GCN Circular 29621

S. Ogata, R. Hosokawa, K. L. Murata, R. Adachi, M. Niwano, F. Ogawa,
N. Nakamura, N. Ito, H. Takamatsu, H. Hara, Y. Yatsu, and N. Kawai
(TokyoTech) report on behalf of the MITSuME collaboration:

We observed the field of GRB 210308A (D'Elia et al.,GCN Circular
#29619) with the optical three color (g', Rc, and Ic) CCD cameras
attached to the MITSuME 50 cm telescope of Okayama Astrophysical
Observatory, Okayama, Japan. The observation with a series of 60 sec
exposures started at 2021-03-08 11:03:59 UT.(4.5 hours after Swift BAT
trigger) We stacked the images with good conditions. We did not detect
the optical afterglow reported previously (D'Elia et al.,GCN Circular
#29619) in all three bands.
We obtained the 5-sigma limits of the stacked images as follows.

T0+[hour] MID-UT T-EXP[sec] 5-sigma limits
------------------------------------------------------------------------------------------------
4.5 11:51:53 4440 g'>17.6, Rc>17.8, Ic>17.8
------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the burst
T-EXP: Total Exposure time

We used the 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).