TITLE: GCN CIRCULAR NUMBER: 29241 SUBJECT: GRB 210104A: MITSuME Ishigaki optical observation DATE: 21/01/04 20:02:05 GMT FROM: Takashi Horiuchi at Ishigakijima Astronomical Obs Takashi Horiuchi, Hidekazu Hanayama (NAOJ), Katsuhiro L. Murata, Yoichi Yatsu, Nobuyuki Kawai (Tokyo Tech) 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) with the optical three color (g', Rc, and Ic) CCD cameras attached to the 105 cm Murikabushi telescope of Ishigakijima Astronomical Observatory, Okinawa, Japan. The observation with a series of 60 sec exposures started on 2021-01-04 13:58:34 UT (150 min after Swift BAT trigger). We detected the point source at the position consistent with the afterglow reported by Swift UVOT and obtained the magnitudes as follows. T0+[min] MID-UT T-EXP[sec] measured magnitudes ------------------------------------------------------------------------------------------------ 150 2021-01-04T14:14:02 1500 g'=19.17+/-0.08, Rc=18.12+/-0.04, Ic=17.96+/-0.09 ------------------------------------------------------------------------------------------------ T0+ : Elapsed time after the burst T-EXP: Total Exposure time The SDSS catalog (DR16) is used for flux calibration.