GCN Circular 31031

R. Hosokawa, Y. Imai, K. L. Murata, M. Niwano, Y. Takamatsu, N. Ito,
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 211023B (S. Dichiara et al. GCN Circular
#30960, V. Lipunov et al. GCN Circular #30964, A.P. Beardmore et al.
GCN Circular #30966, A.P. Beardmore et al. GCN Circular #30967, A.
Pozanenko et al. GCN Circular #30972, Alan M. Watson et al. GCN
Circular #30973, N. P. M. Kuin et al. GCN Circular #30975, Y.-D. Hu et
al. GCN Circular #30976, S. Laha et al. GCN Circular #30979, D. A.
Kann et al. GCN Circular #30992, B. O'Connor et al. GCN Circular
#31000, J. Rastinejad et al. GCN Circular #31002, A. Pozanenko et al.
GCN Circular #31003, B. Grossan et al. GCN Circular #31016) 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 21-10-24 17:18:57 UT (20.2 hours after Swift
trigger). We stacked the images with good conditions. We did not
detect any uncatalogued sources within the UVOT error region (S.
Dichiara et al. GCN Circular #30960). We obtained the 5-sigma limits
of the stacked images as follows.

T0+[days] MID-UT T-EXP[sec] 5-sigma limits
------------------------------------------------------------------------------------------------
2.9 2021-10-26 19:25:18 2160 g'>19.5, Rc>19.6, Ic>18.7
3.9 2021-10-27 19:10:47 4020 g'>19.5, Rc>20.1, Ic>19.4
4.9 2021-10-28 19:02:53 7020 g'>19.8, Rc>20.1, Ic>19.3
5.9 2021-10-29 18:56:59 6120 g'>20.3, Rc>20.5, Ic>19.9
------------------------------------------------------------------------------------------------
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).