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, 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).