TITLE: GCN CIRCULAR NUMBER: 39264 SUBJECT: GRB 250209A: MAXI/GSC detection (possible detection of an X-ray flare from 1RXS J145110.3+310648) DATE: 25/02/10 06:51:48 GMT FROM: Hitoshi Negoro at Nihon University/MAXI team negoro.hitoshi@nihon-u.ac.jp

H. Negoro, H. Nishio (Nihon U.), T. Mihara (RIKEN), M. Serino (AGU), M. Nakajima, Y. Kudo, H. Shibui, K. Takagi, H. Takahashi, K. Tatano (Nihon U.), S. Yamada, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, 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 detected a relatively soft X-ray transient source at 11:00 UT on February 9, 2025. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (222.844 deg, 30.986 deg) = (14 51 22, +30 59 09) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.35 deg and 0.32 deg, respectively. The roll angle of the long axis from the north direction is 54.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 133 +- 34 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) = (221.544, 29.670) deg = (14 46 10, +29 40 12) (J2000) (R.A., Dec) = (222.164, 29.349) deg = (14 48 39, +29 20 56) (J2000) (R.A., Dec) = (223.910, 31.810) deg = (14 55 38, +31 48 35) (J2000) (R.A., Dec) = (223.280, 32.140) deg = (14 53 07, +32 08 24) (J2000) There was no significant excess flux in the previous transit at 09:27 UT and in the next transit at 12:33 UT with an upper limit of 20 mCrab for each.

This transient event is positionally consistent with a nearby M4.0Ve star G 166-49 (a.k.a. 1RXS J145110.3+310648) and GRB 190306A (GCN Circ. 23937, 23941). This and the relatively soft X-ray spectrum suggest that this event originates from an X-ray flare from G 166-49. If so, the luminosity of the flare is about 6.0e31 erg/cm2/s for 13 pc distance, and that GRB 190306A might be an X-ray flare from this source though GRB 190306A showed a harder spectrum. We also designates this event GRB 250209A because of the un-identification.

We crosspost this to the ATel and the GCN Circular for both communities.

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