TITLE: GCN CIRCULAR NUMBER: 40556 SUBJECT: GRB 250527B / MAXI J1450-527 : MAXI/GSC detection DATE: 25/05/27 22:44:00 GMT FROM: taiki.kawamuro@riken.jp T. Kawamuro (Osaka U.), M. Nakajima (Nihon U.), M. Serino (AGU), W. Iwakiri (Chiba U.), H. Negoro, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.), M. Shidatsu, Y. Niida, C. Kang, T. Nakamoto (Ehime U.), I. Takahashi, M. Niwano, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), S. Yamada (Tohoku U.) The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 2025-05-27 17:38:05 UT. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (222.622 deg, -52.704 deg) = (14 50 29, -52 42 14) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.12 deg and 0.11 deg, respectively. The roll angle of long axis from the north direction is 90.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 350 +- 36 mCrab (4.0-10.0keV, 1 sigma error). Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners: (222.154, -51.832) deg = (14 48 36, -51 49 55) (J2000) (221.831, -51.930) deg = (14 47 19, -51 55 48) (J2000) (222.883, -53.207) deg = (14 51 31, -53 12 25) (J2000) (223.212, -53.106) deg = (14 52 50, -53 06 21) (J2000) There was no significant excess flux in the previous transit at 17:17 UT and in the next transit at 18:50 UT with an upper limit of 20 mCrab for each. View this GCN Circular online at https://gcn.nasa.gov/circulars/40556. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...