Follow-up Observations for IceCube-170922A: Detection of Rapid Near-Infrared Variability and Intensive Monitoring of TXS 0506+056

Tomoki Morokuma, Yousuke Utsumi, Kouji Ohta, Masayuki Yamanaka, Koji S. Kawabata, Yoshiyuki Inoue, Masaomi Tanaka, Michitoshi Yoshida, Ryosuke Itoh, Mahito Sasada, Nozomu Tominaga, Hiroki Mori, Miho Kawabata, Tatsuya Nakaoka, Maiko Chogi, Taisei Abe, Ruochen Huang, Naoki Kawahara, Hiroki Kimura, Hiroki Nagashima, Kengo Takagi, Yuina Yamazaki, Wei Liu, Ryou Ohsawa, Shigeyuki Sako, Katsuhiro L. Murata, Kumiko Morihana, Christina K. Gilligan, Keisuke Isogai, Mariko Kimura, Yasuyuki Wakamatsu, Ryuhei Ohnishi, Masaki Takayama, Satoshi Honda, Yoshiki Matsuoka, Takuji Yamashita, Shigehiro Nagataki, Yasuyuki T. Tanaka

Published 2020-11-10Version 1

We present our follow-up observations to search for an electromagnetic counterpart of the IceCube high-energy neutrino, IceCube-170922A. Monitoring observations of a likely counterpart, TXS 0506+056, are also described. First, we quickly took optical and near-infrared images of 7 flat-spectrum radio sources within the IceCube error region right after the neutrino detection and found a rapid flux decline of TXS 0506+056 in Kanata/HONIR J-band data. Motivated by this discovery, intensive follow-up observations of TXS 0506+056 are continuously done, including our monitoring imaging observations, spectroscopic observations, and polarimetric observations in optical and near-infrared wavelengths. TXS 0506+056 shows a large amplitude (~1.0 mag) variability in a time scale of several days or longer, although no significant variability is detected in a time scale of a day or shorter. TXS 0506+056 also shows a bluer-when-brighter trend in optical and near-infrared wavelengths. Structure functions of variabilities are examined and indicate that TXS 0506+056 is not a special blazar in terms of optical variability. Polarization measurement results of TXS 0506+056 are also discussed.