GCN Circular 29675

R. Hosokawa, K. L. Murata, R. Adachi, 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 210318B (Troja et al. GCN Circular
#29663, Svinkin et al. GCN Circular #29664, Stamatikos et al. GCN
Circular #29666, Lipunov et al. GCN Circular #29667, Malacaria et al.
GCN Circular #29668) 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 2021-03-19 19:11:03 UT.(40.4
hours after Swift BAT trigger) We stacked the images with good
conditions. We did not find any new point sources within the Swift XRT
circle (Troja et al. GCN Circular #29663) 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
------------------------------------------------------------------------------------------------
40.4 19:50:32 2040 g'>14.3, Rc>14.2, Ic>13.4
------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the burst
T-EXP: Total Exposure time
We used the UCAC4 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).