TITLE: GCN CIRCULAR NUMBER: 29608 SUBJECT: GRB 210306A: MITSuME Akeno optical observation DATE: 21/03/07 06:38:39 GMT FROM: Ryohei Hosokawa at Tokyo Institute of Technology R. Hosokawa, R. Adachi, K. L. Murata, M. Niwano, 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 210306A (D'Elia et al. GCN #29597, Watson et al. GCN 29598, Romanov GCN #29599, Strausbaugh et al. GCN #29600, Ohno et al. GCN #29602, Hentunen et al. GCN #29603, Stecklum et al. GCN #29604, Veres et al. GCN #29605, Lipunov et al. GCN #29606, Zhu et al. GCN #29607) 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 with a series of 60 sec exposures started at 2021-03-06 09:12:03 UT (5.3 hours after Swift BAT trigger). We stacked the images with good conditions. We marginally detected the point source at the position consistent with the afterglow detected previously(D'Elia et al. GCN #29597, Watson et al. GCN 29598, Romanov GCN #29599, Strausbaugh et al. GCN #29600, Hentunen et al. GCN #29603, Stecklum et al. GCN #29604, Zhu et al. GCN #29607). We measured the magnitudes as follows. T0+[hour] MID-UT T-EXP[sec] measured magnitudes ------------------------------------------------------------------------------------------------ 5.3 11:38:10 8160 g'=20.0+/-0.4, Rc=20.2+/-0.4, Ic=19.0+/-0.3 ------------------------------------------------------------------------------------------------ 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. 2021, PASJ, Vol.73, Issue 1, Pages 4-24; https://github.com/MNiwano/Eclaire).