GCN Circular 30509

M. Niwano, R. Hosokawa, K. L. Murata, N. Ito, H. Takamatsu, Y. Imai, 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 210725A (Bernardini et al. GCN Circular 
#30501, WeiKang Zheng et al. GCN Circular #30503, S. Torii et al. GCN 
Circular #30507) 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 2021-07-25 10:26:07 UT (6.7 
hours after Swift BAT trigger). We stacked the images with good 
conditions. We did not detect any uncatalogued sources within the XRT 
error region (Bernardini et al. GCN Circular #30501). We obtained the 
5-sigma limits of the stacked images as follows.

T0+[hour] MID-UT T-EXP[sec] 5-sigma limits
------------------------------------------------------------------------------------------------
6.7 2021-07-25 11:21:56.58 900 g'>18.6, Rc>18.9, Ic>18.4
------------------------------------------------------------------------------------------------
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).