TITLE: GCN CIRCULAR NUMBER: 23004 SUBJECT: GRB 180720B: MITSuME Ishigaki optical observations DATE: 18/07/22 00:04:42 GMT FROM: Ryosuke Itoh at Tokyo Institute of Tech. T. Horiuchi, H. Hanayama, M. Honma (IAO, NAOJ), R. Itoh, K. L. Murata, Y. Tachibana, S. Harita, K. Morita K. Shiraishi, K. Iida, M. Oeda, R. Adachi, S. Niwano, Y. Yatsu, and N. Kawai (Tokyo Tech) report on behalf of the MITSuME collaboration: We observed the optical counterpart of GRB 180720B (Siegel et al., GCN 22973) with the optical three color (g', Rc, and Ic) CCD cameras attached to the Murikabushi 1m telescope of Ishigakijima Astronomical Observatory, Japan. The observation started on 2018-07-20 17:11:26 UT, (~2.8 hours after the burst). We detected the previously reported afterglow (Martone et al., GCN 22976, Sasada et al., GCN 22977, Reva et al., GCN 22979) in the Rc and Ic band. The photometric results of the OT are listed below. We used UCAC-4 catalog for flux calibration. #T0+[hour] MID-UT T-EXP[sec] Rc Ic ------------------------------------------------------------------- 2.8 17:14:58 240 16.23+/-0.03 16.24+/-0.05 ------------------------------------------------------------------- T0+ : Elapsed time after the burst [hour] T-EXP: Total Exposure time [sec]