TITLE: GCN CIRCULAR NUMBER: 39660 SUBJECT: GRB 250311A/MAXI J1457-026: MAXI/GSC detection DATE: 25/03/11 03:47:32 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University kawakubo@phys.aoyama.ac.jp

K. Tatano, H. Negoro, M. Nakajima (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 01:55:22 UT on March 11, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (224.318 deg, -2.613 deg) = (14 57 16, -02 36 46) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.33 deg and 0.28 deg, respectively. The roll angle of long axis from the north direction is 9.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 75 +- 18 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: (223.607, -1.537) deg = (14 54 25, -01 32 13) (J2000) (223.164, -1.982) deg = (14 52 39, -01 58 55) (J2000) (225.099, -3.901) deg = (15 00 23, -03 54 03) (J2000) (225.542, -3.455) deg = (15 02 10, -03 27 18) (J2000)

There was no significant excess flux in the previous transit at 00:22 UT and in the next transit at 03:28 UT with an upper limit of 20 mCrab for each.

We note that the source position is consistent with the position of GRB 250305A reported by Fermi-GBM (#39597). We encourage follow-up observations.

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