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. 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. The fourth long-term operation (∼18 months; O4) of LVK planned to start in May 2023 will give us next promising opportunities to observe various NS mergers. We perform EM follow-up observations with Subaru Telescope coordinated with various other telescopes. In particular, wide-field and deep imaging capability of HSC will play a very unique role in the world in searching for EM emission from GW sources.
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