TITLE: GCN CIRCULAR NUMBER: 29860 SUBJECT: GRB 210413B: MITSuME Okayama optical upper limits DATE: 21/04/21 09:41:19 GMT FROM: Naohiro Ito at Tokyo Tech N. Ito, K. L. Murata, R. Hosokawa, M. Niwano, H. Takamatsu, Y. Yatsu, and N.Kawai (TokyoTech) report on behalf of the MITSuME collaboration: We observed the field of GRB 210413B (Fermi/GBM TRIGGER 640050417, P. A. Evans et al. GCN Circular #29821, Gregory S.H. Paek et al. GCN Circular #29825, J.A. Kennea et al. GCN Circular #29827) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm telescope of Okayama Astronomical Observatory, Okayama, Japan. The observation with a series of 60 sec exposures started at 2021-04-17 13:31:10 UT (85.8 hours after the Fermi/GBM trigger 640050417). We stacked the images with good conditions. We did not find any transients within the BAT error region (A. Tohuvavohu et al. GCN Circular #29819) 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 ------------------------------------------------------------------------------------------------ 86.8 15:06 2340 g'>17.4, Rc>17.6, 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. 2021, PASJ, Vol.73, Issue 1, Pages 4-24; https://github.com/MNiwano/Eclaire).