TITLE: GCN CIRCULAR NUMBER: 39190 SUBJECT: GRB 250207B: MAXI/GSC detection DATE: 25/02/07 04:26:02 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University kawakubo@phys.aoyama.ac.jp

K. Tatano, M. Nakajima, H. Negoro (Nihon U.), M. Serino, Y. Kawakubo (AGU), Y. Kudo, H. Shibui, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Mihara, S. Yamada, S. Wang, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, H. Hiramatsu, H. Nishikawa, Y. Kondo, S. Sasao, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.), M. Shidatsu, Y. Niida (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, Y. Okada, K. Fujiwara (Kyoto U.), M. Yamauchi, Y. Otsuki, T. Hasegawa, M. Nishio (Miyazaki U.), K. Yamaoka (Nagoya U.),　 M. Sugizaki (Kanazawa U.),　W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 02:23:36 UT on February 07, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (54.362 deg, 5.325 deg) = (03 37 26, +05 19 30) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.27 deg and 0.24 deg, respectively. The roll angle of long axis from the north direction is 124.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 218 +- 41 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: (R.A., Dec) = (53.893, 6.160) deg = (03 35 34, +06 09 36) (J2000) (R.A., Dec) = (53.522, 5.823) deg = (03 34 05, +05 49 22) (J2000) (R.A., Dec) = (54.372, 4.895) deg = (03 37 29, +04 53 41) (J2000) (R.A., Dec) = (54.743, 5.230) deg = (03 38 58, +05 13 48) (J2000) There was no significant excess flux in the previous transit at 00:50 UT and in the next transit at 03:56 UT with an upper limit of 20 mCrab for each.

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