GCN Circular 29936

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

We observed the field of GRB 210504A (Beardmore et al. GCN Circular #29929, Lien
 et al. GCN Circular #29930) 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 2021-05-04 14:55:16 UT (1.0 hour after the BAT
trigger, Lien et al. GCN Circular #29930). We stacked the images with
good conditions. We did not detect any uncatalogued sources within the
XRT error circle (Beardmore et al. GCN Circular #29929) 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
------------------------------------------------------------------------------------------------
1.1 2021-05-04 15:01 300 g���>15.9, Rc>16.2, Ic>15.9
2.8 2021-05-04 16:42 2580 g���>18.3, Rc>18.7, Ic>18.3
------------------------------------------------------------------------------------------------
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).