GCN Circular 30220

R. Noto, R. Hosokawa, K. L. Murata, M. Niwano, N. Ito, H. Takamatsu,
Y. Imai, S. Sato, M. Takaku, R. Yamaguchi, Y. Yatsu, and N. Kawai
(TokyoTech) report on behalf of the MITSuME collaboration:

We observed the field of GRB 210610B (Page et al. GCN #30170, Ursi et
al. GCN #30195, Frederiks et al. GCN #30196, Malacaria et al. GCN
#30199) with the optical three color (g', Rc, and Ic) CCD cameras
attached to the MITSuME 50 cm telescope Akeno. The observation with a
series of 60 sec exposures started at 2021-06-11 10:33:20 UT (14.7
hours after Swift BAT trigger). We stacked the images with good
conditions. We detected the afterglow reported previously (Page et al.
GCN #30170, Kumar et al. GCN #30174, Rumyantsev et al. GCN #30175,
Lipunov et al. GCN #30176, Hu et al. GCN #30177, Rumyantsev et al. GCN
#30178, Wet et al. GCN #30180, Romanov et al. GCN #30181, Fynbo et al.
GCN #30182, Moskvitin et al. GCN #30187, Becerra et al. GCN #30190,
Mong et al. GCN #30193, Postigo et al. GCN #30194, Marchini et al. GCN
#30198, Dutta et al. GCN #30201, Zheng et al. GCN #30204, Vreeswijk et
al. GCN #30205, Perley et al. GCN #30206, Pankov et al. GCN #30212,
Pankov et al. GCN #30215, Perley et al. GCN #30216, Laskar et al. GCN
#30217, Alexander et al. GCN #30218)

We measured the magnitudes as follows.
MID-UT T-EXP[sec] measured magnitudes
------------------------------------------------------------------------------------------------
2021-06-11 12:51:05 9780 g'=19.3+/-0.2, Rc=18.9+/-0.1, Ic=18.8+/-0.1
------------------------------------------------------------------------------------------------
T-EXP: Total Exposure time

We used 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).