Showing 1–50 of 322 results for author: Branchesi, M

Detection of TeV emission during early afterglow from poorly localized GRBs with ground based IACTs

Authors: S. Macera, B. Banerjee, M. Seglar-Arroyo, J. Green, G. Oganesyan, P. Tiwari, A. Ierardi, M. Branchesi, F. Aharonian, S. Mohnani, D. Miceli, F. Schüssler, A. Berti

Abstract: Gamma-ray bursts (GRBs) are among the most luminous and rapidly evolving transients in the Universe. While space-based instruments have extended GRB observations up to energies of $\sim$100 GeV, the detection of very-high-energy (VHE; $E>100$ GeV) emission from ground-based telescopes, especially during prompt or/and the early afterglow phase, remains challenging. These difficulties arise from the… ▽ More Gamma-ray bursts (GRBs) are among the most luminous and rapidly evolving transients in the Universe. While space-based instruments have extended GRB observations up to energies of $\sim$100 GeV, the detection of very-high-energy (VHE; $E>100$ GeV) emission from ground-based telescopes, especially during prompt or/and the early afterglow phase, remains challenging. These difficulties arise from the rapid temporal decay of GRB afterglows, strong attenuation by the extragalactic background light (EBL), observational latency, and the typical poor sky localization provided by MeV-detectors such as Fermi/GBM. In this work, we investigate the prospects for detecting TeV ($\sim$100 GeV--1 TeV) emission from poorly localized GRBs by adopting optimized follow-up strategies based on rapid tiling of large localization regions. We simulate a realistic population of GRBs informed by more than fifteen years of Fermi/GBM and Swift/XRT detections and recent progresses in the afterglow emission modeling. Using these simulations, we evaluate the detectability of GRB early afterglows by the next-generation Imaging Atmospheric Cherenkov Telescopes, equipped with larger field-of-view (FoV), as a function of latency, exposure time, and observational strategy. Our strategy can significantly enhance the detection rate; for instruments such as ASTRI and LACT, it increases by up to a factor of two compared to strategies limited to well-localized (Swift-like) events. For CTAO, our proposed approach provides up to four VHE detections per year. △ Less

Submitted 8 April, 2026; originally announced April 2026.

Comments: 20 pages, 20 figures

Narrowband searches for continuous gravitational waves from known pulsars in the first two parts of the fourth LIGO--Virgo--KAGRA observing run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, A. Adam, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith , et al. (1831 additional authors not shown)

Abstract: Rotating non-axisymmetric neutron stars (NSs) are promising sources for continuous gravitational waves (CWs). Such CWs can, if detected, inform us about the internal structure and equation of state of NSs. Here, we present a narrowband search for CWs from known pulsars, for which an efficient and sensitive matched-filter search can be applied. Narrowband searches are designed to be robust to misma… ▽ More Rotating non-axisymmetric neutron stars (NSs) are promising sources for continuous gravitational waves (CWs). Such CWs can, if detected, inform us about the internal structure and equation of state of NSs. Here, we present a narrowband search for CWs from known pulsars, for which an efficient and sensitive matched-filter search can be applied. Narrowband searches are designed to be robust to mismatches between the electromagnetic (EM) and gravitational emissions, in contrast to fully targeted searches where the CW emission is assumed to be phase-locked to the EM one. In this work, we search for the CW counterparts emitted by 34 pulsars using data from the first and second parts of the fourth LIGO--Virgo--KAGRA observing run. This is the largest number of pulsars so far targeted for narrowband searches in the advanced detector era. We use the 5n-vector narrowband pipeline, which applies frequency-domain matched filtering. In previous searches, it covered a narrow range in the frequency -- frequency time derivative ($f$ -- $\dot{f}$) space. Here, we also explore a range in the second time derivative of the frequency $\ddot{f}$ around the value indicated by EM observations. Additionally, for the first time, we target sources in a binary system with this kind of search. We find no evidence for CWs and therefore set upper limits on the strain amplitude emitted by each pulsar, using simulated signals added in real data. For 20 analyses, we report an upper limit below the theoretical spin-down limit. The tightest constraint is for pulsar PSR J0534+2200 (the Crab pulsar), for which our strain upper limit on the CW amplitude is $\lesssim 2\%$ of its spin-down limit, corresponding to less than $0.04\%$ of the spin-down power being radiated in the CW channel. △ Less

Submitted 26 March, 2026; originally announced March 2026.

Comments: 30 pages, 6 figures, submitted to ApJ

Report number: LIGO-P2500612

Searches for Continuous Gravitational Waves from Supernova Remnants in the first part of the LIGO-Virgo-KAGRA Fourth Observing run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1742 additional authors not shown)

Abstract: We present results from directed searches for continuous gravitational waves from a sample of 15 nearby supernova remnants, likely hosting young neutron star candidates, using data from the first eight months of the fourth observing run (O4) of the LIGO-Virgo-KAGRA Collaboration. The analysis employs five pipelines: four semi-coherent methods -- the Band-Sampled-Data directed pipeline, Weave and t… ▽ More We present results from directed searches for continuous gravitational waves from a sample of 15 nearby supernova remnants, likely hosting young neutron star candidates, using data from the first eight months of the fourth observing run (O4) of the LIGO-Virgo-KAGRA Collaboration. The analysis employs five pipelines: four semi-coherent methods -- the Band-Sampled-Data directed pipeline, Weave and two Viterbi pipelines (single- and dual-harmonic) -- and PyStoch, a cross-correlation-based pipeline. These searches cover wide frequency bands and do not assume prior knowledge of the targets' ephemerides. No evidence of a signal is found from any of the 15 sources. We set 95\% confidence-level upper limits on the intrinsic strain amplitude, with the most stringent constraints reaching $\sim 4 \times 10^{-26}$ near 300 Hz for the nearby source G266.2$-$1.2 (Vela Jr.). We also derive limits on neutron star ellipticity and $r$-mode amplitudes for the same source, with the best constraints reaching $\lesssim 10^{-7}$ and $\lesssim 10^{-5}$, respectively, at frequencies above 400 Hz. These results represent the most sensitive wide-band directed searches for continuous gravitational waves from supernova remnants to date. △ Less

Submitted 2 April, 2026; v1 submitted 26 March, 2026; originally announced March 2026.

Isolated or Dynamical? Tracing Black Hole Binary Formation through the Population of Gravitational-Wave Sources

Authors: Manuel Arca Sedda, Lavinia Paiella, Cristiano Ugolini, Filippo Santoliquido, Benedetta Mestichelli, Ilaria Usai, Filippo Simonato, Marica Branchesi

Abstract: The population of binary black hole (BBH) mergers observed by the LIGO-Virgo-KAGRA (LVK) collaboration offers a window into the cosmic evolution of compact binaries and their formation. We employ the semi-analytic population-synthesis code B-POP to model BBHs assembled through isolated binary evolution and dynamical interactions in young, globular, and nuclear star clusters. Our framework incorpor… ▽ More The population of binary black hole (BBH) mergers observed by the LIGO-Virgo-KAGRA (LVK) collaboration offers a window into the cosmic evolution of compact binaries and their formation. We employ the semi-analytic population-synthesis code B-POP to model BBHs assembled through isolated binary evolution and dynamical interactions in young, globular, and nuclear star clusters. Our framework incorporates star formation history, metallicity evolution, and single and binary stellar evolution to quantify their impact on the observable properties of the BBH population and on the relative contribution of distinct formation channels. Our models are characterized by a merger rate, $\mathcal{R} = 17.5-24.1\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$, broadly consistent with LVK constraints. Moreover, the predicted distributions of primary mass, mass ratio, and effective inspiral spin parameter are compatible with those inferred from current LVK observations. Our primary-mass distribution is dominated by isolated binaries at $m_1 < 20$ M$_\odot$, while dynamically assembled first- and higher-generation mergers dominate at larger masses. As a consequence, the sub-population of mergers with $m_1 > 45$ M$_\odot$ exhibits a nearly flat mass-ratio distribution and distinctive spin properties. We leverage our models to explore how: (i) the fraction of stars in isolated binaries and the fraction of stellar mass bound in clusters regulate the merger rate; (ii) common-envelope physics shapes the primary-mass distribution and its redshift evolution; (iii) the inclusion of stellar-collision products enhances the formation of higher-generation mergers; and (iv) the natal spin distribution influences the effective spin. Using our models to assess possible origins of selected GW events, we illustrate how the complexity of the underlying astrophysical processes can hinder the possibility to draw definitive conclusions. △ Less

Submitted 20 March, 2026; originally announced March 2026.

Comments: 22 pages of main text, 7 pages of appendices, subm. to PRD

Advanced Virgo Plus for O5 -- Design Report Overview

Authors: F. Acernese, A. Agapito, D. Agarwal, I. -L. Ahrend, L. Aiello, A. Ain, S. Albanesi, W. Ali, C. Alléné, A. Allocca, W. Amar, A. Amato, F. Amicucci, C. Amra, M. Andia, T. Andrić, S. Ansoldi, S. Antier, E. Z. Appavuravther, M. Arca Sedda, F. Arciprete, F. Armato, N. Arnaud, L. Asprea, M. Assiduo , et al. (556 additional authors not shown)

Abstract: This document presents an overview of the design, implementation, and expected performance of the Advanced Virgo Plus (AdV+) upgrades in view of the O5 observing run. Following the experience gained during the O4 commissioning and operations, the Virgo Collaboration has revised the upgrade strategy to address limitations associated with marginally stable recycling cavities. The O5 upgrade program… ▽ More This document presents an overview of the design, implementation, and expected performance of the Advanced Virgo Plus (AdV+) upgrades in view of the O5 observing run. Following the experience gained during the O4 commissioning and operations, the Virgo Collaboration has revised the upgrade strategy to address limitations associated with marginally stable recycling cavities. The O5 upgrade program combines elements from the original AdV+ Phase II project with new design solutions, including the implementation of stable recycling cavities, a major modification to the central interferometer layout, and a comprehensive renewal of critical subsystems. The planned upgrades are organized in two steps, targeting progressive improvements in operational stability, noise reduction, and detector sensitivity. Key developments include new vacuum infrastructures, suspensions, mirrors, optical configurations, quantum noise reduction systems, and high-power laser technologies. The resulting configuration is expected to significantly enhance the interferometer performance, enabling a substantial increase in astrophysical reach and scientific return during O5. △ Less

Submitted 31 March, 2026; v1 submitted 20 March, 2026; originally announced March 2026.

GWTC-4.0: Tests of General Relativity. III. Tests of the Remnants

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1757 additional authors not shown)

Abstract: This is the third paper of the set recording the results of the suite of tests of general relativity (GR) performed on the signals from the fourth Gravitational-Wave Transient Catalog (GWTC-4.0), where we focus on the remnants of the binary mergers. We examine for the first time 42 events from the first part of the fourth observing run of the LIGO-Virgo-KAGRA detectors, alongside events from the p… ▽ More This is the third paper of the set recording the results of the suite of tests of general relativity (GR) performed on the signals from the fourth Gravitational-Wave Transient Catalog (GWTC-4.0), where we focus on the remnants of the binary mergers. We examine for the first time 42 events from the first part of the fourth observing run of the LIGO-Virgo-KAGRA detectors, alongside events from the previous observation runs, restricting our analysis to the confident signals, which were measured in at least two detectors and that have false alarm rates $\le 10^{-3} \mathrm{yr}^{-1}$. This paper focuses on seven tests of the coalescence remnants. Three of these are tests of the ringdown and its consistency with the expected quasinormal mode spectrum of a Kerr black hole. Specifically, two tests analyze just the ringdown in the time domain, and the third test analyzes the entire signal in the frequency domain. Four tests allow for the existence of possible echoes arriving after the end of the ringdown, which are not expected in GR. We find overall consistency of the remnants with GR. When combining events by multiplying likelihoods (hierarchically), one analysis finds that the GR prediction lies at the boundary of the $98.6^{+1.4}_{-9.4}\%$ ($99.3^{+0.7}_{-4.5}\%$) credible region, an increase from $93.8^{+6.1}_{-20.0}\%$ ($94.9^{+4.4}_{-18.2}\%$) for GWTC-3.0. Here the ranges of values comes from bootstrapping to account for the finite number of events analyzed and suggest that some of the apparently significant deviation could be attributed to variance due to the finite catalog. Since the significance also decreases to 92.2% (96.2%) when including the more recent very loud event GW250114, there is no strong evidence for a GR deviation. We find no evidence for post-merger echoes in the events that were analyzed. (Abridged) △ Less

Submitted 19 March, 2026; originally announced March 2026.

Comments: As part of the Astrophysical Journal Letters Focus Issue on the Gravitational Wave Transient Catalog

Report number: LIGO-P2500067

GWTC-4.0: Tests of General Relativity. II. Parameterized Tests

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1761 additional authors not shown)

Abstract: In this second of three papers on tests of general relativity (GR) applied to the compact binary coalescence signals in the fourth Gravitational-Wave Transient Catalog (GWTC-4.0), we present the results of the parameterized tests of GR and constraints on line-of-sight acceleration. We include events up to and including the first part of the fourth observing run (O4a) of the LIGO Virgo KAGRA detect… ▽ More In this second of three papers on tests of general relativity (GR) applied to the compact binary coalescence signals in the fourth Gravitational-Wave Transient Catalog (GWTC-4.0), we present the results of the parameterized tests of GR and constraints on line-of-sight acceleration. We include events up to and including the first part of the fourth observing run (O4a) of the LIGO Virgo KAGRA detectors. As in the other two papers in this series, we restrict our analysis to the 42 confident signals, measured by at least two detectors, that have false alarm rates $\le 10^{-3} \mathrm{yr}^{-1}$ from O4a, in addition to the 49 such events from previous observing runs. This paper focuses on the eight tests that constrain parameterized deviations from the expected GR (or unaccelerated) values. These include modifications of post-Newtonian (PN) parameters, spin-induced quadrupole moments different from those of a binary black hole, and possible dispersive or birefringent propagation effects. Overall, we find no evidence for physics beyond GR, for spin-induced quadrupole moments different from those of a Kerr black hole in GR, or for line of sight acceleration, with more than 90% of the events including the null result (no deviation) within their 90% credible intervals. We discuss possible systematics affecting the other events and tests, even though they are statistically not surprising, given noise. We improve the bounds on deviations from the GR PN coefficients by factors of 1.2-5.5 and provide illustrative translations to constraints on some modified theories. Also, we update the bound on the mass of the graviton, at 90% credibility, to $m_g \leq 1.92\times 10^{-23} \mathrm{eV}/c^2$. Thus, we see that GR holds, and many of the bounds on possible deviations derived from our events are the best to date. △ Less

Submitted 19 March, 2026; originally announced March 2026.

Comments: As part of the Astrophysical Journal Letters Focus Issue on the Gravitational Wave Transient Catalog

Report number: LIGO-P2500066

GWTC-4.0: Tests of General Relativity. I. Overview and General Tests

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1759 additional authors not shown)

Abstract: The worldwide LIGO-Virgo-KAGRA network of gravitational-wave (GW) detectors continues to increase in sensitivity, thus increasing the quantity and quality of the detected GW signals from compact binary coalescences. These signals allow us to perform ever-more sensitive tests of general relativity (GR) in the dynamical and strong-field regime of gravity. This paper is the first of three, where we p… ▽ More The worldwide LIGO-Virgo-KAGRA network of gravitational-wave (GW) detectors continues to increase in sensitivity, thus increasing the quantity and quality of the detected GW signals from compact binary coalescences. These signals allow us to perform ever-more sensitive tests of general relativity (GR) in the dynamical and strong-field regime of gravity. This paper is the first of three, where we present the results of a suite of tests of GR using the binary signals included in the fourth GW Transient Catalog (GWTC-4.0), i.e., up to and including the first part of the fourth observing run of the detectors (O4a). We restrict our analysis to the 91 confident signals, henceforth called events, that were measured by at least two detectors, and have false alarm rates $\le 10^{-3} \mathrm{yr}^{-1}$. These include 42 events from O4a. This first paper presents an overview of the methods, selection of events and GR tests, and serves as a guidemap for all three papers. Here we focus on the four general tests of consistency, where we find no evidence for deviations from our models. Specifically, for all the events considered, we find consistency of the residuals with noise. The final mass and final spin as inferred from the low- and high-frequency parts of the waveform are consistent with each other. We also find no evidence for deviations from the GR predictions for the amplitudes of subdominant GW multipole moments, or for non-GR modes of polarization. We thus find that GR, without new physics beyond it, is still consistent with these GW events. The results of the two additional papers in this trio also find overall consistency with vacuum GR, with more than 90% of the events being consistent with GR at the 90% credible level. While one of the ringdown analyses finds the GR value in the tails for its combined results, this may be due in part to catalog variance. △ Less

Submitted 19 March, 2026; originally announced March 2026.

Comments: As part of the Astrophysical Journal Letters Focus Issue on the Gravitational Wave Transient Catalog

Report number: LIGO-P2500065

All-sky Searches for Continuous Gravitational Waves from Isolated Neutron Stars in the Data from the First Part of the Fourth LIGO-Virgo-KAGRA Observing Run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, A. Adam, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith , et al. (1804 additional authors not shown)

Abstract: We present results from an all-sky search for continuous gravitational waves, using three different methods applied to the first eight months of LIGO data from the fourth LIGO-Virgo-KAGRA Collaboration s observing run. We aim at signals potentially emitted by rotating, non-axisymmetric isolated neutron star in the Milky Way. The analysis spans a frequency range from 20 Hz to 2000 Hz and accommodat… ▽ More We present results from an all-sky search for continuous gravitational waves, using three different methods applied to the first eight months of LIGO data from the fourth LIGO-Virgo-KAGRA Collaboration s observing run. We aim at signals potentially emitted by rotating, non-axisymmetric isolated neutron star in the Milky Way. The analysis spans a frequency range from 20 Hz to 2000 Hz and accommodates frequency derivative magnitudes up to $10^{-8}$ Hz/s. No statistically significant periodic gravitational wave signals were detected. We establish 95% confidence-level (CL) frequentist upper limits on the dimensionless strain amplitudes. The most stringent population-averaged strain upper limits reach 9.7 $\times$ $10^{-26}$ near 290 Hz, matching the best previous constraints from 250 to $\sim$1700 Hz while extending coverage to a much broader spin-down range. At higher frequencies, the new limits improve upon previous results by factors of approximately $\sim$1.6. These constraints are applied to three astrophysical scenarios: 1) the distribution of galactic neutron stars as a function of spin frequency and ellipticity; 2) the contribution of millisecond pulsars to the GeV excess near the galactic center; and 3) the possible dark matter fraction composed of nearby inspiraling primordial binary black holes with asteroid-scale masses. △ Less

Submitted 14 March, 2026; originally announced March 2026.

Comments: 45 pages, 17 figures

Report number: LIGO-P2500416

Teen TITANS simulations -- I. Inefficient intermediate-mass black hole seeding via stellar collisions in young massive clusters

Authors: Benedetta Mestichelli, Sara Rastello, Michela Mapelli, Manuel Arca Sedda, Marica Branchesi

Abstract: Young massive clusters (YMCs) provide favorable environments for frequent stellar collisions, potentially leading to the formation of very massive stars (VMSs) and seeds of intermediate-mass black holes (IMBHs). We investigate the role of repeated stellar collisions in YMCs using TITANS, a new suite of 18 direct $N$-body simulations. Our models span cluster masses $10^5 - 10^6\,\rm M_\odot$, half-… ▽ More Young massive clusters (YMCs) provide favorable environments for frequent stellar collisions, potentially leading to the formation of very massive stars (VMSs) and seeds of intermediate-mass black holes (IMBHs). We investigate the role of repeated stellar collisions in YMCs using TITANS, a new suite of 18 direct $N$-body simulations. Our models span cluster masses $10^5 - 10^6\,\rm M_\odot$, half-mass densities $ρ_{\rm h}=100 - 10^5\,\rm M_\odot\,pc^{-3}$, and include high primordial binary fractions, consistent with observations of massive stars in young clusters. Overall, our simulations assume cluster properties that are typical of YMCs in the low-redshift Universe. We find that repeated stellar collisions are efficient only in the densest clusters with short relaxation times and are absent in systems with $ρ_{\rm h}<500\,\rm M_\odot\,pc^{-3}$ and $t_{\rm rh}>1.3\,\rm Gyr$. Rapid mass segregation allows massive stars to sink to the cluster center, merge, and undergo subsequent collisions, even in clusters with long core-collapse times. However, collision chains are typically triggered by primordial binary mergers and usually involve only two collisions. In our simulations, only three VMSs form through repeated collisions and reach $m_*>330\,\rm M_\odot$, while most VMSs have $m_*<300\,\rm M_\odot$ and form through primordial binary mergers. None constitute viable IMBH seeds, as their helium cores fall in the (pulsational) pair-instability regime. We form five IMBHs from stellar collisions involving stars at different evolutionary stages, while the dominant channel is the merger of stellar-mass black holes, producing twelve IMBHs. For properties typical of local YMCs, stellar collision chains are therefore inefficient in producing IMBHs more massive than $140\,\rm M_\odot$, as most collisionally formed VMSs attain masses that fall in the pair-instability regime. △ Less

Submitted 19 February, 2026; originally announced February 2026.

Comments: 14 pages, 12 figures, 2 tables. Comments welcome

Constraining Binary Neutron Star Populations using Short Gamma-Ray Burst Observations

Authors: Alessio Ludovico De Santis, Samuele Ronchini, Filippo Santoliquido, Marica Branchesi

Abstract: The landmark multi-messenger observations of the binary neutron star (BNS) merger GW170817 provided firm evidence that such mergers can produce short gamma-ray bursts (sGRBs). However, the limited number of BNS detections by current gravitational-wave (GW) observatories raises the question of whether BNS mergers alone can account for the full observed sGRB population. We analyze a comprehensive se… ▽ More The landmark multi-messenger observations of the binary neutron star (BNS) merger GW170817 provided firm evidence that such mergers can produce short gamma-ray bursts (sGRBs). However, the limited number of BNS detections by current gravitational-wave (GW) observatories raises the question of whether BNS mergers alone can account for the full observed sGRB population. We analyze a comprehensive set of 64 BNS population synthesis models with a Monte Carlo-based framework to reproduce the properties of sGRBs detected by Fermi-GBM over the past 16 years. We consider three jet geometry scenarios: a universal structured jet calibrated to GW170817, a universal top-hat jet, and a non-universal top-hat jet with distributions of core opening angles. Our results show that models characterized by low local BNS merger rates ($R_{BNS}(0) \lesssim 50$ Gpc$^{-3}$ yr$^{-1}$) predict too few observable sGRBs to reproduce the Fermi-GBM population, effectively disfavoring them as sole progenitors. Even when relaxing assumptions on jet geometry, low-rate models remain viable only for wide jets ($θ_c \ge 15^\circ$), in tension with the narrow jet cores ($θ_c \approx 6^\circ$) inferred from sGRB afterglow observations. In contrast, models with local merger rates of order $R_{BNS}(0) \approx 100$ Gpc$^{-3}$ yr$^{-1}$ successfully reproduce the observed sGRB population, assuming a plausible fraction of BNS mergers launch relativistic jets and realistic jet geometries. This analysis highlights the power of combining GW observations of BNS mergers with electromagnetic observations of sGRBs to place robust constraints on the BNS merger population and to assess their role as progenitors of sGRBs. △ Less

Submitted 13 February, 2026; originally announced February 2026.

deci-Hz Gravitational Wave Observations on the Moon and Beyond

Authors: Emanuele Berti, Marica Branchesi, Alessandra Buonanno, Alessandra Corsi, Daniel J. D'Orazio, Jan Harms, Jason M. Hogan, Francesco Iacovelli, Karan Jani, Marc Kamionkowski, Kentaro Komori, Konstantinos Kritos, Andrea Maselli, M. Coleman Miller, Chiara M. F. Mingarelli, Volker Quetschke, B. S. Sathyaprakash, David H. Shoemaker, Joseph Silk, Jacob P. Slutsky, James Ira Thorpe, James Trippe, Daniele Vetrugno, Stefano Vitale

Abstract: This document summarizes talks and discussions from the workshop "deci-Hz Gravitational Wave Observations on the Moon and Beyond" that took place at Johns Hopkins University between September 1 and September 3, 2025. The workshop focused on experimental proposals to observe gravitational waves in the deci-Hz band, including lunar detectors, laser interferometers in space, and atom interferometry;… ▽ More This document summarizes talks and discussions from the workshop "deci-Hz Gravitational Wave Observations on the Moon and Beyond" that took place at Johns Hopkins University between September 1 and September 3, 2025. The workshop focused on experimental proposals to observe gravitational waves in the deci-Hz band, including lunar detectors, laser interferometers in space, and atom interferometry; gravitational wave sources in the deci-Hz frequency band; and the multi-messenger and multi-band astronomy that would be enabled by these observations. △ Less

Submitted 5 February, 2026; originally announced February 2026.

Comments: 64 pages, 1 figure

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

Authors: Gor Oganesyan, Om Sharan Salafia, Emanuele Sobacchi, Samanta Macera, Giancarlo Ghirlanda, Lara Nava, Annarita Ierardi, Biswajit Banerjee, Alessio Mei, Stefano Ascenzi, Marica Branchesi

Abstract: A small fraction of X-ray photons from gamma-ray bursts (GRBs), after escaping the relativistic jet, are scattered by electrons in the circumburst medium. Subsequent photon-photon absorption between the incoming MeV $γ$-rays and the back-scattered X-rays generate electron-positron pairs, enriching the surrounding medium with leptons. We investigate how these back-scattered photons modify the promp… ▽ More A small fraction of X-ray photons from gamma-ray bursts (GRBs), after escaping the relativistic jet, are scattered by electrons in the circumburst medium. Subsequent photon-photon absorption between the incoming MeV $γ$-rays and the back-scattered X-rays generate electron-positron pairs, enriching the surrounding medium with leptons. We investigate how these back-scattered photons modify the prompt GRB spectrum through $γ-γ$ absorption. In a dense and pair-loaded wind environment, the emerging spectra exhibit a broad attenuation structure, whose morphology is sensitive to the low-energy spectral index $α$. In particular, spectra with $α> -1$ develop a pronounced, saddle-shaped absorption between 1 and 100 MeV (rest frame). Such external MeV absorption could account for the spectral curvature seen in some bright GRBs, and may point to enhanced mass loss from their progenitor stars, consistent with early observations of core-collapse supernovae. △ Less

Submitted 20 January, 2026; originally announced January 2026.

Comments: submitted

Deep Search for Joint Sources of Gravitational Waves and High-Energy Neutrinos with IceCube During the Third Observing Run of LIGO and Virgo

Authors: The IceCube Collaboration, R. Abbasi, M. Ackermann, J. Adams, S. K. Agarwalla, J. A. Aguilar, M. Ahlers, J. M. Alameddine, S. Ali, N. M. Amin, K. Andeen, C. Argüelles, Y. Ashida, S. Athanasiadou, S. N. Axani, R. Babu, X. Bai, J. Baines-Holmes, A. Balagopal V., S. W. Barwick, S. Bash, V. Basu, R. Bay, J. J. Beatty, J. Becker Tjus , et al. (2193 additional authors not shown)

Abstract: The discovery of joint sources of high-energy neutrinos and gravitational waves has been a primary target for the LIGO, Virgo, KAGRA, and IceCube observatories. The joint detection of high-energy neutrinos and gravitational waves would provide insight into cosmic processes, from the dynamics of compact object mergers and stellar collapses to the mechanisms driving relativistic outflows. The joint… ▽ More The discovery of joint sources of high-energy neutrinos and gravitational waves has been a primary target for the LIGO, Virgo, KAGRA, and IceCube observatories. The joint detection of high-energy neutrinos and gravitational waves would provide insight into cosmic processes, from the dynamics of compact object mergers and stellar collapses to the mechanisms driving relativistic outflows. The joint detection of multiple cosmic messengers can also elevate the significance of the common observation even when some or all of the constituent messengers are sub-threshold, i.e. not significant enough to declare their detection individually. Using data from the LIGO, Virgo, and IceCube observatories, including sub-threshold events, we searched for common sources of gravitational waves and high-energy neutrinos during the third observing run of Advanced LIGO and Advanced Virgo detectors. Our search did not identify significant joint sources. We derive constraints on the rate densities of joint sources. Our results constrain the isotropic neutrino emission from gravitational-wave sources for very high values of the total energy emitted in neutrinos (> $10^{52} - 10^{54}$ erg). △ Less

Submitted 28 January, 2026; v1 submitted 12 January, 2026; originally announced January 2026.

Comments: Data release at: https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/34B5AP

Probing Cosmic Expansion and Early Universe with Einstein Telescope

Authors: Angelo Ricciardone, Mairi Sakellariadou, Archisman Ghosh, Alessandro Agapito, M. Celeste Artale, Michael Bacchi, Tessa Baker, Marco Baldi, Nicola Bartolo, Andrea Begnoni, Enis Belgacem, Marek Biesiada, Jose J. Blanco-Pillado, Tomasz Bulik, Marica Branchesi, Gianluca Calcagni, Giulia Capurri, Carmelita Carbone, Roberto Casadio, J. A. R. Cembranos, Andrea Cozzumbo, Ivan De Martino, Jose M. Diego, Emanuela Dimastrogiovanni, Guillem Domènech , et al. (30 additional authors not shown)

Abstract: Over the next two decades, gravitational-wave (GW) observations are expected to evolve from a discovery-driven endeavour into a precision tool for astrophysics, cosmology, and fundamental physics. Current second-generation ground-based detectors have established the existence of compact-binary mergers and enabled GW multi-messenger astronomy, but they remain limited in sensitivity, redshift reach,… ▽ More Over the next two decades, gravitational-wave (GW) observations are expected to evolve from a discovery-driven endeavour into a precision tool for astrophysics, cosmology, and fundamental physics. Current second-generation ground-based detectors have established the existence of compact-binary mergers and enabled GW multi-messenger astronomy, but they remain limited in sensitivity, redshift reach, frequency coverage, and duty cycle. These limitations prevent them from addressing many fundamental open questions in cosmology. By the 2040s, wide-field electromagnetic surveys will have mapped the luminous Universe with unprecedented depth and accuracy. Nevertheless, key problems including the nature of dark matter, the physical origin of cosmic acceleration, the properties of gravity on cosmological scales, and the physical conditions of the earliest moments after the Big Bang will remain only partially constrained by electromagnetic observations alone. Progress on these fronts requires access to physical processes and epochs that do not emit light. Gravitational waves provide a unique and complementary observational channel: they propagate over cosmological distances largely unaffected by intervening matter, probe extreme astrophysical environments, and respond directly to the geometry of spacetime. In this context, next-generation GW observatories such as the Einstein Telescope (ET) will be transformative for European astronomy. Operating at sensitivities and frequencies beyond existing detectors, ET will observe binary black holes and neutron stars out to previously inaccessible redshifts, enable continuous high signal-to-noise monitoring of compact sources, and detect gravitational-wave backgrounds of astrophysical and cosmological origin. Together with space-based detectors, ET will play a central role in advancing our understanding of cosmic evolution and fundamental physics. △ Less

Submitted 9 January, 2026; originally announced January 2026.

Comments: 4 pages. White paper submitted to the ESO Expanding Horizons Call on behalf of ET OSB Div2 - Cosmology

Comparing next-generation detector configurations for high-redshift gravitational wave sources with neural posterior estimation

Authors: Filippo Santoliquido, Jacopo Tissino, Ulyana Dupletsa, Marica Branchesi, Jan Harms

Abstract: The coming decade will be crucial for determining the final design and configuration of a global network of next-generation (XG) gravitational-wave (GW) detectors, including the Einstein Telescope (ET) and Cosmic Explorer (CE). In this study and for the first time, we assess the performance of various network configurations using neural posterior estimation (NPE) implemented in Dingo-IS-a method b… ▽ More The coming decade will be crucial for determining the final design and configuration of a global network of next-generation (XG) gravitational-wave (GW) detectors, including the Einstein Telescope (ET) and Cosmic Explorer (CE). In this study and for the first time, we assess the performance of various network configurations using neural posterior estimation (NPE) implemented in Dingo-IS-a method based on normalizing flows and importance sampling that enables fast and accurate inference. We focus on a specific science case involving short-duration, massive and high-redshift binary black hole (BBH) mergers with detector-frame chirp masses $M_{\mathrm{d}} > 100$ M$_\odot$. These systems encompass early-Universe stellar and primordial black holes, as well as intermediate-mass black-hole binaries, for which XG observatories are expected to deliver major discoveries. Validation against standard Bayesian inference demonstrates that NPE robustly reproduces complex and disconnected posterior structures across all network configurations. For a network of two misaligned L-shaped ET detectors (2L MisA), the posterior distributions on luminosity distance can become multimodal and degenerate with the sky position, leading to less precise distance estimates compared to the triangular ET configuration. However, the number of sky-location multimodalities is substantially lower than the eight expected with the triangular ET, resulting in improved sky and volume localization. Adding CE to the network further reduces sky-position degeneracies, and the better performance of the 2L MisA configuration over the triangle remains evident. △ Less

Submitted 23 December, 2025; originally announced December 2025.

Comments: 23 pages, 12 figures, 5 tables. Comments are welcome

Constraints on gravitational waves from the 2024 Vela pulsar glitch

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1752 additional authors not shown)

Abstract: Among known neutron stars, the Vela pulsar is one of the best targets for gravitational-wave searches. It is also one of the most prolific in terms of glitches, sudden frequency changes in a pulsar's rotation. Such glitches could cause a variety of transient gravitational-wave signals. Here we search for signals associated with a Vela glitch on 29 April 2024 in data of the two LIGO detectors from… ▽ More Among known neutron stars, the Vela pulsar is one of the best targets for gravitational-wave searches. It is also one of the most prolific in terms of glitches, sudden frequency changes in a pulsar's rotation. Such glitches could cause a variety of transient gravitational-wave signals. Here we search for signals associated with a Vela glitch on 29 April 2024 in data of the two LIGO detectors from the fourth LIGO--Virgo--KAGRA observing run. We search both for seconds-scale burst-like emission, primarily from fundamental (f-)mode oscillations, and for longer quasi-monochromatic transients up to four months in duration, primarily from quasi-static quadrupolar deformations. We find no significant detection candidates, but for the first time we set direct observational upper limits on gravitational strain amplitude that are stricter than what can be indirectly inferred from the overall glitch energy scale. We discuss the short- and long-duration observational constraints in the context of specific emission models. These results demonstrate the potential of gravitational-wave probes of glitching pulsars as detector sensitivity continues to improve. △ Less

Submitted 21 January, 2026; v1 submitted 19 December, 2025; originally announced December 2025.

Comments: main paper: 16 pages and 7 figures; total with appendices: 40 pages and 14 figures. Submitted to ApJ. Data release at https://doi.org/10.5281/zenodo.17735648

Report number: LIGO-P2500086

Uncovering the population of compact binary mergers and their formation pathways with gravitational waves through the Einstein Telescope

Authors: M. Arca-Sedda, I. Dvorkin, G. Franciolini, M. C. Artale, M. Branchesi, E. Bortolas, M. Colpi, V. De Luca, A. Ghosh, M. Maggiore, M. Mapelli, B. Mestichelli, M. Mezcua, S. Nissanke, L. Paiella, A. Riotto, F. Santoliquido, N. Tamanini, R. Schneider, C. Ugolini, M. P. Vaccaro, K. Yakut

Abstract: Ground-based gravitational-wave (GW) observatories have transformed our view of compact-object mergers, yet their reach still limits a comprehensive reconstruction of the processes that generate these systems. Only next-generation observatories, with order-of-magnitude improvements in sensitivity and access to lower frequencies, will be capable of radically extending this detection horizon. GW obs… ▽ More Ground-based gravitational-wave (GW) observatories have transformed our view of compact-object mergers, yet their reach still limits a comprehensive reconstruction of the processes that generate these systems. Only next-generation observatories, with order-of-magnitude improvements in sensitivity and access to lower frequencies, will be capable of radically extending this detection horizon. GW observations will make it possible to detect the complete population of binary black hole (BBH) mergers out to redshifts of $z \simeq 100$. This capability will deliver an unprecedented map of merger events across cosmic time and enable precise reconstruction of their mass and spin distributions, while for several thousand events the signal-to-noise ratio will surpass 100, enabling precision physics of BHs and neutron stars (NSs). The access to lower frequencies will also open the intermediate-mass window, detecting systems of order $\sim 10^3 M_\odot$, potentially in coordination with multi-band observations from LISA. At higher redshifts, where Population III stars have so far remained beyond reach - even for the James Webb Space Telescope - GW observations by next-generation detectors will routinely provide observations of BH mergers thought to originate from these primordial stellar populations. Such measurements are expected to play a central role in clarifying the early assembly of supermassive black holes. A single detection of a binary BH system at $z \gtrsim 30$, or of a compact object with sub-solar mass and no tidal deformability, would constitute strong evidence for the existence of primordial black holes. Such a discovery would have profound consequences for our understanding of dark matter and the early Universe. Ultimately, the GW observations will become revolutionary for identifying the physical channels responsible for compact binary formation. △ Less

Submitted 19 December, 2025; originally announced December 2025.

Comments: 4 pages. White paper submitted to the ESO Expanding Horizons Call on behalf of ET OSB Div3

ESO Expanding Horizon White Paper: Revealing the properties of matter at supranuclear densities with gravitational waves

Authors: Tim Dietrich, Tanja Hinderer, Micaela Oertel, Conrado A. Torres, Nils Andersson, Dániel Barta, Andreas Bauswein, Béatrice Bonga, Marica Branchesi, G. Fiorella Burgio, Stefano Burrello, Prasanta Char, Sylvain Chaty, Maria Colonna, Daniela Doneva, Anthea F. Fantina, Tobias Fischer, Juan Garcia-Bellido, Archisman Ghosh, Bruno Giacomazzo, Fabian Gittins, Vanessa Graber, Francesca Gulminelli, Jan Harms, Kostas Kokkotas , et al. (18 additional authors not shown)

Abstract: Understanding dense matter under extreme conditions is one of the most fundamental puzzles in modern physics. Complex interactions give rise to emergent, collective phenomena. While nuclear experiments and Earth - based colliders provide valuable insights, much of the quantum chromodynamics phase diagram at high density and low temperature remains accessible only through astrophysical observations… ▽ More Understanding dense matter under extreme conditions is one of the most fundamental puzzles in modern physics. Complex interactions give rise to emergent, collective phenomena. While nuclear experiments and Earth - based colliders provide valuable insights, much of the quantum chromodynamics phase diagram at high density and low temperature remains accessible only through astrophysical observations of neutron stars, neutron star mergers, and stellar collapse. Astronomical observations thus offer a direct window to the physics on subatomic scales with gravitational waves presenting an especially clean channel. Next-generation gravitational - wave observatories, such as the Einstein Telescope, would serve as unparalleled instruments to transform our understanding of neutron star matter. They will enable the detection of up to tens of thousands of binary neutron star and neutron star - black hole mergers per year, a dramatic increase over the few events accessible with current detectors. They will provide an unprecedented precision in probing cold, dense matter during the binary inspiral, exceeding by at least an order of magnitude what current facilities can achieve. Moreover, these observatories will allow us to explore uncharted regimes of dense matter at finite temperatures produced in a subset of neutron star mergers, areas that remain entirely inaccessible to current instruments. Together with multimessenger observations, these measurements will significantly deepen our knowledge of dense nuclear matter. △ Less

Submitted 5 January, 2026; v1 submitted 18 December, 2025; originally announced December 2025.

Comments: 5 pages, 2 figures. White paper submitted to ESO Expanding Horizons call

GWTC-4.0: Searches for Gravitational-Wave Lensing Signatures

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1744 additional authors not shown)

Abstract: Gravitational waves can be gravitationally lensed by massive objects along their path. Depending on the lens mass and the lens--source geometry, this can lead to the observation of a single distorted signal or multiple repeated events with the same frequency evolution. We present the results for gravitational-wave lensing searches on the data from the first part of the fourth LIGO--Virgo--KAGRA ob… ▽ More Gravitational waves can be gravitationally lensed by massive objects along their path. Depending on the lens mass and the lens--source geometry, this can lead to the observation of a single distorted signal or multiple repeated events with the same frequency evolution. We present the results for gravitational-wave lensing searches on the data from the first part of the fourth LIGO--Virgo--KAGRA observing run (O4a). We search for strongly lensed events in the newly acquired data by (1) searching for an overall phase shift present in an image formed at a saddle point of the lens potential, (2) looking for pairs of detected candidates with consistent frequency evolution, and (3) identifying sub-threshold counterpart candidates to the detected signals. Beyond strong lensing, we also look for lensing-induced distortions in all detected signals using an isolated point-mass model. We do not find evidence for strongly lensed gravitational-wave signals and use this result to constrain the rate of detectable strongly lensed events and the merger rate density of binary black holes at high redshift. In the search for single distorted lensed signals, we find one outlier: GW231123_135430, for which we report more detailed investigations. While this event is interesting, the associated waveform uncertainties make its interpretation complicated, and future observations of the populations of binary black holes and of gravitational lenses will help determine the probability that this event could be lensed. △ Less

Submitted 4 February, 2026; v1 submitted 18 December, 2025; originally announced December 2025.

Comments: 36 pages (including refs), 15 figures

Report number: LIGO-P2500419

Normal or transitional? The evolution and properties of two type Ia supernovae in the Virgo cluster

Authors: L. Izzo, C. Gall, N. Khetan, N. Earl, J. Hjorth, W. B. Hoogendam, Y. Q. Ni, A. Sedgewick, S. M. Ward, Y. Zenati, K. Auchettl, S. Bhattacharjee, S. Benetti, M. Branchesi, E. Cappellaro, A. Catapano, K. C. Chambers, D. A. Coulter, K. W. Davis, M. Della Valle, S. Dhawan, T. de Boer, G. Dimitriadis, R. J. Foley, M. Fulton , et al. (25 additional authors not shown)

Abstract: Type Ia supernovae (SNe Ia) are among the most precise cosmological distance indicators used to study the expansion history of the Universe. The vast increase of SN Ia data due to large-scale astrophysical surveys has led to the discovery of a wide variety of SN Ia sub-classes, such as transitional and fast-declining SNe Ia. However, their distinct photometric and spectroscopic properties differen… ▽ More Type Ia supernovae (SNe Ia) are among the most precise cosmological distance indicators used to study the expansion history of the Universe. The vast increase of SN Ia data due to large-scale astrophysical surveys has led to the discovery of a wide variety of SN Ia sub-classes, such as transitional and fast-declining SNe Ia. However, their distinct photometric and spectroscopic properties differentiate them from the population of normal SNe Ia such that their use as cosmological tools remains challenged. Here, we present a high-cadenced photometric and spectroscopic dataset of two SNe Ia, SNe 2020ue and 2020nlb, which were discovered in the nearby Virgo cluster of galaxies. Our study shows that SN 2020nlb is a normal SN Ia whose unusually red color is intrinsic, arising from a lower photospheric temperature rather than interstellar reddening, providing clear evidence that color diversity among normal SNe Ia can have a physical origin. In contrast, SN 2020ue has photometric properties, such as color evolution and light-curve decay rate, similar to those of transitional SNe, spectroscopically it is more aligned with normal SNe Ia. This is evident from spectroscopic indicators such as the pseudo-equivalent width of \ion{Si}{II} lines. Thus, such SNe Ia that are photometrically at the edge of the standard normal SNe Ia range may be missed in cosmological SNe Ia samples. Our results highlight that spectroscopic analysis of SNe Ia around peak brightness is crucial for identifying intrinsic color variations and constructing a more complete and physically homogeneous SN Ia sample for precision cosmology. △ Less

Submitted 29 November, 2025; originally announced December 2025.

Comments: 24 pages, 21 figures, 7 tables. Accepted for publication in A&A

Journal ref: A&A 706, A381 (2026)

Search for planetary-mass ultra-compact binaries using data from the first part of the LIGO--Virgo--KAGRA fourth observing run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1743 additional authors not shown)

Abstract: We present a search for gravitational waves from inspiraling, planetary-mass ultra-compact binaries using data from the first part of the fourth observing run of LIGO, Virgo and KAGRA. Finding no evidence of such systems, we determine the maximum distance reach for such objects and their merger rate densities, independently of how they could have formed. Then, we identify classes of primordial bla… ▽ More We present a search for gravitational waves from inspiraling, planetary-mass ultra-compact binaries using data from the first part of the fourth observing run of LIGO, Virgo and KAGRA. Finding no evidence of such systems, we determine the maximum distance reach for such objects and their merger rate densities, independently of how they could have formed. Then, we identify classes of primordial black-hole mass distributions for which these rate limits can be translated into relevant constraints on the mass distribution of primordial black holes, assuming that they compose all of dark matter, in the mass range $[10^{-6},10^{-3}]M_\odot$. Our constraints are consistent with existing microlensing results in the planetary-mass range, and provide a complementary probe to sub-solar mass objects. △ Less

Submitted 5 December, 2025; v1 submitted 24 November, 2025; originally announced November 2025.

Comments: 6 pages (main) + 7 pages (appendix) + refs; 8 figures

Report number: P2500248

All-sky search for continuous gravitational-wave signals from unknown neutron stars in binary systems in the first part of the fourth LIGO-Virgo-KAGRA observing run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1743 additional authors not shown)

Abstract: We present the results of a blind all-sky search for continuous gravitational-wave signals from neutron stars in binary systems using data from the first part of the fourth observing run (O4a) using LIGO detectors data. Rapidly rotating, non-axisymmetric neutron stars are expected to emit continuous gravitational waves, whose detection would significantly improve our understanding of the galactic… ▽ More We present the results of a blind all-sky search for continuous gravitational-wave signals from neutron stars in binary systems using data from the first part of the fourth observing run (O4a) using LIGO detectors data. Rapidly rotating, non-axisymmetric neutron stars are expected to emit continuous gravitational waves, whose detection would significantly improve our understanding of the galactic neutron star population and matter under extreme conditions, while also providing valuable tests of general relativity. Neutron stars in binary systems likely constitute a substantial fraction of the unobserved galactic population and, due to potential mass accretion, may emit stronger gravitational-wave signals than their isolated counterparts. This search targets signals from neutron stars with frequencies in the 100-350 Hz range, with orbital periods between 7 and 15 days and projected semi-major axes between 5 and 15 light-seconds. The analysis employs the GPU-accelerated fasttracks pipeline. No credible astrophysical signals were identified, and, in the absence of a detection, we report search sensitivity estimates on the population of neutron stars in binary systems in the Milky Way. △ Less

Submitted 4 December, 2025; v1 submitted 20 November, 2025; originally announced November 2025.

Comments: 24 pages, 18 figures, 6 tables

Report number: LIGO-P2500437

The Advanced X-ray Imaging Satellite (AXIS) Community Science Book

Authors: Michael Koss, Nafisa Aftab, Steven W. Allen, Roberta Amato, Hongjun An, Igor Andreoni, Timo Anguita, Riccardo Arcodia, Thomas Ayres, Matteo Bachetti, Maria Cristina Baglio, Arash Bahramian, Marco Balboni, Ranieri D. Baldi, Solen Balman, Aya Bamba, Eduardo Banados, Tong Bao, Iacopo Bartalucci, Antara Basu-Zych, Rebeca Batalha, Lorenzo Battistini, Franz Erik Bauer, Andy Beardmore, Werner Becker , et al. (373 additional authors not shown)

Abstract: The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with ~1.5'' imaging over a wide 24' field of… ▽ More The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with ~1.5'' imaging over a wide 24' field of view and an order of magnitude greater collecting area than Chandra in the 0.3-12 keV band. Combining sharp imaging, high throughput, and rapid response capabilities, AXIS will open new windows on virtually every aspect of modern astrophysics, exploring the birth and growth of supermassive black holes, the feedback processes that shape galaxies, the life cycles of stars and exoplanet environments, and the nature of compact stellar remnants, supernova remnants, and explosive transients. This book compiles 138 community-contributed science cases developed by five Science Working Groups focused on AGN and supermassive black holes, galaxy evolution and feedback, compact objects and supernova remnants, stellar physics and exoplanets, and time-domain and multi-messenger astrophysics. Together, these studies establish the scientific foundation for next-generation X-ray exploration in the 2030s and highlight strong synergies with facilities of the 2030s, such as JWST, Roman, Rubin/LSST, SKA, ALMA, ngVLA, and next-generation gravitational-wave and neutrino networks. △ Less

Submitted 6 January, 2026; v1 submitted 31 October, 2025; originally announced November 2025.

Comments: 592 pages, 226 figures, version 2

Direct multi-model dark-matter search with gravitational-wave interferometers using data from the first part of the fourth LIGO-Virgo-KAGRA observing run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1745 additional authors not shown)

Abstract: Gravitational-wave detectors can probe the existence of dark matter with exquisite sensitivity. Here, we perform a search for three kinds of dark matter -- dilatons (spin-0), dark photons (spin-1) and tensor bosons (spin-2) -- using three independent methods on the first part of the most recent data from the fourth observing run of LIGO--Virgo--KAGRA. Each form of dark matter could have interacted… ▽ More Gravitational-wave detectors can probe the existence of dark matter with exquisite sensitivity. Here, we perform a search for three kinds of dark matter -- dilatons (spin-0), dark photons (spin-1) and tensor bosons (spin-2) -- using three independent methods on the first part of the most recent data from the fourth observing run of LIGO--Virgo--KAGRA. Each form of dark matter could have interacted with different standard-model particles in the instruments, causing unique differential strains on the interferometers. While we do not find any evidence for a signal, we place the most stringent upper limits to-date on each of these models. For scalars with masses between $[4\times 10^{-14},1.5\times 10^{-13}]$ eV that couple to photons or electrons, our constraints improve upon those from the third observing run by one order of magnitude, with the tightest limit of $\sim 10^{-20}\,\text{GeV}^{-1}$ at a mass of $\sim2\times 10^{-13}\text{ eV}$. For vectors with masses between $[7\times 10^{-13},8.47\times 10^{-12}]$ eV that couple to baryons, our constraints supersede those from MICROSCOPE and Eöt-Wash by one to two orders of magnitude, reaching a minimum of $\sim 5\times 10^{-24}$ at a mass of $\sim 10^{-12}$ eV. For tensors with masses of $[4\times 10^{-14},8.47\times 10^{-12}]$ eV (the full mass range analyzed) that couple via a Yukawa interaction, our constraints surpass those from fifth-force experiments by four to five orders of magnitude, achieving a limit as low as $\sim 8\times 10^{-9}$ at $\sim2\times 10^{-13}$ eV. Our results show that gravitational-wave interferometers have become frontiers for new physics and laboratories for direct multi-model dark-matter detection. △ Less

Submitted 11 December, 2025; v1 submitted 30 October, 2025; originally announced October 2025.

Comments: 9 pages, 1 figure (+1 figure in appendix)

Report number: P2500252

GW241011 and GW241110: Exploring Binary Formation and Fundamental Physics with Asymmetric, High-Spin Black Hole Coalescence

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1761 additional authors not shown)

Abstract: We report the observation of gravitational waves from two binary black hole coalescences during the fourth observing run of the LIGO--Virgo--KAGRA detector network, GW241011 and GW241110. The sources of these two signals are characterized by rapid and precisely measured primary spins, non-negligible spin--orbit misalignment, and unequal mass ratios between their constituent black holes. These prop… ▽ More We report the observation of gravitational waves from two binary black hole coalescences during the fourth observing run of the LIGO--Virgo--KAGRA detector network, GW241011 and GW241110. The sources of these two signals are characterized by rapid and precisely measured primary spins, non-negligible spin--orbit misalignment, and unequal mass ratios between their constituent black holes. These properties are characteristic of binaries in which the more massive object was itself formed from a previous binary black hole merger, and suggest that the sources of GW241011 and GW241110 may have formed in dense stellar environments in which repeated mergers can take place. As the third loudest gravitational-wave event published to date, with a median network signal-to-noise ratio of $36.0$, GW241011 furthermore yields stringent constraints on the Kerr nature of black holes, the multipolar structure of gravitational-wave generation, and the existence of ultralight bosons within the mass range $10^{-13}$--$10^{-12}$ eV. △ Less

Submitted 30 October, 2025; originally announced October 2025.

Comments: Data available from Zenodo (https://zenodo.org/records/17343574) or the Gravitational-Wave Open Science Center (https://gwosc.org)

Report number: LIGO-P2500402

Journal ref: Astrophys. J. Letters, 993, L21 (2025)

Cosmological and High Energy Physics implications from gravitational-wave background searches in LIGO-Virgo-KAGRA's O1-O4a runs

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1747 additional authors not shown)

Abstract: We search for gravitational-wave background signals produced by various early Universe processes in the Advanced LIGO O4a dataset, combined with the data from the earlier O1, O2, and O3 (LIGO-Virgo) runs. The absence of detectable signals enables powerful constraints on fundamental physics. We derive gravitational-wave background energy density upper limits from the O1-O4a data to constrain parame… ▽ More We search for gravitational-wave background signals produced by various early Universe processes in the Advanced LIGO O4a dataset, combined with the data from the earlier O1, O2, and O3 (LIGO-Virgo) runs. The absence of detectable signals enables powerful constraints on fundamental physics. We derive gravitational-wave background energy density upper limits from the O1-O4a data to constrain parameters associated with various possible processes in the early Universe: first-order phase transitions, cosmic strings, domain walls, stiff equation of state, axion inflation, second-order scalar perturbations, primordial black hole binaries, and parity violation. In our analyses, the presence of an astrophysical background produced by compact (black hole and neutron star) binary coalescences throughout the Universe is also considered. We address the implications for various cosmological and high energy physics models based on the obtained parameter constraints. We conclude that LIGO-Virgo data already yield significant constraints on numerous early Universe scenarios. △ Less

Submitted 7 November, 2025; v1 submitted 30 October, 2025; originally announced October 2025.

Comments: Main paper: 45 pages, 20 figures

Report number: LIGO-PP2500150

Directional Search for Persistent Gravitational Waves: Results from the First Part of LIGO-Virgo-KAGRA's Fourth Observing Run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1743 additional authors not shown)

Abstract: The angular distribution of gravitational-wave power from persistent sources may exhibit anisotropies arising from the large-scale structure of the Universe. This motivates directional searches for astrophysical and cosmological gravitational-wave backgrounds, as well as continuous-wave emitters. We present results of such a search using data from the first observing run through the first portion… ▽ More The angular distribution of gravitational-wave power from persistent sources may exhibit anisotropies arising from the large-scale structure of the Universe. This motivates directional searches for astrophysical and cosmological gravitational-wave backgrounds, as well as continuous-wave emitters. We present results of such a search using data from the first observing run through the first portion of the fourth observing run of the LIGO-Virgo-KAGRA Collaborations. We apply gravitational-wave radiometer techniques to generate skymaps and search for both narrowband and broadband persistent gravitational-wave sources. Additionally, we use spherical harmonic decomposition to probe spatially extended sources. No evidence of persistent gravitational-wave signals is found, and we set the most stringent constraints to date on such emissions. For narrowband point sources, our sensitivity estimate to effective strain amplitude lies in the range $(0.03 - 8.4) \times 10^{-24}$ across all sky and frequency range $(20 - 160)$ Hz. For targeted sources -- Scorpius X-1, SN 1987A, the Galactic Center, Terzan 5, and NGC 6397 -- we constrain the strain amplitude with best limits ranging from $\sim 1.1 \times 10^{-25}$ to $6.5 \times 10^{-24}$. For persistent broadband sources, we constrain the gravitational-wave flux $F_{α, \hat{n}}^{95\%, \mathrm{UL}}(25\, \mathrm{Hz}) < (0.008 - 5.5) \times 10^{-8}\, \mathrm{erg\, cm^{-2}\, s^{-1}\, Hz^{-1}}$, depending on the sky direction $\hat{n}$ and spectral index $α=0,\,2/3,\,3$. Finally, for extended sources, we place upper limits on the strain angular power spectrum $C_\ell^{1/2} < (0.63 - 17) \times 10^{-10} \,\mathrm{sr}^{-1}$. △ Less

Submitted 20 October, 2025; originally announced October 2025.

Comments: Main paper: 11 pages and 4 figures; Total with appendices: 39 pages and 12 figures

Report number: LIGO-P250038

Early X-ray emission of short Gamma-Ray Bursts: insights into physics and multi-messenger prospects

Authors: Annarita Ierardi, Gor Oganesyan, Stefano Ascenzi, Marica Branchesi, Biswajit Banerjee, Samuele Ronchini

Abstract: Early X-ray emission of Gamma-Ray Bursts (GRBs) traces the transition between the prompt emission and the afterglow radiation, and its rapid flux decline is often interpreted as the tail of the prompt emission. As such, it can offer insights into the emission mechanisms active during the prompt emission and the physics of GRB jets. In this work, we focus on merger-driven GRBs, which are sources of… ▽ More Early X-ray emission of Gamma-Ray Bursts (GRBs) traces the transition between the prompt emission and the afterglow radiation, and its rapid flux decline is often interpreted as the tail of the prompt emission. As such, it can offer insights into the emission mechanisms active during the prompt emission and the physics of GRB jets. In this work, we focus on merger-driven GRBs, which are sources of gravitational waves (GWs) detectable by ground-based interferometers, such as LIGO, Virgo, and KAGRA. We present a systematic analysis of the early X-ray emission ($t < 10^3 \ \mathrm{s}$) of a sample of 16 merger-driven GRB candidates detected by the Neil Gehrels Swift Observatory (hereafter, Swift). We performed a time-resolved spectral analysis of soft and hard X-ray data (0.3-150 keV) by fitting two curved spectral models to the spectra: a physical synchrotron model and an empirical smoothly broken power law model. We characterized the evolution of the peak energy and bolometric flux, and derived the intrinsic properties of the 10 bursts with measured redshift. We discovered a tight correlation between the rest-frame peak energy of the spectra and the isotropic-equivalent luminosity. Specifically, we obtained $ν_{c,z} \propto L_{\rm iso}^{(0.64 \pm 0.03)}$ when adopting the synchrotron model, and $E_{p,z} \propto L_{\rm iso}^{(0.58 \pm 0.04)}$ when adopting the smoothly broken power law. Both relations were extrapolated to the typical prompt emission energies and well describe the properties of short GRBs detected in the MeV gamma-rays. These results suggest a common origin for the prompt and steep-decay emissions in merger-driven GRBs, and rule out high-latitude emission as the dominant process shaping the early X-ray tails. Finally, we assessed the detectability of these sources with the Wide-field X-ray Telescope onboard the Einstein Probe mission. △ Less

Submitted 17 October, 2025; originally announced October 2025.

Comments: Submitted to A&A

Journal ref: A&A 708, A190 (2026)

Encapsulating Textual Contents into a MOC data Structure for Advanced Applications

Authors: Giuseppe Greco, Thomas Boch, Pierre Fernique, Manon Marchand, Mark Allen, Francois Xavier Pineau, Matthieu Baumann, Marco Molinaro, Roberto De Pietri, Marica Branchesi, Steven Schramm, Gergely Dalya, Elahe Khalouei, Barbara Patricelli, Giulia Stratta

Abstract: Context. The Multi-Order Coverage map (MOC) is a widely adopted standard promoted by the International Virtual Observatory Alliance (IVOA) to support data sharing and interoperability within the Virtual Observatory (VO) ecosystem. This hierarchical data structure efficiently encodes and visualizes irregularly shaped regions of the sky, enabling applications such as cross-matching large astronomica… ▽ More Context. The Multi-Order Coverage map (MOC) is a widely adopted standard promoted by the International Virtual Observatory Alliance (IVOA) to support data sharing and interoperability within the Virtual Observatory (VO) ecosystem. This hierarchical data structure efficiently encodes and visualizes irregularly shaped regions of the sky, enabling applications such as cross-matching large astronomical catalogs. Aims. This study aims to explore potential enhancements to the MOC data structure by encapsulating textual descriptions and semantic embeddings into sky regions. Specifically, we introduce "Textual MOCs", in which textual content is encapsulated, and "Semantic MOCs" that transform textual content into semantic embeddings. These enhancements are designed to enable advanced operations such as similarity searches and complex queries and to integrate with generative artificial intelligence (GenAI) tools. Method. We experimented with Textual MOCs by annotating detailed descriptions directly into the MOC sky regions, enriching the maps with contextual information suitable for interactive learning tools. For Semantic MOCs, we converted the textual content into semantic embeddings, numerical representations capturing textual meanings in multidimensional spaces, and stored them in high-dimensional vector databases optimized for efficient retrieval. Results. The implementation of Textual MOCs enhances user engagement by providing meaningful descriptions within sky regions. Semantic MOCs enable sophisticated query capabilities, such as similarity-based searches and context-aware data retrieval. Integration with multimodal generative AI systems allows for more accurate and contextually relevant interactions supporting both spatial, semantic and visual operations for advancing astronomical data analysis capabilities. △ Less

Submitted 14 October, 2025; originally announced October 2025.

Comments: Published in Astronomy and Computing; 11 pages, 4 figures

Journal ref: Astron. Comput. 54 (2026) 101014

From X-rays to High-Energy Gamma-rays: A Comprehensive Multi-Wavelength Study of Early Gamma-Ray Burst Afterglows

Authors: P. Tiwari, B. Banerjee, D. Miceli, G. Oganesyan, A. Ierardi, S. Macera, M. Branchesi, L. Nava, S. Mohnani, S. Agarwal, A. Shukla

Abstract: Gamma-ray Bursts (GRBs) generate powerful relativistic jets that inject a large amount of energy into their surrounding environment, producing blast waves that accelerate particles to high energies. The GRB afterglow radiation provides a powerful means to investigate the microphysics of relativistic shocks and to probe the medium surrounding the progenitor of the burst. In this study, we present a… ▽ More Gamma-ray Bursts (GRBs) generate powerful relativistic jets that inject a large amount of energy into their surrounding environment, producing blast waves that accelerate particles to high energies. The GRB afterglow radiation provides a powerful means to investigate the microphysics of relativistic shocks and to probe the medium surrounding the progenitor of the burst. In this study, we present a comprehensive multiwavelength analysis of 31 GRBs observed between 2008 and 2024 from the Neil Gehrels Swift Observatory (X-ray Telescope and Burst Alert Telescope) and the Fermi Large Area Telescope, covering photon energies from 0.3 keV to 300 GeV. Our goal is to characterize the broadband spectral properties of GRB afterglows in soft X-rays, hard X-rays, and high-energy gamma rays. We investigate correlations between spectral shape and energy output across different parts of the spectrum. The observed emission is modeled using a forward shock scenario that includes both synchrotron and synchrotron self-Compton (SSC) radiation losses. The results favor an SSC-dominated radiation model in a wind-like medium, consistent with expectations for long-duration GRBs. Crucially, this work provides new benchmarks for the microphysical parameters governing the emission, particularly indicating a notably low magnetic energy fraction, which refines previous estimates. By modeling broadband data, this study offers one of the most detailed SSC analyses in a wind-like environment to date. Notably, our results naturally account for the comparable energy output observed in both the soft X-ray and TeV bands, consistent with the previously detected TeV-GRBs. △ Less

Submitted 23 February, 2026; v1 submitted 6 October, 2025; originally announced October 2025.

Comments: Comments are welcome, to be submitted soon

Binary black holes in magnetized AGN disks

Authors: Raj Kishor Joshi, Aryan Bhake, Biswajit Banerjee, Bhargav Vaidya, Milton Ruiz, Antonios Tsokaros, Andrea Mignone, Marica Branchesi, Amit Shukla, Miljenko Čemeljić

Abstract: Stellar-mass binary black hole (BBH) mergers occurring within the disks of active galactic nuclei (AGN) are promising sources for gravitational waves detectable by the LIGO, Virgo, and KAGRA (LVK) interferometers. Some of these events have also been potentially associated with transient electromagnetic flares, indicating that BBH mergers in dense environments may be promising sources of multi-mess… ▽ More Stellar-mass binary black hole (BBH) mergers occurring within the disks of active galactic nuclei (AGN) are promising sources for gravitational waves detectable by the LIGO, Virgo, and KAGRA (LVK) interferometers. Some of these events have also been potentially associated with transient electromagnetic flares, indicating that BBH mergers in dense environments may be promising sources of multi-messenger signals. To investigate the prospects for electromagnetic emission from these systems, we study the dynamics of accretion flows onto BBHs embedded in AGN disks using numerical simulations. Although recent studies have explored this scenario, they often employ simplified disk models that neglect magnetic fields. In this work, we examine how strong magnetic fields influence and regulate the accretion onto such binary systems. In this context, we conduct three-dimensional magnetohydrodynamical local shearing-box simulations of a binary black hole system embedded within a magnetized disk of an AGN. We observe that the dynamically important magnetic fields can drive the formation of well-collimated outflows capable of penetrating the vertical extent of the AGN disk. However, outflow generation is not ubiquitous and strongly depends on the radial distance of the binary from the supermassive black hole (SMBH). In particular, binaries placed at a larger distance from the central SMBH show relatively more transient accretion and the formation of stronger spiral shocks. Furthermore, accretion behavior onto the binary system via individual circum-singular disks (CSDs) is also modulated by local AGN disk properties. Our simulations highlight the importance of shear velocity in the amplification of the toroidal magnetic field component, which plays a crucial role in governing the outflow strength. △ Less

Submitted 20 September, 2025; originally announced September 2025.

Comments: Accepted for publication in A&A

Journal ref: A&A 703, A304 (2025)

Looking into the faintEst WIth MUSE (LEWIS): Exploring the nature of ultra-diffuse galaxies in the Hydra-I cluster IV. A study of the Globular Cluster population in four UDGs

Authors: Marco Mirabile, Michele Cantiello, Marina Rejkuba, Steffen Mieske, Enrichetta Iodice, Chiara Buttitta, Maria Luisa Buzzo, Johanna Hartke, Goran Doll, Luca Rossi, Magda Arnaboldi, Marica Branchesi, Giuseppe D'Ago, Jesus Falcon-Barroso, Katja Fahrion, Duncan A. Forbes, Marco Gullieuszik, Michael Hilker, Felipe S. Lohmann, Maurizio Paolillo, Gabriele Riccio, Tom Richtler, Marilena Spavone

Abstract: As old stellar systems, globular clusters (GCs) are key fossil tracers of galaxy formation and interaction histories. This paper is part of the LEWIS project, an integral-field spectroscopic survey of ultra-diffuse galaxies (UDGs) in the Hydra I cluster. We use MUSE spectroscopy and new VIRCAM $H$-band imaging data to study the GC populations and dark matter content in four dwarf galaxies. We retr… ▽ More As old stellar systems, globular clusters (GCs) are key fossil tracers of galaxy formation and interaction histories. This paper is part of the LEWIS project, an integral-field spectroscopic survey of ultra-diffuse galaxies (UDGs) in the Hydra I cluster. We use MUSE spectroscopy and new VIRCAM $H$-band imaging data to study the GC populations and dark matter content in four dwarf galaxies. We retrieved line-of-sight velocities for all sources in the observed MUSE fields. Since the spectroscopic measurements are limited to relatively bright sources, we developed a multi-band photometric procedure to identify additional GC candidates too faint for spectroscopic confirmation. GC candidates were selected using a combination of photometric properties and morphometric criteria. Additionally, the $H$-band observations were used to constrain the stellar masses of the studied galaxies. Based on the spectroscopic classification, we confirm one GC in UDG3, two in UDG7, and four in UDG11, while UDG9 has no spectroscopically confirmed bright GCs. We identify four intra-cluster GCs in the vicinity of UDG3 and UDG11, and one ultra-compact dwarf with a radial velocity only $Δv = -85 \pm 10\mathrm{km\ s^{-1}}$ relative to UDG7, suggesting it may be bound to it. Considering completeness corrections and accounting for possible contamination, from photometry we estimate that the number of GCs ranges between 0 and $\sim40$ for the investigated UDGs. Their specific frequencies suggest that three out of four UDGs are either GC-rich, similar to those in the Coma cluster, or belong to an intermediate population as seen in the Perseus cluster. Dark matter content estimates, inferred from GC counts and stellar mass, indicate that these galaxies are dark-matter dominated, with dynamical-to-stellar mass ratios of $M_{\mathrm{dyn}} / M_\star \sim 10-1000$. △ Less

Submitted 17 September, 2025; originally announced September 2025.

Comments: 25 pages, 24 figures, 7 tables. Accepted for publication in A&A

Journal ref: A&A 705, A117 (2026)

Assembling GW231123 in star clusters through the combination of stellar binary evolution and hierarchical mergers

Authors: Lavinia Paiella, Cristiano Ugolini, Mario Spera, Marica Branchesi, Manuel Arca Sedda

Abstract: GW231123 is the most massive binary black hole (BBH) merger detected to date by the LIGO-Virgo-KAGRA collaboration. With at least one black hole (BH) in the upper-mass gap and both BHs exhibiting high spins ($χ_{1,2} \gtrsim 0.8$), this event challenges standard isolated binary evolution models. A compelling alternative is a dynamical origin in star clusters, where stellar binaries and hierarchica… ▽ More GW231123 is the most massive binary black hole (BBH) merger detected to date by the LIGO-Virgo-KAGRA collaboration. With at least one black hole (BH) in the upper-mass gap and both BHs exhibiting high spins ($χ_{1,2} \gtrsim 0.8$), this event challenges standard isolated binary evolution models. A compelling alternative is a dynamical origin in star clusters, where stellar binaries and hierarchical mergers may both contribute to the formation of similar BBHs. In this work, we investigate the formation of GW231123-like events in different cluster environments using the B-POP semi-analytic population synthesis code. We find that low-metallicity environments ($Z \lesssim 0.002$) are ideal for producing BBH mergers similar to GW231123. In young and globular clusters, these BBHs have components formed in stellar binaries, whilst in nuclear clusters there is also a significant contribution from BHs built-up via hierarchical mergers. Natal spins of BHs formed in stellar binaries are crucial to find GW231123 analogs. In particular, our models suggest that BHs from stellar binaries are likely characterized by high-spins. Simulated GW231123-like systems exhibit short delay times, $t_\mathrm{del} \sim 0.1 - 1$ Gyr, which suggests their progenitors formed close to the inferred merger redshift ($z = 0.39^{+0.27}_{-0.24}$). We argue that star clusters in metal-poor dwarf galaxies or Milky Way-like galaxies are ideal nurseries, inferring an upper limit to the local merger rate of $\mathcal{R} \sim 1.6\times10^{-3} - 0.16$ yr$^{-1}$ Gpc$^{-3}$ for nuclear clusters, $\sim 0.036 - 0.72$ yr$^{-1}$ Gpc$^{-3}$ for globular clusters, and $4\times10^{-4}-0.041$ yr$^{-1}$ Gpc$^{-3}$ for young clusters. △ Less

Submitted 12 September, 2025; originally announced September 2025.

Comments: 11 pages, 6 figure, 1 table

GW250114: testing Hawking's area law and the Kerr nature of black holes

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1763 additional authors not shown)

Abstract: The gravitational-wave signal GW250114 was observed by the two LIGO detectors with a network matched-filter signal-to-noise ratio of 80. The signal was emitted by the coalescence of two black holes with near-equal masses $m_1 = 33.6^{+1.2}_{-0.8}\,M_\odot$ and $m_2 = 32.2^{+0.8}_{-1.3}\,M_\odot$, and small spins $χ_{1,2} \leq 0.26$ (90% credibility) and negligible eccentricity $e \leq 0.03$. Post-… ▽ More The gravitational-wave signal GW250114 was observed by the two LIGO detectors with a network matched-filter signal-to-noise ratio of 80. The signal was emitted by the coalescence of two black holes with near-equal masses $m_1 = 33.6^{+1.2}_{-0.8}\,M_\odot$ and $m_2 = 32.2^{+0.8}_{-1.3}\,M_\odot$, and small spins $χ_{1,2} \leq 0.26$ (90% credibility) and negligible eccentricity $e \leq 0.03$. Post-merger data excluding the peak region are consistent with the dominant quadrupolar $(\ell = |m| = 2)$ mode of a Kerr black hole and its first overtone. We constrain the modes' frequencies to $\pm 30\%$ of the Kerr spectrum, providing a test of the remnant's Kerr nature. We also examine Hawking's area law, also known as the second law of black hole mechanics, which states that the total area of the black hole event horizons cannot decrease with time. A range of analyses that exclude up to 5 of the strongest merger cycles confirm that the remnant area is larger than the sum of the initial areas to high credibility. △ Less

Submitted 9 September, 2025; originally announced September 2025.

Comments: 6 pages, 5 figures (plus supplement)

Report number: LIGO-P2500421

Observing Double White Dwarfs with the Lunar GW Antenna

Authors: Giovanni Benetti, Marica Branchesi, Jan Harms, Jean-Pierre Zendri

Abstract: The Lunar Gravitational Wave Antenna (LGWA) is a proposed gravitational-wave detector that will observe in the decihertz (dHz) frequency region. In this band, binary white dwarf systems are expected to merge, emitting gravitational waves. Detecting this emission opens new perspectives for understanding the Type Ia supernova progenitors and for investigating dense matter physics. In this paper, we… ▽ More The Lunar Gravitational Wave Antenna (LGWA) is a proposed gravitational-wave detector that will observe in the decihertz (dHz) frequency region. In this band, binary white dwarf systems are expected to merge, emitting gravitational waves. Detecting this emission opens new perspectives for understanding the Type Ia supernova progenitors and for investigating dense matter physics. In this paper, we present the capabilities of LGWA to detect and localize short-period double white dwarfs in terms of sky locations and distances. The analysis employs realistic spatial distributions and merger rates, as well as binary-mass distributions informed by population-synthesis models. The simulated population of double white dwarfs is generated using the SeBa stellar-evolution code, coupled with dedicated sampling algorithms. The performance of the LGWA detector, both in terms of signal detectability and parameter estimation, is assessed using standard gravitational-wave data analysis techniques, including Fisher matrix methods, as implemented in the GWFish and Legwork codes. The analysis indicates that, over 10 years of observation, LGWA could detect approximately 30 monochromatic Galactic sources and 10 extragalactic mergers, demonstrating the unique potential of decihertz gravitational-wave detectors to access and characterize extragalactic DWD populations. This will open new avenues for understanding Type Ia supernova progenitors and the physics of DWDs. △ Less

Submitted 22 December, 2025; v1 submitted 9 September, 2025; originally announced September 2025.

Comments: 20 pages, 13 figures, 4 tables. Paper restructured with unchanged results. Appendices and references added

Directed searches for gravitational waves from ultralight vector boson clouds around merger remnant and galactic black holes during the first part of the fourth LIGO-Virgo-KAGRA observing run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1747 additional authors not shown)

Abstract: We present the first directed searches for long-transient and continuous gravitational waves from ultralight vector boson clouds around known black holes (BHs). We use LIGO data from the first part of the fourth LIGO-Virgo-KAGRA observing run. The searches target two distinct types of BHs and use two new semicoherent methods: hidden Markov model (HMM) tracking for the remnant BHs of the mergers GW… ▽ More We present the first directed searches for long-transient and continuous gravitational waves from ultralight vector boson clouds around known black holes (BHs). We use LIGO data from the first part of the fourth LIGO-Virgo-KAGRA observing run. The searches target two distinct types of BHs and use two new semicoherent methods: hidden Markov model (HMM) tracking for the remnant BHs of the mergers GW230814_230901 and GW231123_135430 (referred to as GW230814 and GW231123 in this study), and a dedicated method using the Band Sampled Data (BSD) framework for the galactic BH in the Cygnus X-1 binary system. Without finding evidence of a signal from vector bosons in the data, we estimate the mass range that can be constrained. For the HMM searches targeting the remnants from GW231123 and GW230814, we disfavor vector boson masses in the ranges $[0.94, 1.08]$ and $[2.75, 3.28] \times 10^{-13}$ eV, respectively, at 30% confidence, assuming a 1% false alarm probability. Although these searches are only marginally sensitive to signals from merger remnants at relatively large distances, future observations are expected to yield more stringent constraints with high confidence. For the BSD search targeting the BH in Cygnus X-1, we exclude vector boson masses in the range $[0.85, 1.59] \times 10^{-13}$ eV at 95% confidence, assuming an initial BH spin larger than 0.5. △ Less

Submitted 14 September, 2025; v1 submitted 8 September, 2025; originally announced September 2025.

Comments: 33 pages, 4 figures

Report number: LIGO-P2500256

GWTC-4.0: Constraints on the Cosmic Expansion Rate and Modified Gravitational-wave Propagation

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1750 additional authors not shown)

Abstract: We analyze data from 142 of the 218 gravitational-wave (GW) sources in the fourth LIGO-Virgo-KAGRA Collaboration (LVK) Gravitational-Wave Transient Catalog (GWTC-4.0) to estimate the Hubble constant $H_0$ jointly with the population properties of merging compact binaries. We measure the luminosity distance and redshifted masses of GW sources directly; in contrast, we infer GW source redshifts stat… ▽ More We analyze data from 142 of the 218 gravitational-wave (GW) sources in the fourth LIGO-Virgo-KAGRA Collaboration (LVK) Gravitational-Wave Transient Catalog (GWTC-4.0) to estimate the Hubble constant $H_0$ jointly with the population properties of merging compact binaries. We measure the luminosity distance and redshifted masses of GW sources directly; in contrast, we infer GW source redshifts statistically through i) location of features in the compact object mass spectrum and merger rate evolution, and ii) identifying potential host galaxies in the GW localization volume. Probing the relationship between source luminosity distances and redshifts obtained in this way yields constraints on cosmological parameters. We also constrain parameterized deviations from general relativity which affect GW propagation, specifically those modifying the dependence of a GW signal on the source luminosity distance. Assuming our fiducial model for the source-frame mass distribution and using GW candidates detected up to the end of the fourth observing run (O4a), together with the GLADE+ all-sky galaxy catalog, we estimate $H_0 = 76.6^{+13.0}_{-9.5} (76.6^{+25.2}_{-14.0})$ km s$^{-1}$ Mpc$^{-1}$. This value is reported as a median with 68.3% (90%) symmetric credible interval, and includes combination with the $H_0$ measurement from GW170817 and its electromagnetic counterpart. Using a parametrization of modified GW propagation in terms of the magnitude parameter $Ξ_0$, we estimate $Ξ_0 = 1.2^{+0.8}_{-0.4} (1.2^{+2.4}_{-0.5})$, where $Ξ_0 = 1$ recovers the behavior of general relativity. △ Less

Submitted 7 October, 2025; v1 submitted 4 September, 2025; originally announced September 2025.

Comments: As part of the Astrophysical Journal Letters Focus Issue on the Gravitational Wave Transient Catalog

Report number: LIGO-P2400152

Upper Limits on the Isotropic Gravitational-Wave Background from the first part of LIGO, Virgo, and KAGRA's fourth Observing Run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1751 additional authors not shown)

Abstract: We present results from the search for an isotropic gravitational-wave background using Advanced LIGO and Advanced Virgo data from O1 through O4a, the first part of the fourth observing run. This background is the accumulated signal from unresolved sources throughout cosmic history and encodes information about the merger history of compact binaries throughout the Universe, as well as exotic physi… ▽ More We present results from the search for an isotropic gravitational-wave background using Advanced LIGO and Advanced Virgo data from O1 through O4a, the first part of the fourth observing run. This background is the accumulated signal from unresolved sources throughout cosmic history and encodes information about the merger history of compact binaries throughout the Universe, as well as exotic physics and potentially primordial processes from the early cosmos. Our cross-correlation analysis reveals no statistically significant background signal, enabling us to constrain several theoretical scenarios. For compact binary coalescences which approximately follow a 2/3 power-law spectrum, we constrain the fractional energy density to $Ω_{\rm GW}(25{\rm Hz})\leq 2.0\times 10^{-9}$ (95% cred.), a factor of 1.7 improvement over previous results. Scale-invariant backgrounds are constrained to $Ω_{\rm GW}(25{\rm Hz})\leq 2.8\times 10^{-9}$, representing a 2.1x sensitivity gain. We also place new limits on gravity theories predicting non-standard polarization modes and confirm that terrestrial magnetic noise sources remain below detection threshold. Combining these spectral limits with population models for GWTC-4, the latest gravitational-wave event catalog, we find our constraints remain above predicted merger backgrounds but are approaching detectability. The joint analysis combining the background limits shown here with the GWTC-4 catalog enables improved inference of the binary black hole merger rate evolution across cosmic time. Employing GWTC-4 inference results and standard modeling choices, we estimate that the total background arising from compact binary coalescences is $Ω_{\rm CBC}(25{\rm Hz})={0.9^{+1.1}_{-0.5}\times 10^{-9}}$ at 90% confidence, where the largest contribution is due to binary black holes only, $Ω_{\rm BBH}(25{\rm Hz})=0.8^{+1.1}_{-0.5}\times 10^{-9}$. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Comments: 31 pages, 7 figures

Report number: LIGO-P2500349

GWTC-4.0: Population Properties of Merging Compact Binaries

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, D. Agarwal, M. Agathos, M. Aghaei Abchouyeh, O. D. Aguiar, S. Ahmadzadeh, L. Aiello, A. Ain, P. Ajith, T. Akutsu, S. Albanesi, R. A. Alfaidi , et al. (1783 additional authors not shown)

Abstract: We detail the population properties of merging compact objects using 158 mergers from the cumulative Gravitational-Wave Transient Catalog 4.0, which includes three types of binary mergers: binary neutron star, neutron star--black hole binary, and binary black hole mergers. We resolve multiple over- and under-densities in the black hole mass distribution: features persist at primary masses of… ▽ More We detail the population properties of merging compact objects using 158 mergers from the cumulative Gravitational-Wave Transient Catalog 4.0, which includes three types of binary mergers: binary neutron star, neutron star--black hole binary, and binary black hole mergers. We resolve multiple over- and under-densities in the black hole mass distribution: features persist at primary masses of $10\,M_\odot$ and $35\,M_\odot$ with a possible third feature at $\sim 20\,M_\odot$. These are departures from an otherwise power-law-like continuum that steepens above $35\,M_\odot$. Binary black holes with primary masses near $10\,M_\odot$ are more likely to have less massive secondaries, with a mass ratio distribution peaking at $q = 0.74^{+0.13}_{-0.13}$, potentially a signature of stable mass transfer during binary evolution. Black hole spins are inferred to be non-extremal, with 90\% of black holes having $χ< 0.57$, and preferentially aligned with binary orbits, implying many merging binaries form in isolation. However, we find a significant fraction, 0.24-0.42, of binaries have negative effective inspiral spins, suggesting many could be formed dynamically in gas-free environments. We find evidence for correlation between effective inspiral spin and mass ratio, though it is unclear if this is driven by variation in the mode of the distribution or the width. (Abridged) △ Less

Submitted 17 September, 2025; v1 submitted 25 August, 2025; originally announced August 2025.

Comments: As part of the Astrophysical Journal Letters Focus Issue on the Gravitational Wave Transient Catalog

Report number: LIGO-P2400004

GWTC-4.0: Updating the Gravitational-Wave Transient Catalog with Observations from the First Part of the Fourth LIGO-Virgo-KAGRA Observing Run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1748 additional authors not shown)

Abstract: Version 4.0 of the Gravitational-Wave Transient Catalog (GWTC-4.0) adds new candidates detected by the LIGO, Virgo, and KAGRA observatories through the first part of the fourth observing run (O4a: 2023 May 24 15:00:00 to 2024 January 16 16:00:00 UTC) and a preceding engineering run. In this new data, we find 128 new compact binary coalescence candidates that are identified by at least one of our s… ▽ More Version 4.0 of the Gravitational-Wave Transient Catalog (GWTC-4.0) adds new candidates detected by the LIGO, Virgo, and KAGRA observatories through the first part of the fourth observing run (O4a: 2023 May 24 15:00:00 to 2024 January 16 16:00:00 UTC) and a preceding engineering run. In this new data, we find 128 new compact binary coalescence candidates that are identified by at least one of our search algorithms with a probability of astrophysical origin $p_{\rm astro} \geq 0.5$ and that are not vetoed during event validation. We also provide detailed source property measurements for 86 of these that have a false alarm rate $< 1 \rm{yr}^{-1}$. Based on the inferred component masses, these new candidates are consistent with signals from binary black holes and neutron star-black hole binaries (GW230518_125908 and GW230529_181500). Median inferred component masses of binary black holes in the catalog now range from $5.79\,M_\odot$ (GW230627_015337) to $137\,M_\odot$ (GW231123_135430), while GW231123_135430 was probably produced by the most massive binary observed in the catalog. For the first time we have discovered binary black hole signals with network signal-to-noise ratio exceeding 30, GW230814_230901 and GW231226_01520, enabling high-fidelity studies of the waveforms and astrophysical properties of these systems. Combined with the 90 candidates included in GWTC-3.0, the catalog now contains 218 candidates with $p_{\rm astro} \geq 0.5$ and not otherwise vetoed, doubling the size of the catalog and further opening our view of the gravitational-wave Universe. △ Less

Submitted 8 September, 2025; v1 submitted 25 August, 2025; originally announced August 2025.

Comments: As part of the Astrophysical Journal Letters Focus Issue on the Gravitational Wave Transient Catalog

Report number: LIGO-P2400386

GWTC-4.0: Methods for Identifying and Characterizing Gravitational-wave Transients

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, D. Agarwal, M. Agathos, M. Aghaei Abchouyeh, O. D. Aguiar, S. Ahmadzadeh, L. Aiello, A. Ain, P. Ajith, S. Akcay, T. Akutsu, S. Albanesi, R. A. Alfaidi , et al. (1787 additional authors not shown)

Abstract: The Gravitational-Wave Transient Catalog (GWTC) is a collection of candidate gravitational-wave transient signals identified and characterized by the LIGO-Virgo-KAGRA Collaboration. Producing the contents of the GWTC from detector data requires complex analysis methods. These comprise techniques to model the signal; identify the transients in the data; evaluate the quality of the data and mitigate… ▽ More The Gravitational-Wave Transient Catalog (GWTC) is a collection of candidate gravitational-wave transient signals identified and characterized by the LIGO-Virgo-KAGRA Collaboration. Producing the contents of the GWTC from detector data requires complex analysis methods. These comprise techniques to model the signal; identify the transients in the data; evaluate the quality of the data and mitigate possible instrumental issues; infer the parameters of each transient; compare the data with the waveform models for compact binary coalescences; and handle the large amount of results associated with all these different analyses. In this paper, we describe the methods employed to produce the catalog's fourth release, GWTC-4.0, focusing on the analysis of the first part of the fourth observing run of Advanced LIGO, Advanced Virgo and KAGRA. △ Less

Submitted 19 February, 2026; v1 submitted 25 August, 2025; originally announced August 2025.

Comments: As part of the Astrophysical Journal Letters Focus Issue on the Gravitational Wave Transient Catalog. Version accepted for publication

Report number: LIGO-P2400300

GWTC-4.0: An Introduction to Version 4.0 of the Gravitational-Wave Transient Catalog

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, D. Agarwal, M. Agathos, M. Aghaei Abchouyeh, O. D. Aguiar, S. Ahmadzadeh, L. Aiello, A. Ain, P. Ajith, S. Akcay, T. Akutsu, S. Albanesi, R. A. Alfaidi , et al. (1786 additional authors not shown)

Abstract: The Gravitational-Wave Transient Catalog (GWTC) is a collection of short-duration (transient) gravitational wave signals identified by the LIGO-Virgo-KAGRA Collaboration in gravitational-wave data produced by the eponymous detectors. The catalog provides information about the identified candidates, such as the arrival time and amplitude of the signal and properties of the signal's source as inferr… ▽ More The Gravitational-Wave Transient Catalog (GWTC) is a collection of short-duration (transient) gravitational wave signals identified by the LIGO-Virgo-KAGRA Collaboration in gravitational-wave data produced by the eponymous detectors. The catalog provides information about the identified candidates, such as the arrival time and amplitude of the signal and properties of the signal's source as inferred from the observational data. GWTC is the data release of this dataset and version 4.0 extends the catalog to include observations made during the first part of the fourth LIGO-Virgo-KAGRA observing run up until 2024 January 31. This paper marks an introduction to a collection of articles related to this version of the catalog, GWTC-4.0. The collection of articles accompanying the catalog provides documentation of the methods used to analyze the data, summaries of the catalog of events, observational measurements drawn from the population, and detailed discussions of selected candidates △ Less

Submitted 23 September, 2025; v1 submitted 25 August, 2025; originally announced August 2025.

Comments: As part of the Astrophysical Journal Letters Focus Issue on the Gravitational Wave Transient Catalog. Update following peer review

Report number: LIGO-P2400293

Open Data from LIGO, Virgo, and KAGRA through the First Part of the Fourth Observing Run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1746 additional authors not shown)

Abstract: LIGO, Virgo, and KAGRA form a network of gravitational-wave observatories. Data and analysis results from this network are made publicly available through the Gravitational Wave Open Science Center. This paper describes open data from this network, including the addition of data from the first part of the fourth observing run (O4a) and selected periods from the preceding engineering run, collected… ▽ More LIGO, Virgo, and KAGRA form a network of gravitational-wave observatories. Data and analysis results from this network are made publicly available through the Gravitational Wave Open Science Center. This paper describes open data from this network, including the addition of data from the first part of the fourth observing run (O4a) and selected periods from the preceding engineering run, collected from May 2023 to January 2024. The public data set includes calibrated strain time series for each instrument, data from additional channels used for noise subtraction and detector characterization, and analysis data products from version 4.0 of the Gravitational-Wave Transient Catalog. △ Less

Submitted 4 November, 2025; v1 submitted 25 August, 2025; originally announced August 2025.

Comments: 26 pages. The version updates Table 3, updates the author list, removes one figure, and updates some text for clarity and grammar

Report number: LIGO-P2500167

Crystal Eye: all sky MeV monitor with high precision real-time localization

Authors: Roberto Aloisio, Uygar Atalay, Biswajit Banerjee, Felicia C. T. Barbato, Elisabetta Bissaldi, Marica Branchesi, Fiamma Capitanio, Elisabetta Casilli, Roberta Colalillo, Ivan De Mitri, Alessio De Santis, Adriano Di Giovanni, Mateo Fernandez Alonso, Giulio Fontanella, Fabio Gargano, Fabio Garufi, Fausto Guarino, Dimitrios Kyratzis, Herman Lima, Francesco Loparco, Francesco Longo, Riccardo Martinelli, Teresa Montaruli, Gor Oganesyan, Javier Rico , et al. (11 additional authors not shown)

Abstract: Crystal Eye is a space-based all-sky monitor optimized for the autonomous detection and localization of transients in the 10 keV to 30 MeV energy range, a region where extensive observations and monitoring of various astrophysical phenomena are required. By focusing on the operating environment and its impact on the observation process, we optimized the detector design and assessed its scientific… ▽ More Crystal Eye is a space-based all-sky monitor optimized for the autonomous detection and localization of transients in the 10 keV to 30 MeV energy range, a region where extensive observations and monitoring of various astrophysical phenomena are required. By focusing on the operating environment and its impact on the observation process, we optimized the detector design and assessed its scientific potential. We explored the use of novel techniques to achieve the science goals of the experiment. We assumed the orbit of a potential future mission at approximately 550 km altitude near the equatorial region with a 20° inclination. In such an orbit, the main background contributions for this kind of detector are from different particles and radiation of cosmic origin and secondaries produced by their interaction in the Earth's atmospheric and geomagnetic environment. We studied the response of the detector in this background environment, using the Geant4 Monte Carlo simulation toolkit. We also calculated other detector performance parameters to estimate its scientific capabilities. The detector effective area and efficiency are calculated for low energy gamma-ray sources and used to estimate its sensitivity to short-duration transient sources. The calculation shows a better effective area and sensitivity by several factors compared to existing instruments of similar type. A method is also developed and discussed to estimate the online transient-localization performance of the detector, suggesting a better localization precision by about an order of magnitude than those typically reported by existing gamma-ray monitors. We present here the simulation study and results of an innovative detector design concept that can make a significant contribution in the multi-messenger era. Moreover, this study can be useful as a technical reference for similar future experiments. △ Less

Submitted 8 September, 2025; v1 submitted 26 July, 2025; originally announced July 2025.

Comments: 27 pages, 13 figures

Journal ref: Astroparticle Physics (2026), 174, 103171

Broadband Modelling of GRB 230812B Afterglow: Implications for VHE $γ$-ray Detection with IACTs

Authors: Shraddha Mohnani, Biswajit Banerjee, Davide Miceli, Lara Nava, Gor Oganesyan, Pawan Tiwari, Annarita Ierardi, Alessio L. De Santis, Samanta Macera, Amit Shukla, Marica Branchesi, Swarna Chatterjee, Sushmita Agarwal, Abhirup Datta, Kuldeep Kumar Yadav, G. C. Anupama

Abstract: A significant fraction of the energy from the $γ$-ray burst (GRB) jets, after powering the keV-MeV emission, forms an ultra-relativistic shock propagating into the circumburst medium. The particles in the medium accelerate through the shock and produce afterglow emission. Recently, a few GRB afterglows have been observed in TeV $γ$-rays by Cherenkov Telescopes. This provides access to broadband sp… ▽ More A significant fraction of the energy from the $γ$-ray burst (GRB) jets, after powering the keV-MeV emission, forms an ultra-relativistic shock propagating into the circumburst medium. The particles in the medium accelerate through the shock and produce afterglow emission. Recently, a few GRB afterglows have been observed in TeV $γ$-rays by Cherenkov Telescopes. This provides access to broadband spectra of GRB afterglows containing rich information about the microphysics of relativistic shocks and the profile of the circumburst medium. Since the transition from synchrotron to inverse Compton regime in afterglow spectra occurs between hard X-rays and the very-high-energy (VHE) $γ$-rays, detection in one of these bands is required to identify the two spectral components. The early afterglow data in the hard X-rays, along with the GeV emission, could accurately constrain the spectral shape and help in capturing the spectral turnover to distinguish the two components. We present the multiwavelength spectral and temporal study, focused on the keV-VHE domain, of GRB 230812B, one of the brightest GRBs detected by Fermi Gamma Ray Burst Monitor (GBM), along with the detection of a 72 GeV photon in Large Area Telescope (LAT) during the early afterglow phase. Through a detailed modelling of the emission within the afterglow external forward shock in a wind-like scenario, we predict optical to high-energy observations up to ~1 day. We emphasize the importance of following up poorly localised GRBs by demonstrating that even without prompt sub-degree localisation, such as in GRB 230812B, it is possible to recover the emission using imaging atmospheric Cherenkov telescopes, thanks to their relatively wider field of view. Moreover, we show that the low energy threshold of Large-Sized Telescope is essential in discovering the VHE component at much higher redshifts, typical of long GRBs. △ Less

Submitted 24 July, 2025; originally announced July 2025.

Comments: Submitted. Comments and Suggestions are welcome

Journal ref: A&A 705, A227 (2026)

Ultra-long MeV transient from a relativistic jet: a tidal disruption event candidate

Authors: Gor Oganesyan, Elias Kammoun, Annarita Ierardi, Alessio Ludovico De Santis, Biswajit Banerjee, Emanuele Sobacchi, Felix Aharonian, Samanta Macera, Pawan Tiwari, Alessio Mei, Shraddha Mohnani, Stefano Ascenzi, Samuele Ronchini, Marica Branchesi

Abstract: On July 2, 2025, the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray space telescope detected three short-duration MeV transients with overlapping sky locations. These events, named as GRB 250702D, B, and E (collectively referred to as DBE), triggered the detector with delays of approximately 1-2 hours between each burst. Follow-up observations of this unusually long MeV transient (lasti… ▽ More On July 2, 2025, the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray space telescope detected three short-duration MeV transients with overlapping sky locations. These events, named as GRB 250702D, B, and E (collectively referred to as DBE), triggered the detector with delays of approximately 1-2 hours between each burst. Follow-up observations of this unusually long MeV transient (lasting >3 hours) by the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array over a period of 10 days revealed a steep temporal decline in soft X-rays ($\propto t^{-1.9 \pm 0.1}$). The time-averaged spectra during the outbursts are well described by a single power law $dN_γ/dE \propto E^{-1.5}$, while upper limits above 100 MeV imply a spectral cutoff between 10 MeV and 100 MeV. Using standard gamma-ray transparency arguments, we derive a lower limit on the bulk Lorentz factor. Combined with the steep decline in X-rays, these constraints point to a relativistic jet origin. The properties of DBE are inconsistent with established GRB spectral-energy correlations, disfavoring classical long GRB progenitors. Instead, the basic characteristics of DBE resemble those of previously reported jetted tidal disruption events (TDEs), though alternative progenitor channels cannot be excluded. In the relativistic TDE scenario, DBE is the first one with detected MeV gamma-ray emission. We argue that the observed emission is most likely produced by synchrotron radiation from sub-TeV electrons. △ Less

Submitted 14 October, 2025; v1 submitted 24 July, 2025; originally announced July 2025.

Comments: Accepted for publication in A&A (Letter)

Journal ref: A&A 703, L2 (2025)

Direct Measurement of the Accretion Disk Formed in Prompt Collapse Mergers with Future Gravitational-Wave Observatories

Authors: Arnab Dhani, Alessandro Camilletti, Alessio Ludovico De Santis, Andrea Cozzumbo, David Radice, Domenico Logoteta, Albino Perego, Jan Harms, Marica Branchesi

Abstract: The production site of heavy r-process elements, such as Gold and Uranium, is uncertain. Neutron star mergers are the only astrophysical phenomenon in which we have witnessed their formation. However, the amount of heavy elements resulting from the merger remains poorly constrained, mainly due to uncertainties on the mass and angular momentum of the disk formed in the merger remnant. Matter accret… ▽ More The production site of heavy r-process elements, such as Gold and Uranium, is uncertain. Neutron star mergers are the only astrophysical phenomenon in which we have witnessed their formation. However, the amount of heavy elements resulting from the merger remains poorly constrained, mainly due to uncertainties on the mass and angular momentum of the disk formed in the merger remnant. Matter accretion from the disk is also thought to power gamma ray-bursts. We discover from numerical relativity simulations that the accretion disk influences the ringdown gravitational-wave signal produced by binaries that promptly collapse to black-hole at merger. We propose a method to \emph{directly} measure the mass of the accretion disk left during black hole formation in binary mergers using observatories such as the Einstein Telescope or Cosmic Explorer with a relative error of 10\% for binaries at a distance of up to 30~Mpc, corresponding to an event rate of 0.001 to 0.25 events per year. △ Less

Submitted 18 July, 2025; originally announced July 2025.

All-sky search for long-duration gravitational-wave transients in the first part of the fourth LIGO-Virgo-KAGRA Observing run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1750 additional authors not shown)

Abstract: We present an all-sky search for long-duration gravitational waves (GWs) from the first part of the LIGO-Virgo-KAGRA fourth observing run (O4), called O4a and comprising data taken between 24 May 2023 and 16 January 2024. The GW signals targeted by this search are the so-called "long-duration" (> 1 s) transients expected from a variety of astrophysical processes, including non-axisymmetric deforma… ▽ More We present an all-sky search for long-duration gravitational waves (GWs) from the first part of the LIGO-Virgo-KAGRA fourth observing run (O4), called O4a and comprising data taken between 24 May 2023 and 16 January 2024. The GW signals targeted by this search are the so-called "long-duration" (> 1 s) transients expected from a variety of astrophysical processes, including non-axisymmetric deformations in magnetars or eccentric binary coalescences. We make minimal assumptions on the emitted GW waveforms in terms of morphologies and durations. Overall, our search targets signals with durations ~1-1000 s and frequency content in the range 16-2048 Hz. In the absence of significant detections, we report the sensitivity limits of our search in terms of root-sum-square signal amplitude (hrss) of reference waveforms. These limits improve upon the results from the third LIGO-Virgo-KAGRA observing run (O3) by about 30% on average. Moreover, this analysis demonstrates substantial progress in our ability to search for long-duration GW signals owing to enhancements in pipeline detection efficiencies. As detector sensitivities continue to advance and observational runs grow longer, unmodeled long-duration searches will increasingly be able to explore a range of compelling astrophysical scenarios involving neutron stars and black holes. △ Less

Submitted 23 July, 2025; v1 submitted 16 July, 2025; originally announced July 2025.

Report number: LIGO-P2500090-v6

GW231123: a Binary Black Hole Merger with Total Mass 190-265 $M_{\odot}$

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, A. G. Abac, I. Abouelfettouh, F. Acernese, K. Ackley, C. Adamcewicz, S. Adhicary, D. Adhikari, N. Adhikari, R. X. Adhikari, V. K. Adkins, S. Afroz, A. Agapito, D. Agarwal, M. Agathos, N. Aggarwal, S. Aggarwal, O. D. Aguiar, I. -L. Ahrend, L. Aiello, A. Ain, P. Ajith, T. Akutsu , et al. (1749 additional authors not shown)

Abstract: On 2023 November 23 the two LIGO observatories both detected GW231123, a gravitational-wave signal consistent with the merger of two black holes with masses $137^{+23}_{-18}\, M_\odot$ and $101^{+22}_{-50}\, M_\odot$ (90\% credible intervals), at luminosity distance 0.7-4.1 Gpc and redshift of $0.40^{+0.27}_{-0.25}$, and a network signal-to-noise ratio of $\sim$20.7. Both black holes exhibit high… ▽ More On 2023 November 23 the two LIGO observatories both detected GW231123, a gravitational-wave signal consistent with the merger of two black holes with masses $137^{+23}_{-18}\, M_\odot$ and $101^{+22}_{-50}\, M_\odot$ (90\% credible intervals), at luminosity distance 0.7-4.1 Gpc and redshift of $0.40^{+0.27}_{-0.25}$, and a network signal-to-noise ratio of $\sim$20.7. Both black holes exhibit high spins, $0.9^{+0.10}_{-0.19}$ and $0.80^{+0.20}_{-0.52}$ respectively. A massive black hole remnant is supported by an independent ringdown analysis. Some properties of GW231123 are subject to large systematic uncertainties, as indicated by differences in inferred parameters between signal models. The primary black hole lies within or above the theorized mass gap where black holes between 60-130 $M_\odot$ should be rare due to pair instability mechanisms, while the secondary spans the gap. The observation of GW231123 therefore suggests the formation of black holes from channels beyond standard stellar collapse, and that intermediate-mass black holes of mass $\sim$200 $M_\odot$ form through gravitational-wave driven mergers. △ Less

Submitted 10 November, 2025; v1 submitted 10 July, 2025; originally announced July 2025.

Comments: 27 pages, 10 figures

Report number: DCC: P2500026-v8

Journal ref: ApJL 993 L25 (2025)