We propose ToO observations of gravitational wave (GW) sources detected with LIGO/Virgo/KAGRA (LVK). The first GW signal from a neutron star (NS) merger was detected in 2017 (GW170817). By intensive electromagnetic (EM) follow-up, the EM counterpart AT2017gfo was also identified, which opened a new era of multi-messenger astronomy. The Subaru Telescope played an important role to identify this source as a genuine EM counterpart and to provide detailed light curves, giving evidence of r-process nucleosynthesis by the NS merger. GW170817/AT2017gfo was a remarkable event, however, it is just one example. More samples are obviously needed to reveal the nature of NS merger including diversity of kilonovae depending on NS mass, binary mass ratio, or viewing angle, its relationship with galaxy properties, and the event rate of NS mergers. We perform EM follow-up observations of NS merger candidates detected in the fourth long-term operation of LVK (O4) with the Subaru Telescope in coordination with various other telescopes. Wide-field and deep imaging capability of Subaru Hyper Suprime-Cam will play a very unique role in the world in searching for EM emission from GW sources.
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