We propose ToO observations of gravitational wave (GW) sources detected with LIGO and Virgo. The state-of-the-art GW detectors, LIGO and Virgo have detected the GW signal from a neutron star (NS) merger GW170817 on August 17 2017. The intense electromagnetic (EM) emission was associated with this event and our observation team J-GEM (Japanese collaboration for Gravitational-wave Electro-Magnetic follow-up) successfully detected and followed up this EM counterpart using Subaru HSC and MOIRCS, and found evidence of r-process nucleosynthesis in the ejecta of the merger. The fourth long-term operation (>12 months; O4) of LIGO and Virgo will start in March 2023. GWs from ∼5 NS–NS mergers are expected to be detected in O4. We plan to make EM follow-up observations with HSC, FOCAS, MOIRCS, IRCS, or HDS of Subaru Telescope in the framework of J-GEM to reveal the nature of NS merger. In particular, wide field and deep imaging capability of HSC will play a very unique role in searching for EM emission from GW sources in the world.
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