Follow-up surveys for gravitational wave events using Subaru/Hyper Suprime-Cam

Abstract

Hyper Suprime-Cam (HSC) is a wide-field imager on the Subaru telescope. Its field of view of 1.77 deg 2 is the largest among current 8m-class telescopes, which makes the Subaru/HSC the most efficient instrument for the optical survey. We, Japanese Collaboration for Gravitational-Wave Electro-Magnetic Follow-up (J-GEM), had performed follow-up surveys for gravitational wave events using Subauru/HSC in the past LIGO-Virgo-KAGRA (LVK) observing runs. We summarize our follow-up surveys. In particular, the follow-up survey for an optical counterpart of a binary neutron star coalescence GW170817 covered 56% credible region and reaches the 50% completeness magnitude of 20.6 mag on average. We found 60 candidates of extragalactic transients and evaluated the probability that these candidates are located inside of the 3D skymap. As a result, we concluded that AT2017gfo is the most-likely and distinguished candidate as the optical counterpart of GW170817 and demonstrated the uniqueness of AT2017gfo. We also conducted the follow-up surveys for binary black hole coalescences, GW151226, S190510g, and GW200224_222234. Although we could not identify promising candidates of their optical counterparts, we put upper limits of optical luminosity of possible counterparts. We also introduce our strategy and the status of follow-up surveys using Subaru/HSC in the LVK O4 run.