http://rss.arxiv.org/rss/astro-ph.CO astro-ph.CO updates on the arXiv.org e-print archive. http://www.rssboard.org/rss-specification en-us Mon, 27 Apr 2026 04:00:11 +0000 rss-help@arxiv.org Mon, 27 Apr 2026 00:00:00 -0400 Sunday Saturday https://arxiv.org/abs/2604.22105 arXiv:2604.22105v1 Announce Type: new Abstract: Fast Radio Bursts (FRBs) probe baryons permeating the cosmic web through their dispersion measures (DMs), which encode the integrated electron density along cosmological sightlines. Using 3,455 unique FRB sources from CHIME/FRB with $\sim 15$ arcmin localizations, we present an anthology of DM correlations with tracers of large-scale structure and baryonic matter at redshifts $z \lesssim 1.5$. We measure statistically significant correlations at $2.6-5\sigma$ with ten probes, including galaxies ($2.8\sigma$), weak gravitational lensing ($2.6\sigma$), cosmic infrared background ($4.0\sigma$), cosmic microwave background (CMB) lensing ($3.3\sigma$), thermal Sunyaev Zel'dovich (tSZ) effect ($3.8\sigma$), X-ray emission tracing galaxy clusters ($5.0\sigma$) and superclusters ($3.3\sigma$), soft X-ray background (SXRB, $4.1\sigma$), and radio continuum emission ($3.2\sigma$). These measurements reveal a consistent picture in which FRB sightlines intersecting overdense environments carry systematically larger DMs. Correlations with hot-gas tracers provide additional leverage on the strength of feedback, as they are strongly weighted towards the dense, bound gas. The measured amplitude of tSZ$\times$DM and SXRB$\times$DM correlations are consistent with theoretical predictions of baryon distribution from a DM-$z$ relation-inferred model with moderate feedback at $\sim 0.5\sigma$ level. Weaker feedback scenario is ruled out at $\sim 3.5\sigma$ by the SXRB$\times$DM correlation. Taken together, these measurements constitute a quantitative multi-tracer foundation for a new era in which FRBs from next generation facilities, such as BURSTT, CHORD, DSA, and SKA, in harmony with other probes, will map the baryon content of the full extent of the cosmic web. oai:arXiv.org:2604.22105v1 astro-ph.CO astro-ph.GA astro-ph.HE Mon, 27 Apr 2026 00:00:00 -0400 new http://creativecommons.org/licenses/by/4.0/ Kritti Sharma, Elisabeth Krause, Vikram Ravi, Dhayaa Anbajagane, Liam Connor, W. L. Kimmy Wu, Simone Ferraro, Sebastian Grandis, David Alonso, Yi-Kuan Chiang, Casey J. Law, Pranjal R. S., Samuel McCarty, Shivam Pandey https://arxiv.org/abs/2604.22213 arXiv:2604.22213v1 Announce Type: new Abstract: We apply caustic skeleton theory to the Manticore-Local simulations, which are Bayesian constrained reconstructions of the Local Universe from the 2M++ galaxy catalogue, and extract the three-dimensional multi-scale caustic skeleton of two canonical weblike structures in our Local Universe, namely the Coma Cluster and the Pisces-Perseus ridge as they represent the most prominent cluster node and filamentary artery in the nearby Universe. We show that the Caustic Skeleton network of caustic singularities accurately reproduces the observed large-scale organisation of galaxies in redshift space for one of the Manticore realisations. The hierarchy of caustic features allows us to establish a multi-scale classification of the large-scale environment in which observed 2M++ galaxies reside. One of the most interesting aspects of the theory is that it predicts two topologically distinct classes of filaments (A_4 swallowtail and D_4 umbilic caustics) that form through fundamentally different folding histories yet appear morphologically similar enough, on the surface, to be overlooked by conventional structure identifiers. We find that the influence of D_4 filaments only becomes increasingly relevant towards smaller scales, and the Pisces-Perseus Supercluster in particular is revealed to be a distinctly D_4-dominated structure compared to the extended Stickman structure around the Coma Cluster. In other words, caustic skeleton theory enables a novel topological characterisation of one of the most studied filamentary complexes in the nearby Universe. [Shortened] oai:arXiv.org:2604.22213v1 astro-ph.CO gr-qc Mon, 27 Apr 2026 00:00:00 -0400 new http://creativecommons.org/licenses/by/4.0/ Amelie Read, Job Feldbrugge, Celine Boehm, Rien van de Weygaert, Benjamin Hertzsch https://arxiv.org/abs/2604.22372 arXiv:2604.22372v1 Announce Type: new Abstract: In modern cosmology, the rapid growth of high-precision observational data, along with significant theoretical advances, has intensified the challenge of identifying a robust, model-independent framework to probe the expansion history of the Universe. In this work, we propose a novel artificial neural network (ANN)-based framework for the non-parametric reconstruction of the late-time cosmic expansion. The framework is trained and validated through a three-stage screening pipeline prior to its application to real observational data. As a demonstration of its effectiveness, we reconstruct the Hubble parameter $H(z)$ using the latest cosmic chronometer measurements. Our results show that the reconstructed expansion history aligns with the predictions of the $\Lambda$CDM model within observational uncertainties, thereby supporting the robustness and reliability of the proposed approach. oai:arXiv.org:2604.22372v1 astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 new http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Yuki Hashimoto, Kazuharu Bamba, Sanjay Mandal https://arxiv.org/abs/2604.22634 arXiv:2604.22634v1 Announce Type: new Abstract: Primordial black holes (PBHs) provide a compelling interpretation for the binary black holes (BBHs) observed by ground-based gravitational-wave (GW) detectors, especially for those BBHs in the theoretical mass gap. In the early Universe, the scalar perturbations required to produce such PBHs inevitably generate scalar-induced GWs (SIGWs). These SIGWs peak in the sub-nanohertz band, and manifest secularly as measurable jerk-like drifts in the second derivative of pulsar spin periods. In this Letter, we perform the first search for SIGWs using pulsar parameter drifts, and place a 95\% confidence-level upper limit on the PBH abundance of $f_{\mathrm{PBH}} < 10^{-10}$ over the mass range $[3 \times 10^{-1}, 4 \times 10^{4}] M_{\odot}$. Our results strongly disfavor a PBH origin for the BBHs currently detected by the LIGO-Virgo-KAGRA (LVK) Collaborations. oai:arXiv.org:2604.22634v1 astro-ph.CO astro-ph.HE Mon, 27 Apr 2026 00:00:00 -0400 new http://creativecommons.org/licenses/by/4.0/ Yan-Chen Bi, Yu-Mei Wu, Qing-Guo Huang https://arxiv.org/abs/2604.22703 arXiv:2604.22703v1 Announce Type: new Abstract: Magnetic fields with field strengths between $10^{-17}\,$G and a few Nanogauss are expected to exist today in the intergalactic medium (IGM). Their origin is unknown, but may be of primordial nature, in which case they would have influenced the thermal and ionization history of the IGM as well as the growth of small-scale matter perturbations. In this work, we revisit constraints on Primordial Magnetic fields (PMFs) by consistently accounting for their energy losses through ambipolar diffusion and decaying turbulences from recombination through the epoch of reionization, which progressively reduces the magnetic field strength over time. We implement these effects in ${\tt HyRec}$ and ${\tt exo21cmFAST}$ to model the interplay between PMFs and astrophysical processes up to reionization. Using a neural-network emulator (${\tt NNERO}$), we perform a MCMC analysis that combines late-time probes of the reionization history and galaxy UV luminosity functions. We find that including PMF energy losses significantly relaxes previous bounds, as the reduced field strength suppresses their imprint on observables. Employing a Fisher matrix analysis, we estimate the sensitivity of the 21$\,$cm signal experiment HERA to the PMFs' imprint on intergalactic medium perturbations and show that 21$\,$cm cosmology could significantly improve on current bounds. Our results highlight the importance of modeling PMF evolution self-consistently with the IGM evolution to extract current bounds and future sensitivities. oai:arXiv.org:2604.22703v1 astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 new http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Ga\'etan Facchinetti, Alexander Korochkin, Laura Lopez-Honorez, Justus Schwagereit https://arxiv.org/abs/2604.22731 arXiv:2604.22731v1 Announce Type: new Abstract: We investigate how multi-band gravitational wave (GW) observations can constrain the uncertainties in the Hubble parameter ($H_0$) using primordial black holes (PBHs) as possible sources. Our framework combines scalar-induced and merger-induced GWs from PBHs, and forecasts on a combination of two future detectors Square Kilometre Array (SKA) and the Einstein Telescope (ET), enabling a multi-band analysis. We perform a statistical forecast of the PBH parameters, $M_{\rm PBH}$ and $f_{\rm PBH}$, using signal-to-noise ratio (SNR) estimates and Fisher matrix analysis. Imposing $\mathrm{SNR} \geq 1$, we identify the accessible PBH parameter space and propagate these uncertainties to estimate the corresponding uncertainties in $H_0$. For $\delta \theta_i/\theta_i \leq 0.1$, with $\theta_i \equiv M_{\rm PBH}(f_{\rm PBH})$, we find $\delta H_0 \lesssim 2~{\rm km\,s^{-1}\,Mpc^{-1}}$ in a conservative approach, improving to $\delta H_0 \lesssim \mathcal{O}(0.1)~{\rm km\,s^{-1}\,Mpc^{-1}}$ for $\delta \theta_i/\theta_i \leq 0.01$ for an optimistic approach of precision measurement. The results are further found to be largely insensitive to the fiducial choice of the $H_0$, with only moderate dependence on the PBH collapse efficiency. These findings demonstrate that multi-band GW observations provide an independent and complementary approach to constraining the uncertainties in $H_0$, with the potential to provide a novel, cosmic distance ladder-independent measure of the Hubble parameter. oai:arXiv.org:2604.22731v1 astro-ph.CO gr-qc hep-ph Mon, 27 Apr 2026 00:00:00 -0400 new http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Setabuddin, Md Riajul Haque, Ratna Koley, Supratik Pal https://arxiv.org/abs/2604.22738 arXiv:2604.22738v1 Announce Type: new Abstract: Although the spin parameter of dark matter halos is well known to follow a log-normal distribution at fixed epoch, its quantitative redshift evolution - encompassing both the mean and the dispersion - remains only partially explored. Prior studies either lack the mass resolution required to establish reliable evolutionary trends or do not provide analytical relations that enable forward modelling. Using a suite of LCDM N-body simulations with controlled resolution across the redshift range 0 < z < 5, we characterise the evolution of the mean and dispersion of the Peebles (lambda) and Bullock (lambda') definitions of spin. We find a mild but statistically robust linear evolution for ln(lambda) and a non-monotonic trend with a turnover at z ~ 1 - 2 for ln lambda', which we verify are unaffected by mass resolution of choice of halo definition. We provide closed-form fitting functions for these trends that allow modellers to draw physically motivated spin values at any redshift within our range of validity. This is a practical, redshift-dependent alternative to the common assumption of a constant spin distribution, and provides a useful input to semi-empirical and semi-analytic models of galaxy formation. oai:arXiv.org:2604.22738v1 astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 new http://creativecommons.org/licenses/by/4.0/ Tomas Riera, Alexander Knebe, Chris Power, Robert Adriel Mostoghiu Paun, Adam Ussing https://arxiv.org/abs/2604.21954 arXiv:2604.21954v1 Announce Type: cross Abstract: The lack of discovery of particle dark matter candidates within the favored mass-window range brings in the motivation for the study of new options brought by Planck-mass dark matter models. Extended supergravity theories predict the existence of non-relativistic gravitinos that could at least in part contribute to the missing mass-energy density of the Universe. The feasibility study for the discovery with DEAP-3600 experiment of Planck mass charged gravitino dark matter is presented. Additionally the expected signal events topology within the detector is discussed. oai:arXiv.org:2604.21954v1 hep-ph astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 cross http://creativecommons.org/licenses/by-nc-nd/4.0/ Micha{\l} Olszewski (for the DEAP Collaboration) https://arxiv.org/abs/2604.21976 arXiv:2604.21976v1 Announce Type: cross Abstract: We present a framework in which three classes of dark matter number-changing processes can affect both the relic abundance via thermal freeze-out in the early universe and the generation of indirect cosmic-ray signals today. These processes are: (i) direct annihilations into Standard Model final states; (ii) annihilations into metastable on-shell mediators that subsequently decay into Standard Model particles; (iii) semi-annihilation processes featuring a dark matter particle in the final state, accompanied by a metastable mediator. A central element of our analysis is the systematic inclusion of semi-annihilation alongside the more commonly considered channels. This setup is largely model-independent, as we only assume the presence of one or more of these processes with unsuppressed $s$-wave contributions. We analyze representative benchmarks for the dominant decay modes of the mediator and show how the resulting injection spectra for $\gamma$ rays, neutrinos, and cosmic-ray antimatter vary with the relative importance of the three classes of processes. As an application, we evaluate the observable $\gamma$-ray fluxes from dwarf spheroidal galaxies in the GeV-TeV window. Finally, we provide explicit model realizations in which multiple processes coexist, and discuss how their interplay shapes indirect detection signatures. Our results provide a consistent connection between early-universe dynamics and present-day observables, revealing distinctive features that arise when multiple dark matter processes contribute simultaneously. oai:arXiv.org:2604.21976v1 hep-ph astro-ph.CO astro-ph.HE Mon, 27 Apr 2026 00:00:00 -0400 cross http://creativecommons.org/licenses/by/4.0/ Francesco D'Eramo, Silvia Manconi, Tommaso Sassi https://arxiv.org/abs/2604.22048 arXiv:2604.22048v1 Announce Type: cross Abstract: Forthcoming Stage-IV dark energy optical surveys, such as LSST, have the ambitious goal of measuring cosmological parameters at sub-percent precision. Realizing their full scientific potential requires very precise measurement of the cosmic shear signal and control of corresponding systematics. In this work, we present a modern implementation of the Bayesian shear inference framework in Schneider et al. (2014), in the case that the PSF and sky background are known. This framework automatically propagates the pixel-noise measurement error from each galaxy into the final shear estimate, and thus requires no external calibration to handle noise bias. As a first application of this new implementation, we infer the cosmic shear posterior from simulated images consisting of isolated exponential galaxies with LSST-like levels of shape and pixel noise. In this simplified scenario, we estimate the absolute multiplicative bias $|m|$ of our approach to be below $0.9 \times 10^{-3} \, [3\sigma]$ when the intrinsic distribution of galaxy properties is known, and below $1.3 \times 10^{-3}\, [3\sigma]$ when these distributions are inferred alongside shear. Both results are within the LSST requirement of $|m| < 2 \times10^{-3}$. Additionally, we make progress towards the algorithm's computational feasibility in the context of modern wide-field surveys, where billions of galaxies must be processed, by leveraging differentiable forward models of galaxies, gradient-based samplers, and GPUs. Our final galaxy-fitting MCMC produces $300$ effective samples of galaxy properties in $0.45$ seconds per galaxy using a single A100 GPU. In the future, we seek to generalize our algorithm to handle selection, detection, and model shear biases so it can be applied to real survey data. oai:arXiv.org:2604.22048v1 astro-ph.IM astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 cross http://creativecommons.org/licenses/by/4.0/ Ismael Mendoza, Axel Guinot, Matthew R. Becker, Camille Avestruz, Jean-Eric Campagne, Natalia Porqueres, Michael Schneider, Eleni Tsaprazi https://arxiv.org/abs/2604.22441 arXiv:2604.22441v1 Announce Type: cross Abstract: Gravitational-wave (GW) observations of compact binary coalescences (CBCs) are traditionally interpreted under the assumption that the binary evolves in isolation. However, in realistic astrophysical environments, brief three-body encounters may perturb the binary's orbital evolution and imprint deviations on the emitted GWs. We develop a physically motivated model for such interactions, retaining Newtonian three-body dynamics supplemented by leading-order ($2.5$PN) radiation-reaction within the binary. We show that such encounters produce a distinctive morphology of dephasing and amplitude modulation in GWs. We search for this kind of distortion from the LIGO--Virgo--KAGRA (LVK) GW catalog GWTC-4 on three events: GW170817, GW190814, and GW230627\_015337, chosen based on high SNR and in-band duration $\gtrsim 10~\mathrm{s}$. We find no statistically significant deviation in the data, which translates into constraints on the absence of any intermediate-mass black hole in the mass range above $\sim 10^2$ M$_\odot$ in the vicinity of these binaries of radius approximately $10^{-1}~\mathrm{AU}$. This arises from robust exclusions arising from fly-by interactions that would dynamically disrupt the binary and are directly ruled out independent of waveform modelling, placing the first upper bound on intermediate-mass black holes near these GW events. In future, with the availability of long-duration GW signals, this new avenue can probe encounters of the binary GW sources with compact objects of lighter masses at distances farther away than 1 AU and hence opens a new window to probe the population of individual compact objects of both astrophysical and primordial origin in astrophysical systems of dense environments ranging from galactic centers to dense globular clusters. oai:arXiv.org:2604.22441v1 astro-ph.HE astro-ph.CO gr-qc Mon, 27 Apr 2026 00:00:00 -0400 cross http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Devesh Giri, Suvodip Mukherjee https://arxiv.org/abs/2604.22460 arXiv:2604.22460v1 Announce Type: cross Abstract: Studying the interstellar medium (ISM) in merging high-redshift galaxies is crucial for understanding early galaxy assembly, star formation, and black hole growth, predicted by hierarchical $\Lambda$CDM models. Deep imaging and spatially resolved spectroscopy with JWST enable unprecedented insight into these processes, even for galaxies in the Epoch of Reionization. We present NIRSpec and MIRI integral field spectroscopy and MIRI imaging of the merging galaxy Gz9p3 at z=9.3 of the UV and optical rest-frame showing a clumpy morphology in the continuum as well as line emission covering the entire galaxy over a range of 5 kpc from the central clump to the tail region. We analyze the integrated spectrum as well as different apertures in the galaxy allowing a spatially resolved characterization of the ionized ISM of this galaxy. We compare our measurements with archival NIRCam imaging and ALMA data. We measure a total star formation rate of 13.4 $\pm$ 1.8 Msun yr$^{-1}$, a metallicity of 12+log(O/H) = 7.84 $\pm$ 0.05 and $\xi_{ion}$= 25.4 $\pm$ 0.1 erg$^{-1}$ Hz and a burstiness parameter of 0.9 $\pm$ 0.1 for the integrated spectrum. We find large spatial differences in these parameters between the central clump and the tail region. The optical [OIII] emission peaks in the main galaxy, the far-infrared [OIII] emission peaks towards the tail, indicating different physical conditions in the ISM of the tail and main galaxy. This study presents the spatially resolved ISM analyses of a galaxy at z>9, revealing nebular line emission and strong spatial variations in star formation, metallicity, physical conditions, and ionizing efficiency. The results indicate a recent, metal-poor starburst in a tail alongside a more evolved, enriched central clump with evidence for extreme excitation. This demonstrates the power of spatially resolved JWST spectroscopy of galaxies in the Epoch of Reionization. oai:arXiv.org:2604.22460v1 astro-ph.GA astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 cross http://creativecommons.org/licenses/by/4.0/ Arjan Bik, Javier \'Alvarez-M\'arquez, Alejandro Crespo G\'omez, Luis Colina, Pablo G. P\'erez-Gonz\'alez, G\"oran \"Ostlin, Carmen Blanco Prieto, Jens Melinder, Danial Langeroodi, Gillian Wright, Hiddo S. B. Algera, Leindert A. Boogaard, Karina Caputi, Steven Gillman, Thomas Greve, Jens Hjorth, Edoardo Iani, Sarah Kendrew, Alvaro Labiano Ortega, Michele Perna, Carlota Prieto Jimenez, John Pye, Pierluigi Rinaldi, Paul van der Werf, Fabian Walter, Florian Pei{\ss}ker, Andreas Eckart, Thomas Henning, Manuel G\"udel https://arxiv.org/abs/2604.22462 arXiv:2604.22462v1 Announce Type: cross Abstract: The multi-messenger exploration of dark matter and physics beyond the Standard Model has emerged as a central direction in modern astro-particle physics, particularly following the discovery of gravitational waves. In this work, we present a comprehensive review and forward-looking perspective on machine-learning-enhanced multi-messenger approaches, combining information from gravitational waves, cosmic rays, gamma rays, neutrinos, and collider experiments. We summarize the current state of the field, discuss recent methodological developments, and outline a coherent research program aimed at integrating heterogeneous datasets within a unified inference framework. Our collaboration proposes here a plan for forthcoming analyses aiming at extracting information on the properties and interactions of dark matter, and finally on its genesis, combining multi-messenger astronomy techniques and inputs from laboratory physics. The main objectives planned in this line of research comprise: i) the multi-messenger analysis of new physics in cosmology, including mainly, but not only, several different models of dark matter; ii) the phenomenology of new physics signatures in ground-based cosmic rays experiments, with cross-correlation to the corresponding physical, astrophysical and cosmological observations; iii) the development of machine learning methods for data analysis in ground-based cosmic rays experiments, in light of the new physics signatures. We note that several groups have explored the use of multi-messenger observations, including gravitational waves, to probe alternative dark matter candidates. The present work builds on these developments by focusing on the role of machine learning in integrating heterogeneous datasets. We foresee that such a cross-fertilizing approach will represent the right path to extract information about the main questions left in fundamental physics. oai:arXiv.org:2604.22462v1 hep-ph astro-ph.CO gr-qc Mon, 27 Apr 2026 00:00:00 -0400 cross http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Andrea Addazi, Konstantin Belotsky, Vitaly Beylin, Timur Bikbaev, Deen Chen, Filippo Fabrocini, Stefano Giagu, Krid Jinklub, Artem Kharakhashyan, Maxim Khlopov, Vladimir Korchagin, Maxim Krasnov, Atharv Mahajan, Antonino Marciano, Andrey Mayorov, Antonio Morais, Roman Pasechnik, Jackson Levi Said, Danila Sopin, Viktor Stasenko, Oem Trivedi https://arxiv.org/abs/2604.22665 arXiv:2604.22665v1 Announce Type: cross Abstract: Dark matter and baryons acquire a relative velocity after decoupling in the early Universe. Baryons are gravitationally unstable only above their Jeans scale, while cold dark matter (CDM) is unstable on all scales. We show for the first time that their relative drift triggers a resonant gravitational instability that drives sound waves in baryons. When the projected DM drift is subsonic, the stable oscillatory branch of baryons resonates with the Doppler-shifted DM mode, producing exponentially growing perturbations whose growth rates exceed the intrinsic CDM growth rate. The instability peaks below the baryon Jeans scale and, in baryon-dominated environments, opens a window of complete stability between the Jeans scale and the resonance. Supersonic drift suppresses growth, as previously noted. The resonant coupling also transfers momentum between the species, creating a non-viscous, collisionless drag. We derive an accurate analytical approximation for the growth rate at resonance and show that the associated timescales range from years to tens of millions of years across diverse environments -- planets, protoplanetary disks, stars, molecular clouds, galaxies, and galaxy clusters -- typically much shorter than their ages. In an expanding FLRW universe, the instability enhances baryon density perturbations at different redshifts for appropriately oriented modes while suppressing the growth of those aligned with the DM stream. The universe thus sings across all scales, and this resonant mechanism provides the means to listen: it offers a novel probe of dark matter through its seismic imprint on astrophysical objects and may explain long-standing puzzles such as the persistence of spiral arms and the heating of the intracluster medium in galaxy clusters. oai:arXiv.org:2604.22665v1 astro-ph.GA astro-ph.CO astro-ph.EP astro-ph.IM astro-ph.SR Mon, 27 Apr 2026 00:00:00 -0400 cross http://creativecommons.org/licenses/by/4.0/ Mohamad Shalaby, Avery Broderick https://arxiv.org/abs/2604.22702 arXiv:2604.22702v1 Announce Type: cross Abstract: Highly magnified stars at cosmological distances ($z \gtrsim 0.7$) become detectable thanks to microlensing by intracluster stars near the critical curves of galaxy clusters. Multi-epoch photometric campaigns targeting caustic crossing galaxies magnified by massive galaxy clusters enable the detection of these objects as transient events. Such stars provide unique opportunities to study stellar populations at early cosmic times, probe the nature of dark matter, reveal small-scale structure in the cluster, and improve lens models. To date, only a few dozen high-redshift stars have been reported, with a single lensed galaxy, the Dragon, holding the current record of 44 detections. These numbers, however, remain insufficient to exploit their full potential. In this paper, owing to the inclusion of new observations, we report the identification of more than 100 magnified stellar events in the Dragon, behind the massive galaxy cluster Abell 370. The relatively low redshift of the Dragon ($z\approx0.725$) facilitates the detection of its most massive stars. Using imaging data from three different cycles (2022--2024) with the James Webb Space Telescope, we apply a time-domain technique to identify flux variations associated with caustic-crossing events. From the spatial distribution of stellar events we constrain the high-end slope of the stellar luminosity function, finding $\beta=2.18^{+0.20}_{-0.30}$. Alternatively, assuming a fixed slope, we constrain the microlens surface mass density. In addition, we examine the parity asymmetry of the detected caustic-crossing events, a proposed probe of wave dark matter, and find that it remains present. We also use the events to trace the regions of highest magnification, offering an alternative way to map the system critical curves. oai:arXiv.org:2604.22702v1 astro-ph.GA astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 cross http://creativecommons.org/licenses/by/4.0/ J. M. Palencia, Fengwu Sun, J. M. Diego, Yoshinobu Fudamoto, Anton M. Koekemoer, Christopher N. A. Willmer, Eduardo Iani, Xiaojing Lin, Justin D. R. Pierel, Alfred Amruth, Tom Broadhurst, W. Chen, Liang Dai, Daniel Espada, Alexei V. Filippenko, Seiji Fujimoto, Mingyu Li, Sung Kei Li, Ashish Kumar Meena, Jordi Miralda-Escud\'e, P. Morilla, Mitchell F. Struble, Hayley Williams, Ruwen Zhou, Adi Zitrin https://arxiv.org/abs/2604.22725 arXiv:2604.22725v1 Announce Type: cross Abstract: We investigate the scalar sector of linear cosmological perturbations in quadratic gravity. Working in the Einstein frame, we derive the equations of motion in a gauge-independent manner and express them in terms of three sets of gauge-invariant variables. This approach allows us to distinguish genuine physical effects from gauge artefacts, which is particularly relevant for assessing the stability of perturbations in this theory. In the superhorizon limit, we obtain the leading-order behaviour of the relevant gauge-invariant variables and analyse the perturbations in several commonly used gauges. We find that the Newtonian gauge exhibits an apparent instability, characterised by the exponential growth of the metric perturbations. However, this growth is non-generic and gauge-dependent; in the other gauges analysed in this work, the perturbations remain well behaved within the perturbative regime. Physical observables can thus be consistently computed, and the apparent instability is identified as a gauge artefact rather than a pathology of the theory. Our analysis also demonstrates how the evolution behaviour of a gauge-invariant variable changes under the frame transformation and clarifies the relation between results obtained in the Jordan and Einstein frames. oai:arXiv.org:2604.22725v1 gr-qc astro-ph.CO hep-ph hep-th Mon, 27 Apr 2026 00:00:00 -0400 cross http://creativecommons.org/licenses/by/4.0/ Adrian Palomares, Ying-li Zhang, Jinsu Kim https://arxiv.org/abs/2509.07453 arXiv:2509.07453v2 Announce Type: replace Abstract: We compute the 3D cross-correlation between the absorption of the $z\sim 2.3$ Lyman-alpha forest measured by the COSMOS Lyman-Alpha Mapping And Tomography Observations (CLAMATO) survey, and 1642 foreground galaxies with spectroscopic redshifts from several different surveys, including 3D-HST, CLAMATO, zCOSMOS-Deep, MOSDEF, and VUDS. For each survey, we compare the measured cross-correlation with models incorporating the galaxy linear bias as well as observed redshift dispersion and systematic redshift offset. The derived redshift dispersion and offsets are generally consistent with those expected from, e.g., spectroscopic redshifts measured with UV absorption lines or NIR emission lines observed with specific instruments, but we find hints of `fingers-of-god' caused by overdensities in the field. We combine our foreground galaxy sample, and split them into 3 bins of robustly-estimated stellar mass in order to study the stellar mass-halo mass relationship. For sub-samples with median stellar masses of $\log_{10}(M_* / M_\odot) = [9.23,9.71,10.21]$, we find galaxy biases of $b_g\approx [2.9, 3.3,4.7]$, respectively. A comparison with mock measurements from the Bolshoi-Planck $N$-body simulation yields corresponding halo masses of $\log_{10}(M_* / M_\odot) \approx [10.3,11.6,12.1]$ for these stellar mass bins. At the low mass end, our results suggest enhanced star formation histories in mild tension with predictions from previous angular correlation and abundance matching-based observations, and the IllustrisTNG simulation. oai:arXiv.org:2509.07453v2 astro-ph.CO astro-ph.GA Mon, 27 Apr 2026 00:00:00 -0400 replace http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Benjamin Zhang, Khee-Gan Lee, Andrei Cuceu, Andreu Font-Ribera, Rieko Momose https://arxiv.org/abs/2510.04179 arXiv:2510.04179v3 Announce Type: replace Abstract: We perform a consistency check of DESI DR2 BAO constraints ($D_M/r_d, D_H/r_d)$ by reconstructing the same quantities from DES supernovae (SNe) in bins with the same effective redshift $z_{\textrm{eff}} \in \{ 0.510, 0.706, 0.934 \}$ and a Planck $r_d$ prior. Through mock analysis we show that $D_M(z_{\rm eff})$ and $D_{H}(z_{\rm eff})$ can be locally reconstructed model agnostically from $\Lambda$CDM and extended models, but only if one employs frequentist methods; purely Bayesian reconstructions from Markov Chain Monte Carlo (MCMC) exhibit bias. We find that the ratio of the three $D_M/r_d$ values at different $z_{\textrm{eff}}$ are consistent with a horizontal, thus confirming that the distance duality relation holds up to calibration. However, the $D_H/r_d$ ratio shows a decreasing trend driven by the $z_{\textrm{eff}} = 0.934$ bin, the significance of which varies from $2.5 \sigma$ with Bayesian methods down to $1.4 \sigma$ with frequentist methods. We show that replacing DES with DES-Dovekie SNe reduces the significance to $1.7 \sigma$ and $1.2 \sigma$ in Bayesian and frequentist approaches, respectively. We conclude that distances reconstructed from SNe show good agreement with DESI BAO distances across the redshifts studied. We also note that $D_M(z_{\rm eff} = 0.510)/r_d$ reconstructed from SNe favours DESI BAO over transversal BAO against a backdrop of a $3.7 \sigma$ disagreement. oai:arXiv.org:2510.04179v3 astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 replace http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Mauricio Lopez-Hernandez, Eoin \'O Colg\'ain, Saeed Pourojaghi, M. M. Sheikh-Jabbari https://arxiv.org/abs/2510.09463 arXiv:2510.09463v2 Announce Type: replace Abstract: Fast radio bursts (FRBs) can provide a measure of the Hubble constant $H_0$ that is independent of the constraints set by the cosmic microwave background (CMB) and the type Ia supernovae (SNIa), thereby arbitrating the Hubble tension. In the literature, the methodology proposed by Macquart et al. has been widely used, in which the contributions to the dispersion measure (DM) from the intergalactic medium (IGM, $\rm DM_{IGM}$) and the host galaxy ($\rm DM_{host}$) are described by probability distribution functions. Within the Macquart et al. methodology, it has been found that the parameter $F$, which quantifies the strength of the baryon feedback in galaxies, must be bound by an artificially narrow prior to result in a Hubble constant $H_0$ that is consistent with the ones derived from the CMB and SNIa studies. A recent study using ${\cal O}(100)$ localized FRBs found that this also causes the fraction of baryon mass in the IGM, $f_{\rm IGM}$, to approach its upper bound $1$. In the present work, using 125 localized FRBs, we find an unusually low $H_0$ when using a model with a loose prior on $F$. This model is in fact strongly preferred to the model with the narrow prior when considering the Bayesian evidence and the Akaike and Bayesian information criteria. Instead of modifying $\sigma_\Delta=Fz^{-0.5}$ in the distribution of $\rm DM_{IGM}$, we explore an alternative method of resolving the tension, by generalizing the distribution of $\rm DM_{host}$ with varying location and scale parameters $\ell$ and $e^\mu$, respectively. We find that $H_0$ can be well consistent with the ones of Planck 2018 and SH0ES for all the models considered in this work, while these generalized models are all strongly preferred to the model with a narrow prior on $F$. Our findings indicate that more realistic distributions of $\rm DM_{host}$ could be the key to using FRBs as an independent measure of $H_0$. oai:arXiv.org:2510.09463v2 astro-ph.CO astro-ph.HE astro-ph.IM gr-qc Mon, 27 Apr 2026 00:00:00 -0400 replace http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Jing-Yi Jia, Da-Chun Qiang, Lin-Yu Li, Hao Wei https://arxiv.org/abs/2510.11555 arXiv:2510.11555v2 Announce Type: replace Abstract: The $O_k$ null test can not only assess whether the cosmic curvature is zero, therefore if true reducing degeneracies between cosmic curvature and other cosmological parameters, but also provide a model-independent check of compatibility between different data sets. However, traditional implementations often require absolute distance data from Type Ia supernovae (SNe Ia) or baryon acoustic oscillation (BAO) measurements, limiting their applicability because such absolute distance data usually are not accessible. The BAO Alcock Paczynski (AP) parameter $F_{AP}$ is a measurement of a distance ratio, making the Dark Energy Spectroscopic Instrument (DESI) AP measurements particularly well suited for the $O_k$ null test because no absolute distance measurements are required. We propose a novel null test of cosmic curvature tailored to DESI BAO data that combines $F_{AP}$ with ratios such as $D_V'/D_V$ or $D_M'/D_M$. Crucially, this construction eliminates the need for absolute distance measurements. We further develop multi-task Gaussian processes to perform the null test. This approach can also be applied to a joint DESI BAO and SNe Ia dataset, and we find that DESI BAO and SNe Ia data are compatible. Although there is $\sim 2\sigma$ evidence of nonzero curvature at low redshift $z\lesssim 0.5$, this result is not conclusive largely due to the lack of observational data in the corresponding redshift range. oai:arXiv.org:2510.11555v2 astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 replace http://creativecommons.org/licenses/by/4.0/ 10.1088/0256-307X/43/5/051101 Chin.Phys.Lett. 43 (2026) 051101 Yungui Gong, Qing Gao, Xuchen Lu, Zhu Yi https://arxiv.org/abs/2601.02458 arXiv:2601.02458v3 Announce Type: replace Abstract: We study a minimal type-I seesaw framework in which a first-order phase transition (FOPT), driven by a singlet scalar, produces right-handed neutrinos (RHNs) through bubble collisions, realizing a cosmic-scale collider that probes ultra-high energy scales. The resulting RHN distribution sources novel low-frequency gravitational-waves (GWs) in addition to the standard bubble-collision contribution. A stable lightest RHN can account for the observed dark matter (DM) relic abundance for masses as low as $M_{1} \equiv m_{\rm DM} \gtrsim 10^{6}\,\mathrm{GeV}$, with the associated novel GW signal accessible in LISA, ET and upcoming LVK detectors. If the RHNs are unstable, their CP-violating decays generate the observed baryon asymmetry via leptogenesis for $M_{1} \gtrsim 10^{11}\,\mathrm{GeV}$ and phase transition temperatures $T_* \gtrsim 10^{6}\,\mathrm{GeV}$, for which the novel GW spectrum is detectable in ET, BBO and upcoming LVK. If RHN decays also populate a dark-sector fermion with mass $m_{\chi} \in [10^{-4},10^{4}],\mathrm{GeV}$, successful co-genesis of baryons and asymmetric dark matter occurs for $T_* \gtrsim 10^{7}\,\mathrm{GeV}$ and $M_{1} \gtrsim 10^{9}\,\mathrm{GeV}$, naturally explaining $\Omega_{\rm DM} \simeq 5\Omega_{\rm B}$. The corresponding GW signals are testable with LISA, ET, and BBO. Finally, we analyze a UV-complete multi-Majoron model, based on a global $U(1)_N \times U(1)_{\rm B-L}$ extension, motivated from the hierarchy of lepton masses, which we dub as Mojaron collider. The corresponding FOPT in this model leaves a distinctive GW signature arising from RHN production during $U(1)_N$ symmetry breaking detectable by BBO, ET and upcoming LVK. Successful leptogenesis is realized for heaviest RHN mass $M_3 \sim 10^{10}\,\mathrm{GeV}$ and a $U(1)_N$ breaking vev $v_2 \sim \mathcal{O}(\mathrm{TeV})$, which sets the seesaw scale. oai:arXiv.org:2601.02458v3 astro-ph.CO gr-qc hep-ph hep-th Mon, 27 Apr 2026 00:00:00 -0400 replace http://arxiv.org/licenses/nonexclusive-distrib/1.0/ Anish Ghoshal, Pratyay Pal https://arxiv.org/abs/2603.09973 arXiv:2603.09973v2 Announce Type: replace Abstract: Observations of Type Ia supernovae (\sne), which probe the late Universe, together with baryon acoustic oscillations (BAO) and the cosmic microwave background (CMB), which probe the intermediate and early epochs, provide complementary constraints on the expansion history of the Universe. In this work, we forecast constraints on dark energy and other extensions to the standard cosmological model by combining the SNIa sample expected from the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST), data from current and forthcoming CMB surveys, and BAO measurements from the Dark Energy Spectroscopic Instrument (DESI). For the CMB, we use temperature, polarization, and lensing power spectra ($TT/EE/TE/\phi\phi$) from South Pole Telescope, the planned Advanced Simons Observatory, and a CMB-S4-like experiment. We derive constraints on $\Lambda {\rm CDM}$ and its extensions involving the dark energy equation of state parameters $(w_{0}, w_{a})$ and the sum of neutrino masses $\sum m_{\nu}$, using a Markov Chain Monte Carlo (MCMC) sampling framework. We find that the LSST Year-3 SNIa sample can improve upon the DES Year-5 dark energy constraints by a factor of $\times2-\times2.5$, with the gains driven primarily by the significantly higher SNIa density in the LSST sample. Similarly, DESI-DR3 shows up to a $\times1.8$ improvement on dark energy parameters over DR2, driven largely by the substantial increase in low-redshift sample. Combining CMB with LSST-Y3-SNIa and DESI-DR3-BAO yields $\sigma(w_{0}) = 0.028$ and $\sigma(w_{a}) = 0.11$ for $w_{0} w_{a} {\rm CDM}$ cosmology with the results being largely independent of the CMB dataset. The constraints weaken by 10%-30% when freeing $\sum m_{\nu}$ and spatial curvature. Moreover, the joint analysis of the three datasets can enable a $2-3\sigma$ detection of $\sum m_{\nu}$. oai:arXiv.org:2603.09973v2 astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 replace http://creativecommons.org/licenses/by/4.0/ Srinivasan Raghunathan, Ayan Mitra, Nikolina \v{S}ar\v{c}evi\'c, Fei Ge, Corentin Ravoux, Christos Georgiou, Ren\'ee Hlo\v{z}ek, Richard Kessler, Gautham Narayan, Paul Rogozenski, Paul Shah, Georgios Valogiannis, Joaquin Vieira, the LSST Dark Energy Science Collaboration https://arxiv.org/abs/2604.03381 arXiv:2604.03381v2 Announce Type: replace Abstract: We investigate whether Big Bang nucleosynthesis (BBN) remains compatible with the Covarying Coupling Constants plus Tired Light (CCC+TL) cosmology. In this framework, only quantities with explicit length dimensionality covary through a universal scaling function $f \left( z \right)$, while dimensionless constants and dimensionless ratios remain invariant. At the redshifts $z$ relevant to BBN, $f \left( z \right )$ approaches a constant plateau $f_{\text{max}} \left( z \right) \simeq 3$, and the tired-light contribution is negligible, so the early-time dynamics reduce to a global rescaling of dimensioned quantities. In particular, the Hubble expansion rate $H$ at fixed temperature $T$ satisfies $H_{\text{CTL}} \left( T \right) = f^{-1}_{\text{max}} H_{\Lambda\text{CDM}}\left( T\right)$, implying a longer cooling time $\Delta t$ between weak freeze-out and the onset of nucleosynthesis by the same factor (CCC+TL labeled as $\textit{CTL}$). We find that BBN predictions are preserved provided the relevant interaction rates $\Gamma$ and decay rates governing the neutron lifetime ${\tau}_n$ share the same plateau scaling as $H$, so that governing combinations such as $\Gamma\text{/}H$ and $\text{exp} \left( -\Delta t \text{/} {\tau}_n \right)$ remain invariant. Implementing these plateau rescalings in the Kawano/NUC123 network (via a single control parameter $\texttt{fctl} \equiv f_{\text{max}}$) yields identical light-element abundances for $\texttt{fctl}= 1$ ($\Lambda$CDM) and $\texttt{fctl} = 3\left( \text{CCC+TL} \right)$ to within $10^{-3} - 10^{-4}$ level, consistent with numerical rounding. We also illustrate that adopting the lower late-time CCC+TL baryon density from the Pantheon+ data fit can reduce the ${}^7\text{Li}$ discrepancy but simultaneously increases D/H, implying that BBN alone does not select between the late-time baryon-density inferences considered here. oai:arXiv.org:2604.03381v2 astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 replace http://creativecommons.org/licenses/by/4.0/ Rajendra P. Gupta, Nikolaos Samaras https://arxiv.org/abs/2405.06513 arXiv:2405.06513v3 Announce Type: replace-cross Abstract: Polarization wiggling is an observational effect of a gravitational field on the polarization of electromagnetic radiation traversing it. We find that in linear gravity, the polarization wiggle rate contributions from scalar, vector and tensor perturbations are independent and gauge invariant. While vector and tensor perturbations do induce polarization wiggling, scalar perturbations do not. This poses two natural questions: Can polarized electromagnetic radiation be used to measure vectorial and tensorial components of gravitational fields directly? And if so, how? Polarization wiggling is studied for an arbitrary vector perturbation to the spacetime metric. In a stationary spacetime, the polarization wiggle rate is proportional to the difference in frame dragging rate around the direction of propagation between radiation emission and measurement events. We show how this can be used to measure the angular momentum of a gravitational source if the emitter orbits the gravitational source on a known orbit. Finally, the polarization wiggling effect induced by a gravitational tensor mode with arbitrary polarization is analyzed. The effect is demonstrated for two cases: A spacetime with a flat Minkowski background and an expanding cosmology with a conformally flat background. In both cases, the polarization wiggling frequency equals the frequency of the gravitational tensor mode, while the other state parameters of the gravitational tensor mode are encoded in the polarization wiggling amplitude and phase of the polarized radiation. We show that measurements of polarization wiggling frequency, amplitude and phase of polarized electromagnetic radiation emitted by multiple sources at known positions from different directions enables all state parameters of a gravitational tensor mode to be determined. oai:arXiv.org:2405.06513v3 gr-qc astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 replace-cross http://arxiv.org/licenses/nonexclusive-distrib/1.0/ 10.1007/s10773-025-05978-8 Int J Theor Phys 64, 112 (2025) Kjell Tangen https://arxiv.org/abs/2406.13394 arXiv:2406.13394v3 Announce Type: replace-cross Abstract: Gravitational wave (GW) birefringence is a remarkable phenomenon that can be used to test the parity violation in gravity. By coupling the fuzzy dark matter (FDM) scalar to the gravitational Chern-Simons term, we explore the GW birefringence effects in the FDM background. In particular, in light of the highly oscillating granular FDM structure at the galactic scale, we are led to investigating the GW propagation in the Chern-Simons gravity over the general nontrivial scalar profile, which is a natural extension of previous studies on the homogeneous and isotropic configurations. As a result, it is found that GWs of both circularly polarized modes propagate in the straight line with the speed of light, and does not show any velocity birefringence. However, when considering the imaginary part of the dispersion relation, GWs exhibit the amplitude birefringence in which one circular polarization is enhanced while the other suppressed. Due to its local nature, the FDM-induced amplitude birefringence factor only depends on the GW frequency without any reliance on the GW propagating distance, which can be used to distinguish this signal from other birefringece mechanisms. More importantly, the birefringence shows a periodic time modulation with the period directly reflecting the FDM scalar mass, which is another smoking gun for testing this model. Finally, we also study the extra-galactic FDM contribution to the GW birefringence, which is shown to be suppressed by the cosmological DM density and thus subdominant compared with the galactic counterpart. oai:arXiv.org:2406.13394v3 gr-qc astro-ph.CO hep-ph Mon, 27 Apr 2026 00:00:00 -0400 replace-cross http://creativecommons.org/licenses/by/4.0/ Da Huang, Ze-Xuan Xiong, Lei-Jian Wang https://arxiv.org/abs/2507.23354 arXiv:2507.23354v2 Announce Type: replace-cross Abstract: The effective number of neutrino species, $N_{\rm eff}$, serves as a key fitting parameter extensively employed in cosmological studies. In this work, we point out a fundamental inconsistency in the conventional treatment of $N_{\rm eff}$ in big bang nucleosynthesis (BBN), particularly regarding its applicability to new physics scenarios where $\Delta N_{\rm eff}$, the deviation of $N_{\rm eff}$ from the standard BBN prediction, is negative. To ensure consistent interpretation, it is imperative to either restrict the allowed range of $N_{\rm eff}$ or systematically adjust neutrino-induced reaction rates based on physically motivated assumptions. As a concrete example, we consider a simple scenario in which a negative $\Delta N_{\rm eff}$ arises from entropy injection into the electromagnetic sector due to the decay of long-lived particles after neutrino decoupling. This process dilutes the neutrino density and suppresses the rate of neutrino-driven neutron-proton conversion. Under this assumption, we demonstrate that the resulting BBN constraints on $N_{\rm eff}$ deviate significantly from those obtained by the conventional, but unphysical, extrapolation of dark radiation scenarios into the $\Delta N_{\rm eff} < 0$ regime. oai:arXiv.org:2507.23354v2 hep-ph astro-ph.CO Mon, 27 Apr 2026 00:00:00 -0400 replace-cross http://creativecommons.org/licenses/by/4.0/ Sougata Ganguly, Tae Hyun Jung, Seokhoon Yun https://arxiv.org/abs/2508.08359 arXiv:2508.08359v4 Announce Type: replace-cross Abstract: Recent developments have yielded significant progress towards systematically understanding loop corrections to de Sitter (dS) correlators. In close analogy with physics in Anti-de Sitter (AdS), large logarithms can result from loops that can be interpreted as corrections to the dimensions of operators. In contrast with AdS, these dimensions are not manifestly real. This implies that the theoretical constraints on the associated correlators are less transparent, particularly in the presence of light scalars. In this paper, we revisit these issues by performing and comparing calculations using the spectral representation approach and the Soft de Sitter Effective Theory (SdSET). We review the general arguments that yield positivity constraints on dS correlators from both perspectives. Our particular focus will be on vertex operators for compact scalar fields, since this case introduces novel complications. We will explain how to resolve apparent disagreements between different techniques for calculating the anomalous dimensions for principal series fields coupled to these vertex operators. Along the way, we will offer new proofs of positivity of the anomalous dimensions, and explain why renormalization group flow associated with these anomalous dimensions in SdSET is the same as resumming bubble diagrams in the spectral representation. oai:arXiv.org:2508.08359v4 hep-th astro-ph.CO hep-ph Mon, 27 Apr 2026 00:00:00 -0400 replace-cross http://creativecommons.org/licenses/by/4.0/ Priyesh Chakraborty, Timothy Cohen, Daniel Green, Yiwen Huang https://arxiv.org/abs/2603.20363 arXiv:2603.20363v2 Announce Type: replace-cross Abstract: The properties of the first metal-free stars remain largely unknown, and so far, the only data-driven constraints on their mass distribution (IMF) come from near-field cosmology. Here, we interpret new observations of the C1 and C2 components of Hebe, the HeII emitter near the galaxy GN-z11. Using a locally calibrated model, we robustly confirm the pristine (PopIII) nature of both components, showing that the measured upper limits on metal lines can only be reproduced by galaxies with $>50\%$ of their stellar mass in PopIII stars. We find that C1 is consistent with a purely PopIII system and adopt a simple parametric approach to infer the implications for the PopIII IMF and stellar mass. The observed $\rm HeII/H_\gamma$ ratio excludes steep IMFs, favoring top-heavy distributions, especially for young stellar ages ($\leq 1$ Myr). Combined with the HeII luminosity, this implies a total PopIII stellar mass of $2 \cdot 10^4 < M_\star/M_\odot < 6 \cdot 10^5$. While degeneracies between IMF, stellar mass, and age remain, adopting the lower stellar masses predicted by simulations ($M_\star < 10^5\,M_\odot$) strengthens the preference for top-heavy IMFs. Combining these results with near-field constraints, which instead exclude the flattest IMFs, we define a data-driven range of viable PopIII IMFs, linking characteristic mass and slope. This work demonstrates that direct observations of high-$z$ PopIII systems can place independent constraints on the IMF of the first stars, opening a new window on their formation and properties. oai:arXiv.org:2603.20363v2 astro-ph.GA astro-ph.CO astro-ph.SR Mon, 27 Apr 2026 00:00:00 -0400 replace-cross http://creativecommons.org/licenses/by/4.0/ Elka Rusta, Stefania Salvadori, Roberto Maiolino, Viola Gelli, Ioanna Koutsouridou, Stefano Carniani, Hannah \"Ubler, Alessandro Marconi, Daniel Schaerer