Prospects of electromagnetic follow-up of neutron star-black hole mergers in the LIGO-India era

Abstract

Neutron star-black hole (NSBH) mergers are promising multimessenger sources, but only a subset is expected to produce detectable electromagnetic (EM) counterparts, depending on the binary mass ratio, black hole (BH) spin, and neutron star (NS) equation of state (EoS). We investigate the prospects for detecting kilonova counterparts to NSBH mergers in the LIGO-India era using the Vera C. Rubin Observatory (Rubin). We simulate GW230529-motivated NSBH populations with two effective tidal deformability ranges, estimate gravitational-wave detections for the LHV (LIGO-Livingston, LIGO-Hanford, Virgo) and LHVA (LHV with LIGO-India at Aundha) detector networks, and model the detectability of the associated kilonova emission with Rubin. We find that adding LIGO-India approximately doubles the number of EM follow-ups, primarily by increasing the duty cycle and reducing the median sky-localization area, and notably increases the number of such detections at large distances. For fixed-exposure strategies, the EM detection rate saturates at exposure times of a few hundred seconds, reflecting the trade-off between depth and sky coverage. We therefore introduce an event-specific exposure-time optimization strategy based on the expected counterpart brightness and sky-localization area. With this strategy, the expected NSBH GW+EM detection rate per year is 4.19(-3.58,+9.66) for the optimistic and 0.79(-0.68,+1.83) for the conservative population with the LHVA network, compared to 1.46(-1.25,+3.37) and 0.27(-0.23,+0.62), respectively, for LHV. The required Rubin follow-up time remains within the expected Target-of-Opportunity allocation. These results show that in the next decade, a few such NSBH follow-ups may take place, with a fraction of them in the redshift range of about 0.1-0.2, which will not only be useful for measuring the Hubble constant but can also help us in probing the Hubble parameter.