GRB 210318B: MITSuME Okayama optical upper limits

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).