoints and tracing backward to the steps required to reach them. Along the way, it highlights alternative approaches, critical unknowns and research priorities, and the near term applications and benefits of terraforming research for planetary science, climate engineering, and sustainable technologies on Earth.
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Solar and Stellar Astrophysics · Cross-listed from physics.space-ph

Influence of Solar Sails on Magnetic Field Measurements in Space Plasmas

Konstantinos Horaites, Juan V. Rodriguez and Ying Liu

Solar sail technology is ready to be deployed in a satellite mission carrying a science-grade magnetometer. In preparation for such a mission, it is essential to characterize the interactions between the sail and the ambient plasma that could affect the magnetometer readings. The solar wind magnetic field is a key parameter in space weather prediction, because it governs the energy-releasing magnetic reconnection process at Earth's magnetopause. This paper investigates the influence of solar sails on the ambient magnetic field, particularly focusing on two critical electromagnetic effects: eddy currents and magnetic pileup. We find the induced eddy currents in the metallic sail can significantly perturb the local magnetic field at high frequencies. We also suggest that magnetic pileup can influence the spacecraft's environment when the sail size is comparable to the electron kinetic scales of the surrounding plasma. This research provides an initial guide for determining when sail-plasma interactions could impact magnetometer performance.
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Solar and Stellar Astrophysics

Ensemble seismic study of the properties of the core of Red Clump stars

Anthony Noll, Sarbani Basu and Saskia Hekker

Red clump stars still pose open questions regarding several physical processes, such as the mixing around the core, or the nuclear reactions, which are ill-constrained by theory and experiments. The oscillations of red clump stars, which are of mixed gravito-acoustic nature, allow us to directly investigate the interior of these stars and thereby better understand their physics. In particular, the measurement of their period spacing is a good probe of the structure around the core. We aim to explain the distribution of period spacings in red clump stars observed by Kepler by testing different prescriptions of core-boundary mixing and nuclear reaction rate. Using the MESA stellar evolution code, we computed several grids of core-helium burning tracks, with varying masses and metallicities. Each of these grids have been computed assuming a certain core boundary mixing scheme, or carbon-alpha reaction rate. We then sampled these grids, in a Monte-Carlo fashion, using observational spectroscopic metallicities and seismic masses priors, in order to retrieve a period spacing distribution that we compared to the observations. We found that the best fitting distribution was obtained when using a "maximal overshoot" core-boundary scheme, which has similar seismic properties as a model whose modes are trapped outside a semi-convective region, and which does not exhibit core breathing pulses at the end of the core-helium burning phase. If no mode trapping is assumed, then no core boundary mixing scheme is compatible with the observations. Moreover, we found that extending the core with overshoot worsens the fit. Additionally, reducing the carbon-alpha reaction rate (by around 15%) improves the fit to the observed distribution. Finally, we noted that an overpopulation of early red clump stars with period spacing values around 250s is predicted by the models but not found in the observations.
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Instrumentation and Methods for Astrophysics

CCAT: Design, Implementation, and Testing of a System to Read Out over 10,000 280 GHz KIDs using RFSoC Electronics

Darshan A. Patel, Yuhan Wang, Cody J. Duell et al. (+26 more)

Over the past decade, kinetic inductance detectors (KIDs) have emerged as a viable superconducting technology for astrophysics at millimeter and submillimeter wavelengths. KIDs spanning 210 - 850 GHz across seven instrument modules will be deployed in the Prime-Cam instrument of CCAT Observatory's Fred Young Submillimeter Telescope at an elevation of 5600 m on Cerro Chajnantor in Chile's Atacama Desert. The natural frequency-division multiplexed readout of KIDs allows hundreds of detectors to be coupled to a single radio frequency (RF) transmission line, but requires sophisticated warm readout electronics. The FPGA-based Xilinx ZCU111 radio frequency system on chip (RFSoC) offers a promising and flexible solution to the challenge of warm readout. CCAT uses custom packaged RFSoCs to read out KIDs in the Prime-Cam instrument. Each RFSoC can simultaneously read out four RF channels with up to 1,000 detectors spanning a 512 MHz bandwidth per channel using the current firmware. We use five RFSoCs to read out the >10,000 KIDs in the broadband 280 GHz instrument module. Here, we describe and demonstrate the readout hardware, software and pipeline for the RFSoC system. We present a detector position map of the 280 GHz module focal plane and preliminary averaged spectral responses of a small subset of detectors from the TiN and first Al arrays. These measurements demonstrate our ability to simultaneously readout thousands of detectors, validate the end-to-end performance of the readout and optical systems, and represent a critical step toward reading out the ~100,000 KIDs in Prime-Cam in its future full capacity configuration.
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All authors Darshan A. Patel, Yuhan Wang, Cody J. Duell, Jason E. Austermann, James Beall, James R. Burgoyne, Scott Chapman, Steve K. Choi, Rodrigo G. Freundt, Eliza Gazda, Christopher Groppi, Zachary B. Huber, Johannes Hubmayr, Ben Keller, Lawrence T. Lin, Philip Mauskopf, Alicia Middleton, Michael D. Niemack, Cody Roberson, Adrian K. Sinclair, Ema Smith, Jeff van Lanen, Anna Vaskuri, Benjamin J. Vaughan, Eve M. Vavagiakis, Michael Vissers, Samantha Walker, Jordan Wheeler, Ruixuan Xie

Solar and Stellar Astrophysics

Radiation magnetohydrodynamics modeling of an impulsively driven chromospheric jet in the solar atmosphere

J. J. González-Avilés

In this paper, we present a numerical simulation of an impulsively driven chromospheric jet in the solar atmosphere using the non-ideal magnetohydrodynamic (MHD) equations coupled with frequency- and angle-averaged radiation transport equations. These include the dynamics of the radiation energy density and radiation flux. The jet is initiated by a localized Gaussian pulse applied to the vertical velocity component in the upper chromosphere (y = 1.75 Mm), producing a collimated plasma structure that exhibits characteristics similar to macrospicules. We focus on the formation and evolution of the chromospheric jet as it propagates through an optically thin region encompassing the upper chromosphere and solar corona, where both the Planck-averaged absorption and Rosseland-averaged scattering opacities are low. Although radiation transport terms only slightly affect the jet's morphology, they play a significant role in governing radiative processes in the corona. In particular, radiation transport contributes to the dissipation of the chromospheric jet, which effectively acts as a radiative cooling mechanism as the jet evolves through the optically thin solar corona.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Classification of $g$-modes for neutron stars with a strong transition: Novel universal relation including slow stable hybrid stars

M. C. Rodriguez, José C. Jiménez and Ignacio F. Ranea-Sandoval

We investigate the behavior of the non-radial gravity-pulsation discontinuity $g$-mode in neutron stars with a strong first-order phase transition which give rise to hybrid-star configurations. These modes are of utmost relevance since they can be potentially excited in isolated as well as binary neutron star systems in the inspiral phase, thus allowing us to indirectly detect the presence of a deconfinement transition. In order to do this, we consider four categories of hybrid stars that present distinctive features in their equations of state. We employ the constant speed of sound parametrization, which accounts for a sharp phase transition between confined hadronic matter and deconfined quark matter. Then, working within the relativistic Cowling approximation to obtain the frequencies of non-radial oscillations, we find that, depending on the hybrid star category, the relations between discontinuity $g$-mode frequencies and masses as well as tidal deformabilities display a highly distinct behavior across the diverse hybrid star categories that appear in the slow hadron-quark conversion regime. This distinct phenomenology provides smoking-gun evidence to clearly distinguish and further classify hybrid stars with a strong transition from purely hadronic stars using upcoming gravitational-wave data. In addition, we present for each of the categories studied the relation between the $g$-mode frequency and the normalized energy density jump. Finally, we present a novel universal relation for the discontinuity $g$-mode able to encompass the four categories including long branches of slow stable hybrid stars and address its asteroseismological capability.
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High Energy Astrophysical Phenomena

The first proper motion measurement of the acceleration regions in the large-scale jets of SS 433 powering the W50 nebula

Naomi Tsuji, Yoshiyuki Inoue, Dmitry Khangulyan et al. (+10 more)

We report on new Chandra ACIS-I observations of the X-ray knots located in the western and eastern lobes of W50 associated with the parsec-scale jets of the Galactic microquasar SS 433. These knots are likely counterparts of the recently detected very-high-energy ($E>100$ GeV) gamma-ray emission by HAWC and H.E.S.S. These findings, together with the ultra-high-energy signal recently reported by the LHAASO collaboration, have established the SS 433/W50 system as a unique jet-driven PeVatron candidate. Combining new and archival Chandra data, we perform the first proper motion search of the X-ray knot structures over a time interval spanning approximately 20 years. We found no statistically significant motion of these knots at the 3$\sigma$ confidence level, and place an upper limit of $<$ 0.019-0.033$c$ (5,800-9,800 km s$^{-1}$) for the speed of the innermost knots at an assumed distance $d=5.5$ kpc. Combined with the velocities reported in the literature, the upstream speed in the shock rest frame would reach several 10$^4$ km s$^{-1}$, suggesting that highly efficient particle acceleration, approaching the Bohm limit, is occurring. The absence of significant motion of the knots suggests the presence of a standing recollimation shock, formed by the balance between the jet pressure and the external pressure. This interpretation is consistent with the expected occurrence of such shocks at 20-30 pc from SS 433, matching the location of the observed knots.
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All authors Naomi Tsuji, Yoshiyuki Inoue, Dmitry Khangulyan, Kaya Mori, Samar Safi-Harb, Takaaki Tanaka, Laura Olivera-Nieto, Brydyn Mac Intyre, Kazuho Kayama, Takeshi Go Tsuru, Hiroyuki Uchida, Tatsuki Fujiwara, Felix Aharonian

Astrophysics of Galaxies

The rise and fall of Little Red Dots could be driven by the environment

Rosa M. Mérida, Gaia Gaspar, Yoshihisa Asada et al. (+9 more)

The Little Red Dot (LRD) paradigm comprises three main unknowns that are intrinsically connected: (1) What is the nature of these sources? (2) How do they form? (3) How do they evolve? Larger spectroscopic samples and high-resolution data are needed to delve deeper into the mechanisms ruling these sources. Understanding their formation and evolution requires identifying the rise and fall of the key features that characterize these systems, such as their compactness and ”V”-shaped spectral energy distributions. In this work, we present a galaxy system nicknamed The Stingray that was identified in the Canadian NIRISS Unbiased Cluster Survey (CANUCS). This group contains three sources at $z_{\mathrm{spec}} = 5.12$, including an active galactic nucleus (AGN), a Balmer break galaxy, and a star-forming satellite. The latter resembles a Building Block System in which interactions boost stellar mass and black hole mass growth beyond what is expected from secular processes alone. The AGN in this system exhibits features indicative of a transitional object, bridging a normal AGN and an LRD phase. These are a blue rest-frame ultraviolet slope, compact size, and a broad H$\alpha$ line (all of which are characteristic of LRDs), but a flatter rest-frame optical slope compared to that observed in LRDs. The features in this source point to the emergence or fading of an LRD, potentially triggered by environmental effects.
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All authors Rosa M. Mérida, Gaia Gaspar, Yoshihisa Asada, Marcin Sawicki, Kiyoaki Christopher Omori, Chris J. Willott, Nicholas S. Martis, Adam Muzzin, Gaël Noirot, Gregor Rihtaršič, Ghassan T. E. Sarrouh, Roberta Tripodi

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

A comprehensive look into the accuracy of SpEC binary black hole waveforms

Taylor Knapp, Katerina Chatziioannou, Keefe Mitman et al. (+4 more)

Numerical relativity simulations provide a full description of the dynamics of binary systems, including gravitational radiation. The waveforms produced by these simulations have a number of applications in gravitational-wave detection and inference. In this work, we revisit the accuracy of the waveforms produced by the Spectral Einstein Code. Motivated by the wide range of waveform applications, we propose and explore three accuracy metrics between simulation resolutions: (i) the generalized frequency-weighted mismatch, (ii) the relative amplitude difference, and (iii) the phase difference at different times. We confirm that numerical errors accumulate over the binary evolution, but the error is not intrinsically larger during the latest, more dynamical stages. Studying errors across the parameter space, we identify a positive correlation between both the mismatch and the phase difference with precessing spin, but little correlation with aligned spin or eccentricity. Lastly, amplitude and phases differences are symmetric upon exchanging resolutions across the catalog, suggesting that the dominant source of error is random, rather than something systematic that affects all waveforms similarly.
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All authors Taylor Knapp, Katerina Chatziioannou, Keefe Mitman, Mark A. Scheel, Michael Boyle, Lawrence E. Kidder, Harald Pfeiffer

Earth and Planetary Astrophysics

JWST occultation reveals unforeseen complexity in Chariklo's ring system

Pablo Santos-Sanz, Altair R. Gomes-Júnior, Bruno E. Morgado et al. (+25 more)

Ring systems have been discovered around several small bodies in the outer Solar System through stellar occultations. While such measurements provide key information about ring geometry and dynamical interactions, little is known about their origins, lifetimes, evolutionary pathways, or compositions. Here we report near-infrared observations with the James Webb Space Telescope (JWST) of a stellar occultation by (10199) Chariklo, a Centaur known to host a double-ring system. Our JWST measurements show that Chariklo's inner dense ring has become significantly more opaque than in previous observations, pointing to ongoing replenishment processes or dynamical restructuring. In contrast, the outer ring exhibits a much weaker near-infrared occultation signature than seen in earlier visible-light detections. This discrepancy may reflect material loss, suggesting that the outer ring could be transient, or may arise from wavelength-dependent opacity. These scenarios, which are not mutually exclusive, point to an unprecedented level of complexity in small-body ring systems, distinct from those observed around any other minor bodies in the Solar System.
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All authors Pablo Santos-Sanz, Altair R. Gomes-Júnior, Bruno E. Morgado, Yucel Kilic, Csilla E. Kalup, Csaba Kiss, Chrystian L. Pereira, Bryan J. Holler, Nicolás Morales, José Luis Ortiz, Bruno Sicardy, Juan Luis Rizos, John Stansberry, Richard G. French, Heidi B. Hammel, Zhong-Yi Lin, Damya Souami, Josselin Desmars, Stefanie N. Milam, Felipe Braga-Ribas, Marcelo Assafin, Gustavo Benedetti-Rossi, Julio I. B. Camargo, René Duffard, Flavia L. Rommel, Estela Fernández-Valenzuela, Noemí Pinilla-Alonso, Mónica Vara-Lubiano

Astrophysics of Galaxies

Evidence of violation of Case B recombination in Little Red Dots

G. P. Nikopoulos, D. Watson, A. Sneppen et al. (+4 more)

Little Red Dots (LRDs) are a new class of compact extragalactic objects, with a v-shaped optical spectral energy distribution breaking close to the Balmer break wavelength, and broad, typically exponentially-shaped lines. They are believed to be supermassive black holes surrounded by very dense, ionized gas, leading us to explore for any departures from Case B recombination by examining the ratios of multiple hydrogen Balmer lines: $H\alpha$, $H\beta$, $H\gamma$, and $H\delta$. We analyze a dozen high-S/N LRDs with JWST/NIRSpec, measuring Balmer ratios in the seven objects with coverage of at least three lines. We decompose the line ratios into their respective broad and narrow components. Broad line ratios are consistent with Case B plus dust extinction in all objects but one, RUBIES EGS-49140, which departs from Case B expectations by more than $5\sigma$. The narrow components are consistent with minimal dust attenuation, while two objects exhibit narrow $H\alpha$/$H\beta \approx 1.8$. Such low decrements are observed in highly ionized density bounded nebulae, associated with starburst environments. Nevertheless, both flat decrement cases can be reconciled assuming an unresolved absorption feature. RUBIES EGS-49140, shows a high broad $H\alpha$/$H\beta$, but $H\gamma$/$H\alpha$ and $H\delta$/$H\alpha$ ratios are lower than expected for extinction-modified Case B, hinting at an unphysically steep dust law. These line ratios may be due to increased optical depth in the Balmer lines, as a direct effect of high density (log$n_e$ > 9) gas surrounding the black hole. If Case B recombination does hold in most LRDs, they must be moderate-to-heavily dust obscured ($A_V\simeq1-8$) while the host-galaxy should be dust-free, suggesting that the extinction in the broad lines is local to the LRD and not due to the general ISM of the host galaxy.
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All authors G. P. Nikopoulos, D. Watson, A. Sneppen, V. Rusakov, K. E. Heintz, J. Witstok, G. Brammer

High Energy Astrophysical Phenomena

Are all Binary Black Holes Detected by LIGO-Virgo-KAGRA Following the Universal Time-Delay Distributions? Probably Not

Samsuzzaman Afroz, Navdha and Suvodip Mukherjee

The delay time distribution (DTD) of binary black hole (BBH) mergers encodes the evolutionary link between stellar formation history and gravitational-wave (GW) emission. We present a non-parametric reconstruction of the DTD using BBHs from the GWTC-4 catalog, employing a grid-based framework that avoids restrictive power-law assumptions. We divide the BBH population into two mass bins, $20$–$40\,M_\odot$ and $40$–$100\,M_\odot$, and reconstruct the DTD independently for each bin using grid-based trajectories, under three different star formation rate density (SFRD) models. We find that both mass bins favor an extended plateau of intermediate-to-long delay times rather than a single characteristic timescale, and that in both bins the reconstructed merger rate $\mathcal{R}(z)$ rises from its local value to a peak at intermediate redshift before declining. The two bins differ in degree rather than kind: the $20$–$40\,M_\odot$ bin is broadly constrained, with a local rate of $R_0 \approx 18$–$23\,\mathrm{Gpc^{-3}\,yr^{-1}}$ and a merger rate peaking at $z \sim 0.8$–$1.05$, while the $40$–$100\,M_\odot$ bin prefers a more sharply defined intermediate delay range of $\sim$2–4 Gyr with a steeper suppression of long delay times, within a noticeably tighter set of high-evidence trajectories, a lower local rate of $R_0 \approx 9$–$13\,\mathrm{Gpc^{-3}\,yr^{-1}}$, and a peak shifted to slightly higher redshift, $z \sim 1.0$–$1.05$. The results indicate that a single power-law DTD may not be sufficient to simultaneously describe both mass bins, pointing toward possible mass-dependent binary evolution pathways that merit further investigation with larger GW catalogs.
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Astrophysics of Galaxies

MEGATRON: Disentangling Physical Processes and Observational Bias in the Multi-Phase ISM of High-Redshift Galaxies

Nicholas Choustikov, Harley Katz, Alex J. Cameron et al. (+10 more)

Now detected out to redshifts of $z\sim 14.5$, the rest-frame ultraviolet and optical spectra of galaxies encode numerous physical properties of the interstellar medium (ISM). Accurately extracting these properties from spectra remains a key challenge that numerical simulations are uniquely suited to address. We present a study of the observed ISM of galaxies in MEGATRON: a suite of cosmological radiation hydrodynamics simulations coupled to on-the-fly non-equilibrium thermochemistry, with multiple prescriptions for star formation/feedback and parsec-scale resolution; capable of directly predicting spectroscopic properties of early galaxies. We find that irrespective of feedback physics used, the ISM of high-redshift galaxies is denser, less metal enriched, and subject to higher ionization parameters and radiation fields compared to similar mass galaxies in the local Universe – in agreement with interpretations of JWST observations. Using common observational techniques to infer bulk galaxy properties, we find that ISM gas density controls the slope of the mass-metallicity relation. Similarly, at the densities reached in some high-redshift galaxies, O32 becomes a density tracer rather than one of ionization parameter. This motivates the use of other line ratios like C43 and N43 to infer the ionization state of the gas. Finally, various feedback models populate different regions of strong-line diagnostic diagrams as the line ratios are sensitive to the feedback-modulated density-temperature structure of the ISM. Therefore, observed strong-line diagnostics can provide a strong constraint on the underlying physics of star formation and feedback in the high-redshift Universe.
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All authors Nicholas Choustikov, Harley Katz, Alex J. Cameron, Aayush Saxena, Julien Devriendt, Adrianne Slyz, Martin P. Rey, Corentin Cadiou, Jeremy Blaizot, Taysun Kimm, Isaac Laseter, Kosei Matsumoto, Joki Rosdahl

Astrophysics of Galaxies

A Walk on the Retrograde Side (WRS) project. II. Chemistry to disentangle in situ and accreted components in Thamnos

E. Ceccarelli, D. Massari, M. Palla et al. (+3 more)

We present the results of the first systematic and dedicated high-resolution chemical analysis of the Thamnos substructure, a candidate relic of the process of hierarchical merger of the Milky Way. The analysis was perfomed in comparison with the Gaia-Sausage-Enceladus (GSE) remnant, within the fully self-consistent and homogeneous framework established by the 'A Walk on the Retrograde Side' (WRS) project. We analysed high-resolution and high signal-to-noise ratio spectra obtained with UVES at VLT for 212 red giant branch stars classified as candidate members of Thamnos and GSE, based on selections in the space of the integrals of motion. We derived precise abundances for 16 atomic species. Compared to GSE, stars attributed to the Thamnos substructure are, on average, more metal-poor, yet most of them show higher [X/Fe] abundance ratios in several elements, such as Na, Mg, Al, Ca, Cu, Zn, as well as lower [Eu/Fe]. The majority of candidate Thamnos stars show chemical signatures more consistent with the in situ Milky Way halo rather than a typical low-mass accreted dwarf galaxy. Our findings are further supported by comparisons with tailored galactic chemical evolution models, which fall short in reproducing the observed enhancement in the $\alpha$-elements, but are able to fit the more metal-poor component present in the Thamnos substructure. These results confirm a high level of contamination in the Thamnos substructure from the in situ population and to a lesser degree from GSE, while still leaving room for a genuine accreted population from a small disrupted dwarf galaxy.
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All authors E. Ceccarelli, D. Massari, M. Palla, A. Mucciarelli, M. Bellazzini, A. Helmi

Solar and Stellar Astrophysics

Phase-resolved optical spectroscopy of the rapidly varying white dwarf ZTF 1851+1714

C. C. Pedersen, M. R. M. Knudsen, K. Valeckas et al. (+3 more)

We report on phase-resolved optical spectroscopy and photometry in the R and B bands of the white dwarf candidate ZTF 185139.81+171430.3. The source has been reported to be variable with a large amplitude of close to 1 magnitude, in the R band, and a short period of 12.37 min. We confirm this period and interpret it as the spin period of the white dwarf. The optical spectrum shows emission lines from hydrogen and helium superposed on a featureless continuum. The continuum changes shape throughout a cycle, such that it is redder when the source is bright. There is tentative evidence for Doppler shifts in the emission lines during the spin cycle with an amplitude of a few tens of km s$^{-1}$. Notably, the H$\alpha$ and H$\beta$ lines exhibit different radial velocity amplitudes, suggesting that they come from different emission regions. We also identify a candidate orbital period of 1.00 hr, based on potential orbital sidebands. These features - Doppler shifts modulated at the spin frequency, brightness variations, and continuum shape changes - are consistent with the accretion curtain model, in which material is funneled from a truncated inner disc along magnetic field lines onto the magnetic poles of the white dwarf.
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All authors C. C. Pedersen, M. R. M. Knudsen, K. Valeckas, L. Izzo, T. M. Tauris, J. P. U. Fynbo

Astrophysics of Galaxies

Asymmetries in stellar streams induced by a galactic merger

Claire Guillaume, Florent Renaud, Nicolas F. Martin et al. (+6 more)

Stellar streams are sensitive to perturbations from, e.g., giant molecular clouds, bars and spiral arms, infalling dwarf galaxies, or globular clusters which can imprint gaps, clumps, spurs, and asymmetries in tails. In addition to these effects, the impact of a galactic major merger on a population of stellar streams remains to be explored. Here, we focus on the emergence and longevity of asymmetries between the leading and trailing tails of streams caused by such interactions. We run collisionless N-body simulations of a Milky Way-like galaxy hosting 36 globular cluster streams and merging with a perturber galaxy. We propose a new asymmetry metric to quantify the structural differences between both tails from their respective cumulative density profiles. We find that the over- and under-densities along streams induced by the merger depend on the orbital characteristics of their progenitors. The non-simultaneity of this effect from stream to stream implies that global asymmetry signatures are less prominent than in individual cases. These population-averaged imprints remain detectable over only 2.5 Gyr but asymmetric signatures can persist over much longer periods for individual streams with wide orbits that have been perturbed prior to coalescence. We thus caution that the interpretation of streams' morphology in the context of dark matter mapping is strongly subject to degeneracies and should be performed considering the merger history of the host.
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All authors Claire Guillaume, Florent Renaud, Nicolas F. Martin, Benoit Famaey, Paola Di Matteo, Guillaume F. Thomas, Salvatore Ferrone, Rodrigo Ibata, Giulia Pagnini

Astrophysics of Galaxies

Preferential Accretion onto the Secondary Black Hole Strengthens Gravitational Wave Signals

Julia M. Comerford and Joseph Simon

Pulsar timing arrays have recently found evidence for nanohertz gravitational waves that are consistent with being produced by a cosmological population of binary supermassive black holes (SMBHs). However, the amplitude of this gravitational wave background is larger than predicted from theoretical and empirical models of SMBH binary populations. We investigate preferential accretion onto the secondary, less massive SMBH of the binary as a potential solution to this discrepancy. We carry out the first observationally-based analysis of the effect of preferential accretion on the SMBH binary population, and we find that preferential accretion onto the secondary SMBH increases the binary SMBH mass ratio, causing many minor galaxy mergers to lead to major SMBH mergers. The fraction of SMBH mergers that are major mergers increases by a factor of 2-3 when preferential accretion is included. Further, we find that only a small amount of preferential accretion (10% total SMBH mass growth) is needed to bring the predicted gravitational wave background amplitude into agreement with observations. Preferential accretion has an even larger effect on gravitational wave signals detected by LISA, which will probe SMBH binaries at higher redshifts where the environment is more gas-rich, and can also help explain the rapid build up of overmassive black holes at high redshifts observed by the James Webb Space Telescope. It also shortens the time to the first detection of an individual SMBH binary emitting continuous waves. Preferential accretion strengthens the gravitational wave signals produced by any binary embedded in a circumbinary disk, including LIGO sources.
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Astrophysics of Galaxies

Comparing XRISM cluster velocity dispersions with predictions from cosmological simulations: are feedback models too ejective?

XRISM Collaboration, Marc Audard, Hisamitsu Awaki et al. (+147 more)

The dynamics of the intra-cluster medium (ICM), the hot plasma that fills galaxy clusters, are shaped by gravity-driven cluster mergers and feedback from supermassive black holes (SMBH) in the cluster cores. XRISM measurements of ICM velocities in several clusters offer insights into these processes. We compare XRISM measurements for nine galaxy clusters (Virgo, Perseus, Centaurus, Hydra A, PKS\,0745–19, A2029, Coma, A2319, Ophiuchus) with predictions from three state-of-the-art cosmological simulation suites, TNG-Cluster, The Three Hundred Project GADGET-X, and GIZMO-SIMBA, that employ different models of feedback. In cool cores, XRISM reveals systematically lower velocity dispersions than the simulations predict, with all ten measurements below the median simulated values by a factor $1.5-1.7$ on average and all falling within the bottom $10\%$ of the predicted distributions. The observed kinetic-to-total pressure ratio is also lower, with a median value of $2.2\%$, compared to the predicted $5.0-6.5\%$ for the three simulations. Outside the cool cores and in non-cool-core clusters, simulations show better agreement with XRISM measurements, except for the outskirts of the relaxed, cool-core cluster A2029, which exhibits an exceptionally low kinetic pressure support ($<1\%$), with none of the simulated systems in either of the three suites reaching such low levels. The non-cool-core Coma and A2319 exhibit dispersions at the lower end but within the simulated spread. Our comparison suggests that the three numerical models may overestimate the kinetic effects of SMBH feedback in cluster cores. Additional XRISM observations of non-cool-core clusters will clarify if there is a systematic tension in the gravity-dominated regime as well.
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All authors XRISM Collaboration, Marc Audard, Hisamitsu Awaki, Ralf Ballhausen, Aya Bamba, Ehud Behar, Rozenn Boissay-Malaquin, Laura Brenneman, Gregory V. Brown, Lia Corrales, Elisa Costantini, Renata Cumbee, Maria Diaz Trigo, Chris Done, Tadayasu Dotani, Ken Ebisawa, Megan E. Eckart, Dominique Eckert, Satoshi Eguchi, Teruaki Enoto, Yuichiro Ezoe, Adam Foster, Ryuichi Fujimoto, Yutaka Fujita, Yasushi Fukazawa, Kotaro Fukushima, Akihiro Furuzawa, Luigi Gallo, Javier A. García, Liyi Gu, Matteo Guainazzi, Kouichi Hagino, Kenji Hamaguchi, Isamu Hatsukade, Katsuhiro Hayashi, Takayuki Hayashi, Natalie Hell, Edmund Hodges-Kluck, Ann Hornschemeier, Yuto Ichinohe, Daiki Ishi, Manabu Ishida, Kumi Ishikawa, Yoshitaka Ishisaki, Jelle Kaastra, Timothy Kallman, Erin Kara, Satoru Katsuda, Yoshiaki Kanemaru, Richard Kelley, Caroline Kilbourne, Shunji Kitamoto, Shogo Kobayashi, Takayoshi Kohmura, Aya Kubota, Maurice Leutenegger, Michael Loewenstein, Yoshitomo Maeda, Maxim Markevitch, Hironori Matsumoto, Kyoko Matsushita, Dan McCammon, Brian McNamara, François Mernier, Eric D. Miller, Jon M. Miller, Ikuyuki Mitsuishi, Misaki Mizumoto, Tsunefumi Mizuno, Koji Mori, Koji Mukai, Hiroshi Murakami, Richard Mushotzky, Hiroshi Nakajima, Kazuhiro Nakazawa, Jan-Uwe Ness, Kumiko Nobukawa, Masayoshi Nobukawa, Hirofumi Noda, Hirokazu Odaka, Shoji Ogawa, Anna Ogorzałek, Takashi Okajima, Naomi Ota, Stephane Paltani, Robert Petre, Paul Plucinsky, Frederick S. Porter, Katja Pottschmidt, Kosuke Sato, Toshiki Sato, Makoto Sawada, Hiromi Seta, Megumi Shidatsu, Aurora Simionescu, Randall Smith, Hiromasa Suzuki, Andrew Szymkowiak, Hiromitsu Takahashi, Mai Takeo, Toru Tamagawa, Keisuke Tamura, Takaaki Tanaka, Atsushi Tanimoto, Makoto Tashiro, Yukikatsu Terada, Yuichi Terashima, Yohko Tsuboi, Masahiro Tsujimoto, Hiroshi Tsunemi, Takeshi Tsuru, Aysegül Tümer, Hiroyuki Uchida, Nagomi Uchida, Yuusuke Uchida, Hideki Uchiyama, Shutaro Ueda, Yoshihiro Ueda, Shinichiro Uno, Jacco Vink, Shin Watanabe, Brian J. Williams, Satoshi Yamada, Shinya Yamada, Hiroya Yamaguchi, Kazutaka Yamaoka, Noriko Yamasaki, Makoto Yamauchi, Shigeo Yamauchi, Tahir Yaqoob, Tomokage Yoneyama, Tessei Yoshida, Mihoko Yukita, Irina Zhuravleva, Weiguang Cui, Stefano Ettori, Skylar Grayson, Annie Heinrich, Hannah McCall, Dylan Nelson, Nobuhiro Okabe, Yuki Omiya, Arnab Sarkar, Evan Scannapieco, Ming Sun, Keita Tanaka, Nhut Truong, Daniel R. Wik, Congyao Zhang, John ZuHone

Solar and Stellar Astrophysics

A Deep Look into the Intermediate-Age Open Cluster NGC 2506: What Binary Systems Reveal About Cluster Distance and Age

K. Yakut, B. Kalomeni, S. Rappaport et al. (+1 more)

Using high-precision observations from the space-based Gaia and TESS missions, complemented by ground-based spectroscopic data and multi-band photometric surveys, we perform a detailed investigation of the Galactic open cluster NGC~2506. We present a new analysis of the intermediate-age open cluster NGC~2506, using joint fits to the radial velocities (RVs) and spectral energy distributions (SEDs) of five double-lined binary systems, including two eclipsing binaries. The analysis yields self-consistent estimates of the cluster's age, distance, and extinction, based on 18 free parameters: 10 stellar masses, 5 orbital inclinations, and common values for age, distance, and $A_V$. The SED fitting incorporates stellar isochrones, and the resulting parameters are examined through HR diagrams (R–$T_{\rm eff}$, R–M, and M–$T_{\rm eff}$) to assess evolutionary consistency. The age we derive for the cluster is $1.94 \pm 0.03$ Gyr for an assumed [Fe/H] = -0.30, and a fitting formula is given for extrapolation to other metallicities. The distance we find from the SED fitting is $3189 \pm 53$ pc, and this is to be compared with our own inference from the Gaia data which is $3105 \pm 75$ pc, based on 919 stars identified as cluster members. Our results demonstrate the power of binary systems in tightly constraining cluster-wide age and distance at this evolutionary stage. This approach represents one of the most accurate characterizations of an intermediate-age open cluster using multiple binary systems.
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All authors K. Yakut, B. Kalomeni, S. Rappaport, Veselin Kostov

Astrophysics of Galaxies

Beyond the stars: Linking H$\alpha$ sizes, kinematics, and star formation in galaxies at $z\approx 4-6$ with JWST grism surveys and $\texttt{geko}$

A. Lola Danhaive, Sandro Tacchella, William McClymont et al. (+15 more)

Understanding how galaxies assemble their mass during the first billion years of cosmic time is a central goal of extragalactic astrophysics, yet joint constraints on their sizes and kinematics remain scarce. We present one of the first statistical studies of the $\mathrm{H}\alpha$ size-mass relation at high redshift with a sample of 213 galaxies at spectroscopic redshifts of $z\approx 4-6$ from the FRESCO and CONGRESS NIRCam grism surveys. We measure the $\mathrm{H}\alpha$ morphology and kinematics of our sample using the novel forward modelling Bayesian inference tool $\texttt{geko}$, and complement them with stellar continuum sizes in the rest-frame FUV, NUV, and optical, obtained from modelling of imaging data from the JADES survey with $\texttt{Pysersic}$. At $z\approx5$, we find that the average H$\alpha$ sizes are larger than the stellar continuum (FUV, NUV and optical), with $r_{\rm e, H\alpha}= 1.17 \pm 0.05$ kpc and $r_{\rm e,cont} \approx 0.9$ kpc for galaxies with $\log(M_{\star} ~\rm [M_{\odot}])= 9.5$. However, we find no significant differences between the stellar continuum sizes at different wavelengths, suggesting that galaxies are not yet steadily growing inside-out at these epochs. Instead, we find that the ratio $r_{\rm e, H\alpha}/r_{\rm e, NUV}$ increases with the distance above the star-forming main sequence ($\Delta \rm MS$), consistent with an expansion of H$\alpha$ sizes during episodes of enhanced star formation caused by an increase in ionising photons. As galaxies move above the star-forming main sequence, we find an increase of their rotational support $v/\sigma$, which could be tracing accreting gas illuminated by the \Ha\ emission. Finally, we find that about half of the elongated systems ($b/a < 0.5$) are not rotationally supported, indicating a potential flattened/prolate galaxy population at high redshift.
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All authors A. Lola Danhaive, Sandro Tacchella, William McClymont, Brant Robertson, Stefano Carniani, Courtney Carreira, Eiichi Egami, Andrew J. Bunker, Emma Curtis-Lake, Daniel J. Eisenstein, Zhiyuan Ji, Benjamin D. Johnson, Marcia Rieke, Natalia C. Villanueva, Christopher N. A. Willmer, Chris Willot, Zihao Wu, Yongda Zhu

High Energy Astrophysical Phenomena

New ultralight scalar particles and the mass-radius relation of white dwarfs – the important role of Sirius B

Kai Bartnick, Detlev Koester, Rolf-Peter Kudritzki et al. (+3 more)

We present the equation of state for two classes of new ultralight particles, a scalar field coupling to electrons and a light $\mathbb{Z}_\mathcal{N}$ QCD axion field coupling to nucleons. Both are potential candidates for dark matter. Using the scalar modified equations of state, we calculate models for white dwarf stars and compare their radii and masses with observed mass-radius data. The comparison results in stringent constraints on the masses of the particles and the coupling parameters. For a wide range of particle masses and coupling parameters, constraints from the white dwarf equation of state surpass existing limits, outperforming also dedicated laboratory searches. The remarkable accuracy of modern white-dwarf mass-radius relation data, exemplified by Sirius B, now allows stringent tests of dense-matter physics and constraints on new particle scenarios.
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All authors Kai Bartnick, Detlev Koester, Rolf-Peter Kudritzki, Konstantin Springmann, Stefan Stelzl, Andreas Weiler

Cosmology and Nongalactic Astrophysics

Modeling gravitational wave sources in the MillenniumTNG simulations

Federico Marinacci, Marco Baldi, Giuliano Iorio et al. (+5 more)

(Edited) We introduce a flexible framework for building gravitational wave (GW) event catalogs in hydrodynamic simulations of galaxy formation. Our framework couples the state-of-the-art binary population synthesis code SEVN with Arepo-GW – a module fully integrated into the moving-mesh code Arepo – to assign merger events of binary compact objects to stellar particles in simulations by stochastically sampling merger tables generated with SEVN. Arepo-GW supports both on-the-fly operation, producing event catalogs during simulations, and post-processing, using snapshots from existing runs. The algorithm is fully parallel and can be adapted to outputs from other simulation codes. To demonstrate the capabilities of our new framework, we applied Arepo-GW in post-processing to simulations from the MillenniumTNG suite, including its flagship box. We investigate key properties of the resulting GW event catalog, built on SEVN predictions, focusing on comoving merger rates, formation efficiencies, delay-time distributions, and progenitor mass and metallicity distributions. We also examine how these properties vary with simulated volume. We find that GW progenitor rates closely track simulated star formation histories and are generally consistent with current observational constraints at low redshift, aside from a factor of $\sim 4.5$ excess in binary black hole mergers. Moreover, our binary black hole merger rates decline more slowly with redshift than current observational estimates for $z \lesssim 1$. Finally, the analysis of progenitor mass functions across different formation channels reveals only mild redshift evolution, while the binary black hole mass function displays features compatible with current observational determinations. These findings highlight the potential of our novel framework to enable detailed predictions for upcoming GW surveys within a full cosmological context.
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All authors Federico Marinacci, Marco Baldi, Giuliano Iorio, M. Celeste Artale, Michela Mapelli, Volker Springel, Sownak Bose, Lars Hernquist

Astrophysics of Galaxies

MUSEQuBES: Physical conditions, origins, and multi-element abundances of the circumgalactic medium of an isolated, star-forming dwarf galaxy at z=0.57

Sean D. Johnson, Nishant Mishra, Sowgat Muzahid et al. (+13 more)

In dwarf galaxy models, outflows expel metal-enriched interstellar medium (ISM) into the circumgalactic medium (CGM) to reproduce their observed low metallicities, but measurements of dwarf CGM properties are scarce. We present a study of the CGM of an isolated dwarf at $z=0.5723$ with a stellar mass of $\approx5\times10^7\rm\,M_{\odot}$ and star-formation rate ($\approx0.05\,\rm M_\odot\,yr^{-1}$) and ISM metallicity ($\rm [O/H]\approx-0.9$) consistent with the star-forming main sequence and mass-metallicity relation. A background quasar sightline with archival UV spectra probes the dwarf's CGM at a projected distance of 28 kpc, corresponding to approximately half of the estimated virial radius. The dwarf's CGM is detected in \ion{H}{1}, intermediate metal ions of \ion{C}{3}, \ion{O}{3}, \ion{O}{4}, and \textcolor{black}{\ion{S}{5}}, and kinematically broader, highly-ionized \ion{O}{6}, but is undetected in \ion{N}{4} and \ion{Ne}{8}. Photoionization modeling of the intermediate ions indicates a modest volume-filling factor ($\sim 6\%$ along the sightline or $\sim 2\%$ globally), and a mass of $\sim2\times10^8 {\rm\,M_\odot}$, $\sim4\times$ higher than the dwarf's stellar mass, but $\sim10\times$ less than the highly ionized CGM. The \ion{O}{6} kinematics are comparable to the dwarf's estimated virial velocity, suggesting it is likely associated with cool, photoionized, and volume-filling CGM, with bulk motion or turbulence dominating over thermal pressure. The metallicity inferred for the intermediate ions is $\rm [O/H]=-0.6$, but with low relative abundances of $\rm [C/O]=-0.6$ and \textcolor{black}{$\rm [N/O]<-1.0$}. The [N/O] is below levels expected of the dwarf's ISM, but consistent with core-collapse supernova ejecta, suggesting that supernova-enriched gas escaped the dwarf without mixing significantly with ISM enriched in nitrogen from evolved, low-mass stars.
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All authors Sean D. Johnson, Nishant Mishra, Sowgat Muzahid, Gwen C. Rudie, Fakhri S. Zahedy, Zhijie Qu, Claude-André Faucher-Giguère, Jonathan Stern, Jennifer I-Hsiu Li, Elise Fuller, Sebastiano Cantalupo, Hsiao-Wen Chen, Ahmad Kadri, Suyash Kumar, Zhuoqi Will Liu, Gregory Walth

Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc

Reducing systematic uncertainties in gravitational-wave population analyses by increasing the detection threshold

Noah E. Wolfe, Matthew Mould, Jack Heinzel et al. (+1 more)

From catalogs of gravitational-wave transients, the population-level properties of their sources and the formation channels of merging compact binaries can be constrained. However, astrophysical conclusions can be biased by misspecification or misestimation of the population likelihood. Despite detection thresholds on the false-alarm rate (FAR) or signal-to-noise ratio (SNR), the current catalog is likely contaminated by noise transients. Further, computing the population likelihood becomes less accurate as the catalog grows. Current methods to address these challenges often scale poorly with the number of events and potentially become infeasible for future catalogs. Here, we evaluate a simple remedy: increasing the significance threshold for including events in population analyses. To determine the efficacy of this approach, we analyze simulated catalogs of up to 1600 gravitational-wave signals from black-hole mergers using full Bayesian parameter estimation with current detector sensitivities. We show that the growth in statistical uncertainty about the black-hole population, as we analyze fewer events but with higher SNR, depends on the source parameters of interest. When the SNR threshold is raised from 11 to 15—reducing our catalog size by two–thirds—we find that statistical uncertainties on the mass distribution only grow by a few 10% and constraints on the spin distribution are essentially unchanged; meanwhile, uncertainties on the high-redshift cosmic merger rate more than double. Simultaneously, numerical uncertainty in the estimate of the population likelihood more than halves, allowing us to ensure unbiased inference without additional computational expense. Our results demonstrate that focusing on higher-significance events is an effective way to facilitate robust astrophysical inference with growing gravitational-wave catalogs.
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All authors Noah E. Wolfe, Matthew Mould, Jack Heinzel, Salvatore Vitale

Solar and Stellar Astrophysics

StarEmbed: Benchmarking Time Series Foundation Models on Astronomical Observations of Variable Stars

Weijian Li, Hong-Yu Chen, Nabeel Rehemtulla et al. (+6 more)

Current time series foundation model (TSFM) training corpora largely omit data with certain complexities like irregular temporal sampling. Astronomical time series of stellar fluxes (light curves) are available in immense quantities and exhibit irregular sampling, multiple variates, and heteroskedasticity. We introduce StarEmbed, the first public benchmark for light curves comprised of real observations of ~40,000 stars expert-labeled across seven classes and evaluations in clustering, classification, and out-of-distribution (OOD) source detection. We benchmark TSFMs with differing architecture and training strategies as well as domain-specific transformers. Our results demonstrate that the Chronos family, despite being pre-trained on regularly sampled non-astronomical data, yields state-of-the-art (SOTA) performance in light curve clustering and OOD detection. While no TSFM strictly surpasses the classification performance of the long-established domain baseline, they do demonstrate excellent generalization abilities. StarEmbed marks a step toward universal light curve embeddings and improved TSFM performance on challenging data.
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All authors Weijian Li, Hong-Yu Chen, Nabeel Rehemtulla, Ved G. Shah, Dongho Kim, Dennis Wu, Qinjie Lin, Adam A. Miller, Han Liu

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

Overlap-aware segmentation for topological reconstruction of obscured objects

J. Schueler, H. M. Araújo, S. N. Balashov et al. (+30 more)

The separation of overlapping objects presents a significant challenge in scientific imaging. While deep learning segmentation-regression algorithms can predict pixel-wise intensities, they typically treat all regions equally rather than prioritizing overlap regions where attribution is most ambiguous. Recent advances in instance segmentation show that weighting regions of pixel overlap in training can improve segmentation boundary predictions in regions of overlap, but this idea has not yet been extended to segmentation regression. We address this with Overlap-Aware Segmentation of ImageS (OASIS): a new segmentation-regression framework with a weighted loss function designed to prioritize regions of object-overlap during training, enabling extraction of pixel intensities and topological features from heavily obscured objects. We demonstrate OASIS in the context of the MIGDAL experiment, which aims to directly image the Migdal effect–a rare process where electron emission is induced by nuclear scattering–in a low-pressure optical time projection chamber. This setting poses an extreme test case, as the target for reconstruction is a faint electron recoil track which is often heavily-buried within the order(s)-of-magnitude brighter nuclear recoil track. Compared to unweighted segmentation regression, we demonstrate OASIS's novel overlap region-targeted loss function weight to be the single most important training weight for improving intensity and topological reconstructions of the low-energy electron tracks that tend to be most dominated by pixel overlap. Averaging over eight training campaigns, we further show the addition of overlap-targeted weights to improve median intensity reconstruction errors from -41.1% to -13.3% for these low-energy electrons. These performance gains demonstrate OASIS as a generalizable methodology for recovering obscured signals in overlap-dominated regions.
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All authors J. Schueler, H. M. Araújo, S. N. Balashov, J. E. Borg, C. Brew, F. M. Brunbauer, C. Cazzaniga, A. Cottle, D. Edgeman, C. D. Frost, F. Garcia, D. Hunt, M. Kastriotou, P. Knights, H. Kraus, A. Lindote, M. Lisowska, D. Loomba, E. Lopez Asamar, P. A. Majewski, T. Marley, C. McCabe, L. Millins, R. Nandakumar, T. Neep, F. Neves, K. Nikolopoulos, E. Oliveri, A. Roy, T. J. Sumner, E. Tilly, W. Thompson, M. A. Vogiatzi

High Energy Astrophysical Phenomena

Rare Transients in Nearby Galaxies Explain Ultra-high-energy Cosmic Rays

Imre Bartos and Marek Kowalski

The origin of ultra-high-energy cosmic rays remains one of the central open questions in astroparticle physics. Recent measurements reveal anisotropies in arrival directions, a rigidity-dependent composition dominated by intermediate-mass nuclei, and striking hemispheric differences in the energy spectra. Here we show that rare transients in nearby galaxies can naturally account for these features. In our fiducial neutron-star merger model, the cosmic ray flux above $25$~EeV is dominated by ten nearby galaxies within $8\,$Mpc. This accounts for the observed hotspots: seven of the ten brightest galaxies coincide with reported excess regions, a chance probability of $p\sim0.001$. Nearby transients can also explain the spectral excess of TA over Auger and modify the rigidity–aligned succession of isotopes.
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