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 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).