GCN Circular 29455

R. Hosokawa, R. Adachi, K. L. Murata, M. Niwano, F. Ogawa, N.
Nakamura, N. Ito, S. Ogata, H. Takamatsu, H. Hara, Y. Yatsu, and N.
Kawai (TokyoTech) report on behalf of the MITSuME collaboration:

We observed the field of GRB 210209A (Laha et al. GCN #29442, Evans et
al. GCN #29443, Perri et al. GCN #29449, Breeveld et al. GCN #29453)
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
2021-02-10 08:49:41 UT(11.2 hour after the Swift BAT trigger). Since
the first 17 images were taken under high airmass conditions, we
stacked the images with good conditions. We did not find any new point
sources within the enhanced Swift XRT circle (Evans et al. GCN #29443)
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
------------------------------------------------------------------------------------------------
11.2 09:14:41 600 g'>17.2,Rc>17.6,Ic>17.1
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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).