GCN Circular 29565

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 210222B (Lipunov et al. GCN #29544, Gropp
et al. GCN #29545, Hu et al. GCN #29546, Strausbaugh et al. GCN
#29547, Mong et al. GCN #29548, Perley et al. GCN #29549, Mong et al.
GCN #29550, Zhu et al. GCN #29551, Fernandez et al. GCN #29552,
Lipunov et al. GCN #29554, Jelinek et al. GCN #29557, Zheng et al. GCN
#29558, Siegel et al. GCN #29559, Page et al. GCN #29560, Ukwatta et
al. GCN #29561, Gokuldass et al. GCN #29562, Belkin et al. GCN #29563)
 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
2021-02-23 11:00:13 UT(12.4hours after the Swift BAT trigger). We
stacked the images with good conditions. We did not detect the optical
afterglow reported previously (Gropp et al. GCN #29545, Hu et al. GCN
#29546, Strausbaugh et al. GCN #29547, Mong et al. GCN #29548, Perley
et al. GCN #29549, Mong et al. GCN #29550, Zhu et al. GCN #29551,
Fernandez et al. GCN #29552, Lipunov et al. GCN #29554, Jelinek et al.
GCN #29557, Siegel et al. GCN #29559, Gokuldass et al. GCN #29562,
Belkin et al. GCN #29563) 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
------------------------------------------------------------------------------------------------
12.4 15:18:25 14100 g'>19.6, Rc>19.9, Ic>19.2
------------------------------------------------------------------------------------------------
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).