TITLE: GCN CIRCULAR NUMBER: 30096 SUBJECT: GRB 210528B: MITSuME Akeno optical upper limits DATE: 21/05/29 13:57:33 GMT FROM: Katsuhiro L. Murata at Nagoya U K. L. Murata, R. Hosokawa, M. Niwano, Y. Imai, N. Ito, H. Takamatsu, 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 210528B (Sugawara et al. GCN Circular #30094, Lipunov et al. GCN Circular #30095) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME telescope Akeno. The observation with a series of 60 sec exposures started at 2021-05-29 11:21:57 UT (12.5 hours after the trigger). We stacked the images with good conditions. We did not detect any uncatalogued sources in all three bands within the elliptical error region (Sugawara et al. GCN Circular #30094) determined under the assumption that the source flux was constant over the transit. We obtained the 5-sigma limits of the stacked images as follows. T0+[hour] MID-UT T-EXP[sec] 5-sigma limits ------------------------------------------------------------------------------------------------ 12.5 2021-05-29 12:13 1560 g'>19.9 12.5 2021-05-29 12:13 2040 Rc>20.2 12.5 2021-05-29 12:13 1500 Ic>19.4 ------------------------------------------------------------------------------------------------ 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).