Sofia Bisero

We present a new Astro-COLIBRI component that converts free-text GCN Circulars into structured, event-linked follow-up records and combines them with reports submitted directly by the community. A continuously running listener associates new Circulars with transient events, applies deterministic pre-analysis, and invokes a schema-constrained large language model to extract photometry, contacts, redshifts, and related metadata. The records are normalized into a common event-level database and exposed through the web and mobile interfaces as report summaries, contact tools, optical-afterglow context figures, and downloadable CSV or VOTable products. All 1775 Circulars of an operational evaluation corpus covering the first half of 2026 completed the workflow without failures. An internal human audit of 210 Circulars found 223 of 231 assessed reports (96.5%) fully confirmed, each of the eight remaining ones containing at least one incorrect field, confined to observation timing and facility attribution. Extracted redshifts agree with three published compilations for 91.6% to 99.3% of the events they share, every disagreement tracing to a limit, a candidate-host estimate, or a later-refined value rather than to a misread Circular. The extraction also recovers 90% to 97% of the redshifts those compilations list for bursts it processed. Applied to the GCN archive since 2016, the pipeline yields 68,393 observations from 26,811 reports across 5787 transient events, and now runs in real time on new Circulars. The reusable parsing and normalization pipeline is released as the open-source Python package astro-colibri-circular-parser .

The propagation of very-high-energy (VHE; E _γ ≥ 100 GeV) gamma rays from extragalactic sources is affected by interactions with photons of the extragalactic background light (EBL), resulting in pair production that attenuates the intrinsic gamma-ray flux. This interaction renders the Universe increasingly opaque to VHE photons at high energies and redshifts. New physics scenarios involving axion-like particles (ALPs) could modify this expected optical depth. In particular, ALPs with masses m _a ∼ 10 eV can decay into two photons over cosmological timescales, thereby contributing to the diffuse EBL. If such ALPs constitute a significant fraction of the dark matter density, their decay would enhance the EBL intensity and consequently increase the gamma-ray optical depth. In this study, we investigate this scenario using a large sample of gamma-ray spectra observed with the High Energy Stereoscopic System (H.E.S.S.) and the Fermi Large Area Telescope. We model the contribution of decaying ALPs to the EBL and assess their impact on the spectra of blazars across redshifts. By comparing these observations with standard EBL models, we place constraints on the properties of heavy ALPs — specifically their mass and photon coupling — and evaluate their viability as a dark matter candidate capable of modifying the gamma-ray transparency of the Universe. From the combined analysis, and under the assumption that ALPs constitute the entire dark matter density, we derive 95% confidence exclusion limits on the photon-ALP coupling down to m _aγ ∼ 7×10 ^-12 GeV ^-1 for masses m _a ∼ 15 eV. These constraints are competitive with existing astrophysical bounds and provide complementary sensitivity to other techniques, closing a previously unconstrained region of parameter space in the m _a ∼ 2.5–20 eV range.

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

Context. Third-generation gravitational wave (GW) observatories, such as the Einstein Telescope (ET) and Cosmic Explorer (CE), will access a large volume of the Universe, and detect hundreds of thousands of binary neutron star (BNS) mergers, reaching distances beyond z ∼ 3. The unique information revealed by joint GW and electromagnetic (EM) detections can be fully exploited only with a dedicated observing strategy and suitably adapted EM facilities. Aims. In this work, we explore the impact of integral field and multi-object spectroscopy (IFS and MOS) with the Wide-field Spectroscopic Telescope (WST) on the detection of EM counterparts from BNS detected by next-generation GW observatories. Methods. We considered populations of BNS mergers assuming different equations of state and mass distributions. We computed the corresponding GW signals for both the ET operating alone and in a network with CE. Kilonova (KN) light curves were assigned using either AT2017gfo-like models or numerical-relativity-informed ones. Gamma-ray burst (GRB) afterglow emission was also included. We considered two main observing strategies: one in synergy with wide-field photometric surveys and a second based on a galaxy-targeted approach, exploiting WST’s high multiplexing capabilities. We also estimated the number of galaxies within the GW error volume. Here, we discuss potential observational challenges and corresponding mitigation strategies. Results. We find that KNe from BNS mergers can be detected with WST up to z ∼ 0.4 and magnitudes m _AB ∼ 25, while GRB afterglows may be observable at higher redshifts, z > 1, for viewing angles of Θ _view ≲ 15°. We show that Target of Opportunity (ToO) observations aimed at KN detections can be optimally scheduled 12–24 hours after the merger. For GRB afterglows, particularly in cases of a poor localisation in the first minutes after the high-energy detection, WST IFS can play a key role in counterpart identification and position refinement. Our results show that mini-IFUs (fibre bundles) and galaxy catalogues that are complete in redshift up to z ≤ 0.5 will be essential for the efficient identification of EM counterparts. We find that the WST contribution is also valuable for events with good sky localisation because the uncertainties in luminosity distance (even at low redshift) can result in large error volumes containing numerous galaxies to target up to several thousands at z < 0.1 and tens of thousands at z < 0.2 for GW detections of ET in a network with CE. Finally, we underline that events at redshifts of z < 0.3 and with sky localisations better than 10deg ² will be ‘golden’ events for WST, making it become possible for WST to cover all the galaxies in the error volume with a limited number of exposures. We estimate these to range from about ten (ET-alone configuration) to hundreds per year (ET + CE configuration). Conclusions. The detection and characterisation of EM counterparts of BNS mergers explored by next-generation interferometers will be challenging. Observational strategies and selection criteria should be defined in advance to prioritise events for follow-up among the potentially thousands per year expected from ET and CE. Detecting their EM counterparts will require facilities capable of covering sky regions of tens of deg ² , reaching depths of AB > 22, and rapidly gathering the data needed to identify the many candidate sources expected for each event. We have shown that spectroscopic facilities with large fields of view, high sensitivity, and high-multiplexing modes are powerful instruments to successfully exploit the new multi-messenger science opportunities that will be opened up thanks to next-generation GW interferometers.

Context. Next-generation gravitational wave (GW) observatories, such as the Einstein Telescope (ET) and Cosmic Explorer, will observe binary neutron star (BNS) mergers across cosmic history, providing precise parameter estimates for the closest ones. Innovative wide-field observatories, such as the Vera Rubin Observatory, will quickly cover large portions of the sky with unprecedented sensitivity to detect faint transients. Aims. This study aims to assess the prospects for detecting optical emissions from BNS mergers with next-generation detectors, considering how uncertainties in neutron star (NS) population properties and microphysics may affect detection rates, while developing realistic observational strategies by ET operating with the Rubin Observatory. Methods. Starting from BNS merger populations exploiting different NS mass distributions and equations of state (EOSs), we modelled the GW and kilonova (KN) signals based on source properties. We modelled KNe ejecta through numerical-relativity informed fits, considering the effect of prompt collapse of the remnant to black hole and new fitting formulas appropriate for more massive BNS systems, such as GW190425. We included optical afterglow emission from relativistic jets consistent with observed short gamma-ray bursts. We evaluated the detected mergers and the source parameter estimations for different geometries of ET, operating alone or in network of current or next-generation GW detectors. Finally, we developed target-of-opportunity strategies to follow up on these events using Rubin and evaluated the joint detection capabilities. Results. ET as a single observatory enables the detection of about ten to a hundred KNe per year by the Rubin Observatory. This improves by a factor of ∼10 already when operating in network with current GW detectors. Detection rate uncertainties are dominated by the poorly constrained local BNS merger rate, and depend to a lesser extent on the NS mass distribution and EOS.