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1. Chasing Gamma-Ray Signals from Binary Neutron Star Coalescences with the Cherenkov Telescope Array: Prospects and Observing Strategies

   https://doi.org/10.3847/1538-4357/ae658c  

   Abe, S; Abhir, J; Abhishek, A; Acero, F; Acharyya, A; Adam, R; Aguasca-Cabot, A; Agudo, I; Albanese, I; Alfaro, J; et al (June 2026, The Astrophysical Journal)

   Abstract The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB 170817A, heralded the birth of multimessenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), which is ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV–TeV GW follow-up strategies for CTAO using a multistep simulation pipeline and to estimate the expected rate of joint GW–GRB detections during observing run O5. Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated. We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV–TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star–black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope. 

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2. AT2025ulz and S250818k: Deep X-Ray and Radio Limits on Off-axis Afterglow Emission and Prospects for Future Discovery

   https://doi.org/10.3847/2041-8213/ae16a6  

   O’Connor, Brendan; Ricci, Roberto; Troja, Eleonora; Palmese, Antonella; Yang, Yu-Han; Ryan, Geoffrey; van_Eerten, Hendrik; Yadav, Muskan; Hall, Xander J; Amsellem, Ariel; et al (December 2025, The Astrophysical Journal Letters)

   Abstract The first joint electromagentic (EM) and gravitational-wave (GW) detection, known as GW170817, marked a critical juncture in our collective understanding of compact object mergers. However, it has now been 8 yr since this discovery, and the search for a second EM-GW detection has yielded no robust discoveries. Recently, on 2025 August 18, the LIGO–Virgo–KAGRA Collaboration reported a low-significance (high-false-alarm rate) binary neutron star merger candidate S250818k. Rapid optical follow-up revealed a single optical candidate AT2025ulz (z= 0.08484) that initially appeared consistent with kilonova emission. We quickly initiated a set of observations with Swift, XMM-Newton, Chandra, and the Very Large Array to search for nonthermal afterglow emission. Our deep X-ray and radio search rules out that the optical rebrightening of AT2025ulz is related to the afterglow onset, reinforcing its classification as a stripped-envelope supernova (SN 2025ulz). We derive constraints on the afterglow parameters for a hypothetical binary neutron star merger at the distance of AT2025ulz (≈400 Mpc) based on our X-ray and radio limits. We conclude that our observational campaign could exclude a GW170817-like afterglow out to viewing angles ofθ_v≈ 12 $\stackrel{\text{°}}{.}$ 5. We briefly discuss the prospects for the future discovery of off-axis afterglows. 

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3. Kilonova Constraints for the LIGO/Virgo/KAGRA Neutron Star Merger Candidate S250206dm: GW-MMADS Observations

   https://doi.org/10.3847/2041-8213/adfd49  

   Hu, Lei; Cabrera, Tomás; Palmese, Antonella; Freeburn, James; Bulla, Mattia; Andreoni, Igor; Hall, Xander J; O’Connor, Brendan; Amsellem, Ariel; Bom, Clécio R; et al (September 2025, The Astrophysical Journal Letters)

   Abstract Gravitational-wave (GW) neutron star mergers with an associated electromagnetic counterpart constitute powerful probes of binary evolution, the production sites of heavy elements, general relativity, and the expansion of the Universe. Only a handful of candidate GW binary mergers during the fourth LIGO/Virgo/KAGRA observing run (O4) so far are believed to include a neutron star. We present optical–near-infrared follow-up observations of the candidate neutron star–black hole GW merger S250206dm. This is the first high-significance mass-gap neutron star–black hole candidate observed by multiple GW detectors (thus having a significantly smaller sky localization than one-detector events), offering the first opportunity to effectively follow up a GW event of this kind. Our GW MultiMessenger Astronomy DECam Survey (GW-MMADS) campaign consisted of a wide-field search using the Dark Energy Camera (DECam) and T80-South (T80S), as well as galaxy-targeted observations using the Southern Astrophysical Research (SOAR) imager and the Fraunhofer Telescope at Wendelstein Observatory. No viable kilonova counterpart was found in our observations. We use our observation depths to place competitive constraints on kilonova models similar to or brighter than the GW170817 kilonova AT 2017gfo within our observed fields, ruling out 100% of such models with SOAR galaxy-targeted observations and ∼43% (48%) with DECam (DECam and T80S). 

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4. Implications of the search for optical counterparts during the second part of the Advanced LIGO’s and Advanced Virgo’s third observing run: lessons learned for future follow-up observations

   https://doi.org/10.1093/mnras/staa1925  

   Coughlin, Michael W; Dietrich, Tim; Antier, Sarah; Almualla, Mouza; Anand, Shreya; Bulla, Mattia; Foucart, Francois; Guessoum, Nidhal; Hotokezaka, Kenta; Kumar, Vishwesh; et al (September 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Joint multimessenger observations with gravitational waves and electromagnetic (EM) data offer new insights into the astrophysical studies of compact objects. The third Advanced LIGO and Advanced Virgo observing run began on 2019 April 1; during the 11 months of observation, there have been 14 compact binary systems candidates for which at least one component is potentially a neutron star. Although intensive follow-up campaigns involving tens of ground and space-based observatories searched for counterparts, no EM counterpart has been detected. Following on a previous study of the first six months of the campaign, we present in this paper the next five months of the campaign from 2019 October to 2020 March. We highlight two neutron star–black hole candidates (S191205ah and S200105ae), two binary neutron star candidates (S191213g and S200213t), and a binary merger with a possible neutron star and a ‘MassGap’ component, S200115j. Assuming that the gravitational-wave (GW) candidates are of astrophysical origin and their location was covered by optical telescopes, we derive possible constraints on the matter ejected during the events based on the non-detection of counterparts. We find that the follow-up observations during the second half of the third observing run did not meet the necessary sensitivity to constrain the source properties of the potential GW candidate. Consequently, we suggest that different strategies have to be used to allow a better usage of the available telescope time. We examine different choices for follow-up surveys to optimize sky localization coverage versus observational depth to understand the likelihood of counterpart detection. 

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5. Radio afterglows from compact binary coalescences: prospects for next-generation telescopes

   https://doi.org/10.1093/mnras/stab1468  

   Dobie, Dougal; Murphy, Tara; Kaplan, David L; Hotokezaka, Kenta; Bonilla Ataides, Juan Pablo; Mahony, Elizabeth K; Sadler, Elaine M (June 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The detection of gravitational waves from a neutron star merger, GW170817, marked the dawn of a new era in time-domain astronomy. Monitoring of the radio emission produced by the merger, including high-resolution radio imaging, enabled measurements of merger properties including the energetics and inclination angle. In this work, we compare the capabilities of current and future gravitational wave facilities to the sensitivity of radio facilities to quantify the prospects for detecting the radio afterglows of gravitational wave events. We consider three observing strategies to identify future mergers – wide field follow-up, targeting galaxies within the merger localization and deep monitoring of known counterparts. We find that while planned radio facilities like the Square Kilometre Array will be capable of detecting mergers at gigaparsec distances, no facilities are sufficiently sensitive to detect mergers at the range of proposed third-generation gravitational wave detectors that would operate starting in the 2030s. 

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