TITLE: GCN CIRCULAR NUMBER: 42392 SUBJECT: GRB 251023B / MAXI J0451+122: MAXI/GSC detection DATE: 25/10/23 10:08:49 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University kawakubo@phys.aoyama.ac.jp

C. Kang (Ehime U.), M. Nakajima (Nihon U.), Y. Kawakubo (AGU), H. Negoro, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, 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 08:49:03 UT on October 23, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (72.930 deg, 12.205 deg) = (04 51 43, +12 12 18) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.15 deg and 0.13 deg, respectively. The roll angle of long axis from the north direction is 71.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 341 +- 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: (72.360, 11.893) deg = (04 49 26, +11 53 34) (J2000) (72.539, 11.711) deg = (04 50 09, +11 42 39) (J2000) (73.436, 12.551) deg = (04 53 44, +12 33 03) (J2000) (73.256, 12.734) deg = (04 53 01, +12 44 02) (J2000) There was no significant excess flux in the previous transit at 07:16 UT with an upper limit of 20 mCrab for each.

View this GCN Circular online at https://gcn.nasa.gov/circulars/42392. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...