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XRISM/Xtend Transient Search (XTS) detected an X-ray flare from the flare star HD 120476

ATel #16990; H. Sugai (Chuo U.), K. Fukushima, K. Hayashi, Y. Kanemaru, S. Ogawa, T. Yoshida (JAXA), M. Audard (U. de Geneve), E. Behar (Technion), S. Inoue (Kyoto U.), Y. Ishihara (Chuo U.), T. Kohmura (TUS), Y. Maeda (JAXA), M. Mizumoto (UTEF), N. Nagashima (Chuo U.), M. Nobukawa (NUE), K. Pottschmidt (UMBC, NASA GSFC, CRESST), M. Shidatsu (Ehime U.), Y. Terada (Saitama U.), Y. Terashima (Ehime U.), Y. Tsuboi (Chuo U.), H. Uchida (Kyoto U.), T. Yanagi (Chuo U.), T. Yoneyama (Chuo U.), M. Yoshimoto (Osaka U.)
on 23 Jan 2025; 07:44 UT
Credential Certification: Tomokage Yoneyama (tyoneyama263@g.chuo-u.ac.jp)

Subjects: X-ray, Binary, Star, Transient, Variables

XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J1349+2659 on 2025-01-21 TT. The source position is determined to be (R.A., Dec.) = (207.270, 26.975), with a systematic error of ∼ 40 arcsec. A plausible counterpart is a binary star system HD 120476, which is composed by K4V and K6V. The source corresponds to an X-ray source EXO 1346.7+2713. HD 120476 is located ∼ 20 arcsec apart from the position of XRISM J1349+2659. All statistical uncertainties in this report will be provided as a 90% confidence level unless stated otherwise. This is the first report of the X-ray flare from this source.

The flare started at 2025-01-21 at ∼ 16:37 TT, and the flux is still increasing at the end of the observation at 2025-01-21 at 18:15.

In order to estimate the source flux, we fit the spectrum in the flare peak phase with an absorbed APEC model with a temperature of kT = 0.2 keV and hydrogen column density NH = 6 × 1021 cm-2. Then, the model flux is calculated as 7 × 10-13 erg s-1 cm-2 (0.4 – 10.0 keV). A systematic error of roughly 20% should be added to the statistical error. Corresponding luminosity is 7 × D13.5pc2 × 1028 erg s-1 by assuming the distance to XRISM J1349+2659 of D13.5 pc.

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.