Multi-messenger observations of binary neutron star mergers: Synergies between next-generation gravitational wave interferometers and wide-field, high-multiplex spectroscopic facilities

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.