TITLE: GCN CIRCULAR NUMBER: 28393 SUBJECT: GRB 200907B: MITSuME Okayama optical upper limits DATE: 20/09/08 01:35:11 GMT FROM: Ryohei Hosokawa at Tokyo Institute of Technology R. Hosokawa, K. L. Murata, R. Adachi, M. Niwano, F. Ogawa, N. Nakamura, N. Ito, S. Ogata, H. Takamatsu, H. Hara, Y. Yatsu, and N. Kawai (TokyoTech) report on behalf of the MITSuME collaboration: We observed the field of GRB 200907B (Beardmore et al., GCN 28384; Evans et al., GCN 28385,28391) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm telescope of Okayama Astrophysical Observatory, Okayama, Japan. The observation with a series of 60 sec exposures started at 2020-09−07 18:52:16UT. We did not find any new point sources within the enhanced Swift/XRT error circle (Evans et al., GCN 28391) in the 60 sec exposure images and the stacked images. We obtained the 5-sigma limits of the stacked images as follows. T0+[sec] MID-UT T-EXP[sec] 5-sigma limits ------------------------------------------------------------------------------------------------ 65 2020-09-07T19:19:48 2100 g'>17.1, Rc>17.9, Ic>17.3 ------------------------------------------------------------------------------------------------ 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., accepted for publication in PASJ, https://arxiv.org/abs/2008.11486; https://github.com/MNiwano/Eclaire).