TITLE: GCN CIRCULAR NUMBER: 39956 SUBJECT: GRB 250331A: MAXI/GSC detection DATE: 25/03/31 03:44:29 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University kawakubo@phys.aoyama.ac.jp
K. Takagi, M. Nakajima (Nihon U.), Y. Kawakubo, M. Serino (AGU), H. Negoro, Y. Kudo, H. Shibui, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), T. Mihara, S. Yamada, 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, 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, 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 01:40:54 UT on March 31, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (124.987 deg, -18.636 deg) = (08 19 56, -18 38 09) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.25 deg and 0.2 deg, respectively. The roll angle of long axis from the north direction is 168.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 146 +- 23 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: (124.498, -17.690) deg = (08 17 59, -17 41 24) (J2000) (124.178, -17.935) deg = (08 16 42, -17 56 05) (J2000) (125.521, -19.513) deg = (08 22 05, -19 30 46) (J2000) (125.843, -19.266) deg = (08 23 22, -19 15 57) (J2000)
There was no significant excess flux in the previous transit at 0:08 UT and in the next transit at 03:14 UT with an upper limit of 20 mCrab for each.
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