TITLE: GCN CIRCULAR NUMBER: 30509 SUBJECT: GRB 210725A: MITSuME Akeno optical upper limits DATE: 21/07/25 14:19:34 GMT FROM: Masafumi Niwano at Tokyo Institute of Tech M. Niwano, R. Hosokawa, K. L. Murata, N. Ito, H. Takamatsu, Y. Imai, 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 210725A (Bernardini et al. GCN Circular #30501, WeiKang Zheng et al. GCN Circular #30503, S. Torii et al. GCN Circular #30507) 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 2021-07-25 10:26:07 UT (6.7 hours after Swift BAT trigger). We stacked the images with good conditions. We did not detect any uncatalogued sources within the XRT error region (Bernardini et al. GCN Circular #30501). We obtained the 5-sigma limits of the stacked images as follows. T0+[hour] MID-UT T-EXP[sec] 5-sigma limits ------------------------------------------------------------------------------------------------ 6.7 2021-07-25 11:21:56.58 900 g'>18.6, Rc>18.9, Ic>18.4 ------------------------------------------------------------------------------------------------ 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).