Optical-Infrared Searches for Identifying the IceCube High-Energy Neutrino Sources

We present our follow-up observations in optical and infrared wavelengths to identify electro-magnetic (EM) counterparts of high-energy neutrinos detected with the IceCube experiment. Our observing facilities include a wide range of telescope apertures from small-size ( ∼ 0 . 5m) to the largest-size ( ∼ 8m) in the world. Unique wide-field instruments are also utilized; Hyper Suprime-Cam (1.8 deg 2 field-of-view) on the 8.2-m Subaru telescope and Tomo-e Gozen (20 deg 2 field-of-view) on the 1.05-m Kiso Schmidt telescope. We first aim for searching for candidates of highly variable blazars (including those with red optical colors dominated by their host galaxies rather than blazar components), peculiar supernovae (SNe), and tidal disruption events (TDEs). We also conduct follow-up spectroscopic observations to identify the nature and determine the redshift of the candidates to claim the coincidence of the source with the neutrino detection. We successfully identified the EM counterpart of the high-energy neutrino IceCube-170922A, TXS 0506+056, with quick detection of the rapid near-infrared brightness change with HONIR on the 1.5-m Kanata telescope. After this variability detection in addition to the Fermi/LAT flux increase, world-wide follow-up observations were intensively conducted and the coincidence with the neutrino detection was found. We found that TXS 0506+056 showed a large-amplitude ( ∼ 1 . 0 mag) variability in >several-day time scale or longer with the bluer-when-brighter trend, although no significant variability was detected in a time scale of < 1-day. Structure function analyses indicate that TXS 0506+056 is not