TWO NEUTRON STAR-BLACK HOLE MERGERS OBSERVED BY THE LIGO-VIRGO COLLABORATION

Abstract

Neutron star–black hole (NSBH) mergers are still very rare among gravitational-wave sources. However, together with binary neutron star (BNS) mergers, they play a crucial role in gravitational-wave astronomy, producing not only gravitational waves but also electromagnetic (EM) radiation, as well as neutrinos and high-energy particles (cosmic rays). In the Fourth Open Gravitational-wave Catalog (4-OGC), out of 94 recorded events, 90 are binary black holes (BBHs), 2 are NSBH mergers, and 2 are BNS mergers. Therefore, despite the limited number of NSBH and BNS mergers, their study remains a highly significant area of research. In this article, we review existing observational data for the events GW200115 (S200115j) and GW200105 (S200105ae), which are confirmed NSBH mergers. Powerful networks have been developed globally to observe EM counterparts of gravitational waves, including the GRANDMA collaboration. The GRANDMA project, in which the Shamakhi Astrophysical Observatory named after N. Tusi participates, has been important for coordinating observations across multiple countries and implementing advanced tracking strategies via joint telescope monitoring. No optical counterparts have been found, but the observational data obtained are useful for establishing constraints on the properties and observational capabilities of NSBHs.