TITLE: GCN CIRCULAR NUMBER: 42498 SUBJECT: GRB 251028A: MAXI/GSC detection DATE: 25/10/28 04:17:17 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University kawakubo@phys.aoyama.ac.jp
H. Sugai (Chuo U.), Y. Kawakubo, M. Serino (AGU), H. Negoro, M. Nakajima, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), 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.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U) report on behalf of the MAXI team:
The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 03:27:06 UT on October 28, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (149.003 deg, 17.518 deg) = (09 56 00, +17 31 04) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.27 deg and 0.21 deg, respectively. The roll angle of long axis from the north direction is 49.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 267 +- 43 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: (148.423, 18.362) deg = (09 53 41, +18 21 43) (J2000) (148.075, 17.954) deg = (09 52 17, +17 57 14) (J2000) (149.164, 17.109) deg = (09 56 39, +17 06 32) (J2000) (149.513, 17.514) deg = (09 58 03, +17 30 50) (J2000)
There was no significant excess flux in the previous transit at 02:15 UT with an upper limit of 20 mCrab.
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