[vsnet-grb-info 27429] GRB 210218A: MITSuME Akeno optical upper limits

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29532
SUBJECT: GRB 210218A: MITSuME Akeno optical upper limits
DATE:    21/02/18 15:33:22 GMT
FROM:    Ogawa Futa at Tokyo Institute of Technology  <ogawa at hp.phys.titech.ac.jp>

F. Ogawa, R. Hosokawa, K. L. Murata, R. Adachi, M. Niwano, 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 210218A (Sbarufatti et al. GCN #29529,
Zhu et al. GCN #29530, Xin et al. GCN #29531)  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 10 sec exposures started at 2021-02-18 13:36:05 UT(46
seconds after the Swift BAT trigger). We stacked the images with good
conditions. We did not find any new point sources within the Swift BAT
circle (Sbarufatti et al. GCN #29529) in all three bands.
We obtained the 5-sigma limits of the stacked images as follows.

T0+[sec] MID-UT T-EXP[sec] 5-sigma limits
------------------------------------------------------------------------------------------------
46 14:03:01 1220 g'>17.3, Rc>17.5, Ic>16.7
------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the burst
T-EXP: Total Exposure time
We used the UCAC4 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).