GCN Circular 29532

F. Ogawa, R. Hosokawa, K. L. Murata, R. Adachi, M. Niwano, 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 210218A (Sbarufatti et al. GCN #29529,
Zhu et al. GCN #29530, Xin et al. GCN #29531)  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 10 sec exposures started at 2021-02-18 13:36:05 UT(46
seconds after the Swift BAT trigger). We stacked the images with good
conditions. We did not find any new point sources within the Swift BAT
circle (Sbarufatti et al. GCN #29529) in all three bands.
We obtained the 5-sigma limits of the stacked images as follows.

T0+[sec] MID-UT T-EXP[sec] 5-sigma limits
------------------------------------------------------------------------------------------------
46 14:03:01 1220 g'>17.3, Rc>17.5, Ic>16.7
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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).