[vsnet-grb-info 27779] GRB 210421A: MITSuME Akeno optical upper limits

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29882
SUBJECT: GRB 210421A: MITSuME Akeno optical upper limits
DATE:    21/04/22 14:52:20 GMT
FROM:    Naohiro Ito at Tokyo Tech  <n.ito at hp.phys.titech.ac.jp>

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

We observed the field of GRB 210421A (M. J. Moss et al. GCN Circular
#29849, V. Lipunov et al. GCN Circular #29850, Alan M. Watson et al.
GCN Circular #29857, P.A. Evans et al. GCN Circular #29865, P.A. Evans
et al. GCN Circular #29875) 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 2021-04-21 09:51:58 UT (9.4 hours after the Swift
BAT trigger) We stacked the images with good conditions. We did not
detect any sources within the enhanced XRT error region (P.A. Evans et
al. GCN Circular #29865) 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
------------------------------------------------------------------------------------------------
9.8 11:25 7020 g'>18.8, Rc>18.8, Ic>18.4
------------------------------------------------------------------------------------------------
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).