GCN Circular 30855

Y. Takamatsu, R. Hosokawa, K. L. Murata, N. Ito, Y. Imai, M. Niwano,
R. Noto, 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 210919A (A. Tohuvavohu et al. GCN
Circular #30846, V. Lipunov et al. GCN Circular #30847, Y.-D. Hu et
al. GCN Circular #30848, R. Strausbaugh et al. GCN Circular #30849,
M.R. Goad et al. GCN Circular #30850, N. Pankov et al. GCN Circular
#30851, D. A. Perley et al. GCN Circular #30852, M. Perri et al. GCN
Circular #30854) 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 2021-09-19 15:22:32 UT
(14.9 hours after Swift trigger). We stacked the images with good
conditions. We did not detect any uncatalogued sources within the XRT
error region (M.R. Goad et al. GCN Circular #30850). We obtained the
5-sigma limits of the stacked images as follows.

T0+[hour] MID-UT T-EXP[sec] 5-sigma limits
------------------------------------------------------------------------------------------------
17.3 2021-09-19 17:48:08.43 11160 g'>19.2, Rc>20.5, Ic>19.6
------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the burst
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).