GW200224_222234 follow-up survey with Subaru & GTC

Given its relatively small localization skymap (71deg^2^ for a 90% credible region; revised to 50deg^2^ in GWTC-3), we performed target- of-opportunity observations using the Subaru/Hyper Suprime-Cam (HSC) in the r2 and z bands. Observations were conducted on 2020 February 25 and 28 and March 23, with the first epoch beginning 12.3hr after the GW detection.

The survey covered the highest-probability sky area of 56.6deg^2^, corresponding to a 91% probability. This was the first deep follow-up (m_r_>~24, m_z_>~23) for a binary-black hole merger covering >90% of the localization. By performing image subtraction and candidate screening including light-curve fitting with transient templates and examples, we found 22 off-nucleus transients that were not ruled out as the counterparts of GW200224_222234 with our Subaru/HSC data alone.