TITLE: GCN CIRCULAR NUMBER: 41129 SUBJECT: GRB 250721A: MAXI/GSC detection DATE: 25/07/21 13:31:42 GMT FROM: Motoko Serino at Aoyama Gakuin U. serino@phys.aoyama.ac.jp

C. Kang (Ehime U.), M. Serino, Y. Kawakubo (AGU), H. Negoro (Nihon U.), M. Nakajima, K. Takagi, H. Takahashi, K. Tatano, 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, H. Sugai, N. Nagashima (Chuo U.), M. Shidatsu, Y. Niida, 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)

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 09:38:05 UT on July 21, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = ( 82.805, -25.592) = (05 31 13, -25 35 31) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.37 deg and 0.32 deg, respectively. The roll angle of long axis from the north direction is 24.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 119 +- 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: (81.815, -25.748) deg = (05 27 15, -25 44 52) (J2000) (82.171, -26.333) deg = (05 28 41, -26 19 58) (J2000) (84.986, -24.908) deg = (05 39 56, -24 54 28) (J2000) (84.621, -24.329) deg = (05 38 29, -24 19 44) (J2000)

There was no significant excess flux in the next transit at 11:10 UT with an upper limit of 20 mCrab.

We also note that a transient was detected by Fermi (GCN Circular #41128), which is probably associated with the event.

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