TITLE: GCN CIRCULAR NUMBER: 30855 SUBJECT: GRB 210919A: MITSuME Akeno optical upper limits DATE: 21/09/20 00:36:51 GMT FROM: Yutaka Takamatsu at Tokyotech <takamatsu@hp.phys.titech.ac.jp> Y. Takamatsu, R. Hosokawa, K. L. Murata, N. Ito, Y. Imai, M. Niwano, 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 210919A (A. Tohuvavohu et al. GCN Circular #30846, V. Lipunov et al. GCN Circular #30847, Y.-D. Hu et al. GCN Circular #30848, R. Strausbaugh et al. GCN Circular #30849, M.R. Goad et al. GCN Circular #30850, N. Pankov et al. GCN Circular #30851, D. A. Perley et al. GCN Circular #30852, M. Perri et al. GCN Circular #30854) 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 2021-09-19 15:22:32 UT (14.9 hours after Swift trigger). We stacked the images with good conditions. We did not detect any uncatalogued sources within the XRT error region (M.R. Goad et al. GCN Circular #30850). We obtained the 5-sigma limits of the stacked images as follows. T0+[hour] MID-UT T-EXP[sec] 5-sigma limits ------------------------------------------------------------------------------------------------ 17.3 2021-09-19 17:48:08.43 11160 g'>19.2, Rc>20.5, Ic>19.6 ------------------------------------------------------------------------------------------------ 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).