TITLE: GCN CIRCULAR NUMBER: 41600 SUBJECT: GRB 250830A / MAXI J0853+337: MAXI/GSC detection DATE: 25/08/30 03:56:44 GMT FROM: Motoko Serino at Aoyama Gakuin U. serino@phys.aoyama.ac.jp
T. Usuki (Ehime U.), M. Nakajima, H. Negoro (Nihon U.), M. Serino (AGU), K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, 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 on a bright uncatalogued X-ray transient source at 00:53:04 UT on August 30, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (133.397 deg, 33.778 deg) = (08 53 35, +33 46 40) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.26 deg and 0.26 deg, respectively. The roll angle of the long axis from the north direction is 34.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 182 +- 40 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) = (132.876, 34.855) deg = (08 51 30, +34 51 17) (J2000) (R.A., Dec) = (132.372, 34.578) deg = (08 49 29, +34 34 40) (J2000) (R.A., Dec) = (133.708, 32.906) deg = (08 54 49, +32 54 21) (J2000) (R.A., Dec) = (134.206, 33.177) deg = (08 56 49, +33 10 37) (J2000) There was no significant excess flux in the previous transit at August 29 23:20 UT and in the next transit at August 30 02:26 UT with an upper limit of 20 mCrab for each.
We tantatively name the source MAXI J0853+337, although we called it MAXI J0853+338 in MAXI new-transient ML.
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