XRISM/Xtend Transient Search (XTS) detected a possible stellar flare from the Galactic center
ATel #16794; 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), 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 30 Aug 2024; 06:51 UT
Credential Certification: Tomokage Yoneyama (tyoneyama263@g.chuo-u.ac.jp)
Subjects: X-ray, Star, Transient
XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J1745-2914 on 2024-08-29 TT. The source position is determined to be (R.A., Dec.) = (266.276, -29.246), with a systematic error of ∼ 40 arcsec. One of the plausible counterparts is an F-type star Gaia DR3 4057091288225954688 at 956 pc of distance. However, there are many other candidates due to pointing toward the Galactic center, and we cannot reject other candidates confidently. Gaia DR3 4057091288225954688 is located – 5 arcsec apart from the position of XRISM J1745-2914. All statistical uncertainties in this report will be provided as a 90% confidence level unless stated otherwise.
Due to a bad time interval of XRISM, the flare start time may not be detected firmly; it began at around 2024-08-29 at 17:16 TT. The flare reached its peak on 2024-08-29 at 17:54 – 18:06 TT. The flare exponentially decayed with an e-folding time of 1.9 (+3.3/-0.9) × 103 sec, which is derived by fitting the 0.4 – 10.0 keV light curve with a constant + exponential model in the QDP software package. However, we note that the flare was still in its decay phase at the end of our data.
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 = 7.6 (+8.4/-3.3) keV and hydrogen column density NH = 2.3 (+0.6/-0.7) × 1023 cm-2. Then, the model flux is calculated as 2.0 (+0.2/-1.7) × 10-11 erg s-1cm-2 (0.4 – 10.0 keV). A systematic error of roughly 20% should be added to the statistical error. Corresponding luminosity is 2.4 (+0.2/-2.0) D1kpc2 × 10 33 erg s-1 by assuming the distance to XRISM J1745-2914 of D1kpc kpc; thus, D1kpc = 0.96, for instance, if the counterpart is Gaia DR3 4057091288225954688 as described above.
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.
