J-GEM near-infrared follow-up observations of the gravitational wave event S240422ed

Abstract We report our near-infrared (NIR) follow-up observations of the gravitational wave (GW) event S240422ed using the Subaru Telescope/MOIRCS. S240422ed was initially classified as a black hole-neutron star merger with >99% probability of electromagnetic wave emission. We started follow-up observations $7.8\:$h after the event. Over two nights, we observed 206 nearby galaxies in Y and $K_{\rm s}$ bands down to about 21.4 and $21.1$ AB mag ($3\sigma$), respectively. The total completeness of our survey based on galaxy B-band luminosity is $22\%$. As a result of our observations, five candidate counterparts were identified. We show that properties of these five objects are not consistent with kilonovae such as AT2017gfo. Four objects are consistent with known classes of transients such as supernovae or dwarf nova outbursts. On the other hand, the nature of the remaining one object, which shows a red color and rapid decline, remains unclear. Although later analyses of GW signal reclassified S240422ed as likely terrestrial noise, our NIR observations provide valuable lessons for future NIR surveys for GW sources. We demonstrate that deep NIR follow-up observations as presented in this work would effectively constrain the presence of red kilonovae even at $200$ Mpc distance. We also discuss the importance of deep and wide NIR reference images and of understanding the properties and frequency of Galactic transients.