TITLE: GCN CIRCULAR NUMBER: 29237 SUBJECT: GRB 210104A: MITSuME Akeno optical observation DATE: 21/01/04 12:44:41 GMT FROM: Ryohei Hosokawa at Tokyo Institute of Technology R. Hosokawa, R. Adachi, M. Niwano, K. L. Murata, F. Ogawa, N. Nakamura, N. Ito, S. Ogata, H. Takamatsu, H. Hara, Y. Yatsu, and N. Kawai (TokyoTech) report on behalf of the MITSuME collaboration: We observed the field of GRB 210104A (The Fermi GBM team , GCN 29232, E. Troja et al., GCN 29233, B. Biltzinger et al., GCN 29234) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm telescope of Akeno Observatory, Yamanashi, Japan. The observation started at 2021-01-04 11:27:54 UT (55s after Swift BAT trigger). We detected the point source at the position consistent with the afterglow detected previously(E. Troja et al., GCN 29233, L. P. Xin et al., GCN 29235, Y.-D. Hu et al., GCN 29236). We measured the magnitudes as follows. T0+[sec] MID-UT T-EXP[sec] measured magnitudes ------------------------------------------------------------------------------------------------ 55 11:28:24 60 g'=14.6+/-0.1, Rc=14.2+/-0.1, Ic=13.8+/-0.1 ------------------------------------------------------------------------------------------------ T0+ : Elapsed time after the burst T-EXP: Total Exposure time We used PS1 catalog for flux calibration. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al., https://doi.org/10.1093/pasj/psaa091, https://arxiv.org/abs/2008.11486; https://github.com/MNiwano/Eclaire).