[vsnet-grb-info 28406] GRB 210725A: MITSuME Akeno optical upper limits

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30509
SUBJECT: GRB 210725A: MITSuME Akeno optical upper limits
DATE:    21/07/25 14:19:34 GMT
FROM:    Masafumi Niwano at Tokyo Institute of Tech  <niwano at hp.phys.titech.ac.jp>

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).