XRISM/Xtend Transient Search (XTS) detected an X-ray brightening from XRISM J1235-3957

ATel #17243; M. Yoshimoto, C. Kang (Ehime U.), K. Fukushima, Y. Kanemaru, S. Ogawa (JAXA), M. Audard (U. de Geneve), E. Behar (Technion), T. Hakamata (Osaka U.), S. Inoue (Kyoto U.), Y. Ishihara (Chuo U.), T. Kiyomoto (Saitama U.), T. Kohmura (TUS), H. Kuramoto (Osaka U.), J. Kurashima (U. of Miyazaki), Y. Maeda (JAXA), H. Matsumoto (Osaka U.), T. Matsushima (U. of Miyazaki), A. Miyamoto (Osaka U.), M. Mizumoto (UTEF), K. Mori (U. of Miyazaki), Y. Motogami (Saitama U.), N. Nagashima (Chuo U.), T. Narita (Kyoto U.), M. Nobukawa (NUE), H. Noda (Tohoku U.), Y. Sakamoto (Tohoku U.), M. Shidatsu (Ehime U.), H. Sugai (Chuo U.), H. Takahashi (Hiroshima U.), T. Takagi (Ehime U.), S. Takatuska (Osaka U.), R. Takemoto (U. of Miyazaki), Y. Terada (Saitama U.), Y. Terashima (Ehime U.), Y. Tsuboi (Chuo U.), H. Uchida (Kyoto U.), T. Yoneyama (Chuo U.)
on 24 Jun 2025; 14:29 UT
Credential Certification: Yoshiaki Kanemaru (kanemaru.yoshiaki@jaxa.jp)

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XRISM/Xtend Transient Search (XTS) detected an X-ray brightening from an X-ray source XRISM J1235-3957 on 2025-6-22 TT. The source position is determined to be (R.A., Dec.) = (188.722, -39.947), with a systematic error of ∼ 40 arcsec. There is no plausible X-ray counterpart to XRISM J1235-3957 within a 2 arcmin radius. All statistical uncertainties in this report are provided as a 90% confidence level unless stated otherwise.

The brightening lasted only about four minutes from 2025-6-22 at ∼ 22:27 to 2025-6-22 at ∼ 22:31 TT. In the brightening, the flux is calculated as (2 ± 1) × 10^-12 erg s^-1 cm^-2 (0.4 – 10.0 keV), using an absorbed power-law model. A systematic error of roughly 20% should be added to the statistical error.

We derived the above systematic error for the flux by comparing our derived values for the sources detected with XTS in several observations with those for the corresponding X-ray counterparts. We estimated the systematic error for the source position from the separations between the detected sources with the corresponding counterparts in the same field of view.