TITLE: GCN CIRCULAR NUMBER: 22993 SUBJECT: GRB 180720B: MAXI/GSC detection DATE: 18/07/21 11:19:21 GMT FROM: H. Negoro at Nihon U. H. Negoro (Nihon U.), A. Tanimoto (Kyoto U.), M. Nakajima, W. Maruyama, A. Sakamaki (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), Y. Ueda, T. Morita, S. Yamada (Kyoto U.), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Tsunemi, T. Yoneyama (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.) report on behalf of the MAXI team: The MAXI/GSC nova alert system triggered on GRB 180720B (Siegel et al., GCN 22973/20975; Bissaldi et al., GCN 22980; Roberts et al., GCN 22981) at 14:28:15 UT on 2018 July 20, 396 sec after the Fermi/GRB trigger. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (0.467 deg, -2.846 deg) = (00 01 52, -02 50 45) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.21 deg and 0.16 deg, respectively. The roll angle of the long axis from the north direction is 166.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The above position is 5.6 arcminutes from the Swift/XRT position of GRB 180720B (Siegel et al. GCN 22973). The X-ray flux averaged over the scan was 191 +- 27 mCrab (4.0-10.0keV, 1 sigma error). MAXI/GSC also detected the source at 25 +/- 12 mCrab in the next scan transit at 16:01. The 2-20 keV GSC spectrum obtained in the scan transit at 14:28 is well described by an absorbed power-law with a photon index of 2.11 +/- 0.25 with n_H fixed at the total Galactic column density to the direction of 3.5e20 atoms/cm2.