TITLE:   GCN CIRCULAR
NUMBER:  29565
SUBJECT: GRB 210222B: MITSuME Akeno optical upper limits
DATE:    21/02/25 08:30:41 GMT
FROM:    Ryohei Hosokawa at Tokyo Institute of Technology  <hosokawa@hp.phys.titech.ac.jp>

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).