TITLE: GCN CIRCULAR NUMBER: 21555 SUBJECT: LIGO/Virgo G298048, Fermi GBM trigger 524666471/170817.529: MAXI/GSC observations DATE: 17/08/18 10:43:45 GMT FROM: Satoshi Sugita at Tokyo Inst. of Tech. S. Sugita, N. Kawai (Tokyo Tech), M. Serino (RIKEN), H. Negoro (Nihon U.), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N.Isobe, R. Shimomukai (JAXA), T. Mihara, M. Sugizaki, S. Nakahira, W. Iwakiri, M. Shidatsu, M. Matsuoka (RIKEN), T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), M. Nakajima, T. Kawase, A. Sakamaki (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team: We examined the MAXI/GSC all-sky X-ray images (2-20 keV) obtained in the orbits following LIGO/Virgo trigger G298048 and the Fermi GBM trigger 524666471/170817.529 at 2017-08-17 12:41:06.47 UTC (GCN 21506). At the trigger time of Fermi GBM, the high-voltage of MAXI/GSC was off, and it was turned on at T0+173 sec. The center of the Fermi GBM localization was located near the pole of ISS orbital rotation, where the MAXI/GSC cannot cover the field. The field of candidate optical counterpart (GCN 21529) was marginally covered by MAXI and was scanned at 12:23 UT (18 min before the trigger), but the OT position was obscured by an ISS structure. The OT candidate position was difficult to cover with MAXI/GSC due to unfavorable orientation of the orbital plane of the ISS, and therefore it was not observed for the following 8 orbits. The first scan by MAXI/GSC was started at 08-18 02:19 UTC (818 min after the LIGO trigger). We did not detect a significant source at this position with a 3-sigma upper limit of 54 mCrab. This limit is higher than the typical value because of high particle background flux. For the remaining area of the G298048 preliminary-LALInference skymap, we obtained a typical 1(3)-sigma upper limit of 11(33) mCrab in the orbit following the trigger. If you require information of X-ray flux by MAXI/GSC at specific coordinates, please contact the submitter of this circular by email.