TITLE: GCN CIRCULAR NUMBER: 29920 SUBJECT: GRB 210424B: MITSuME Akeno optical upper limits DATE: 21/05/01 12:26:07 GMT FROM: Katsuhiro L. Murata at Nagoya U R. Hosokawa, Y. Imai, K. L. Murata, M. Niwano, 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 210424B (IPN triangulation: Hurley et al. GCN Circular #29909, Konus-Wind detection: Svinkin et al. GCN Circular #29911, CALET Gamma-Ray Burst Monitor detection: Yoshida et al. GCN Circular #29913) 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 2021-04-29 16:49:39 UT (5.35 days after the triggers). We stacked the images with good conditions. We did not detect any uncatalogued sources within the IPN triangulation error box (Hurley et al. GCN Circular #29909) in all three bands. We obtained the 5-sigma limits of the stacked images as follows. T0+[day] MID-UT T-EXP[sec] 5-sigma limits ------------------------------------------------------------------------------------------------ 5.35 2021-04-29 17:05 1020 g'>18.5, Rc>18.4, Ic>17.8 ------------------------------------------------------------------------------------------------ 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).