TITLE: GCN CIRCULAR NUMBER: 33690 SUBJECT: GRB 230427A: MITSuME Akeno optical upper limits DATE: 23/04/27 15:34:00 GMT FROM: Ichiro Takahashi at Tokyo Tech I. Takahashi, S. Hayatsu, N. Higuchi, M. Sasada, R. Hosokawa, S. Sato, M. Niwano, H. Seki, H. Takei, Y. Yatsu (Tokyo Tech), K. L. Murata (Kyoto U) and N. Kawai (Riken) report on behalf of the MITSuME collaboration: We observed the field of GRB 230427A (Troja et al. GCN Circular #33687, Beardmore et al. GCN Circular #33688) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm telescope Akeno. The observation started at 2023-04-27 10:33:23 UT (91 seconds after the Swift/BAT trigger). We stacked the images with good conditions. We did not detect any uncatalogued sources within the error region (Troja et al. GCN Circular #33687). We obtained the 5-sigma limits of the stacked images as follows. T0+[sec] | MID-UT | T-EXP[sec] | 5-sigma limits ------------------------------------------------------------------------------------------------ 263 | 2023-04-27 10:36:15 | 160.0 | g'>17.8, Rc>17.4, Ic>17.0 753 | 2023-04-27 10:44:25 | 540.0 | g'>18.7, Rc>18.7, Ic>18.2 2391 | 2023-04-27 11:11:43 | 2220.0 | g'>19.6, Rc>19.4, Ic>19.0 ------------------------------------------------------------------------------------------------ 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).