TITLE: GCN CIRCULAR NUMBER: 29936 SUBJECT: GRB 210504A: MITSuME Akeno optical upper limits DATE: 21/05/04 23:35:54 GMT FROM: Ryohei Hosokawa at Tokyo Institute of Technology R. Hosokawa, K. L. Murata, M. Niwano, N. Ito, H. Takamatsu, Y. Imai, S. Sato, M. Takaku, R. Noto, R. Yamaguchi, Y. Yatsu, and N. Kawai (TokyoTech) report on behalf of the MITSuME collaboration: We observed the field of GRB 210504A (Beardmore et al. GCN Circular #29929, Lien et al. GCN Circular #29930) 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 2021-05-04 14:55:16 UT (1.0 hour after the BAT trigger, Lien et al. GCN Circular #29930). We stacked the images with good conditions. We did not detect any uncatalogued sources within the XRT error circle (Beardmore et al. GCN Circular #29929) 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 ------------------------------------------------------------------------------------------------ 1.1 2021-05-04 15:01 300 g’>15.9, Rc>16.2, Ic>15.9 2.8 2021-05-04 16:42 2580 g’>18.3, Rc>18.7, Ic>18.3 ------------------------------------------------------------------------------------------------ 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).