XRISM/Xtend Transient Search (XTS) detected a possible stellar flare from UCAC2 15735923
ATel #16728; T. Yoshida, K. Fukushima, K. Hayashi, Y. Kanemaru, S. Ogawa (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. Yoneyama (Chuo U.), M. Yoshimoto (Osaka U.)
on 21 Jul 2024; 07:29 UT
Credential Certification: Tomokage Yoneyama (tyoneyama263@g.chuo-u.ac.jp)
Subjects: X-ray, Star, Transient
Referred to by ATel #: 16731
XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J1138-3742 on 2024-07-19 TT. The source position is determined to be (R.A., Dec.) = (174.415 deg, -37.707 deg), with a systematic error of -40 arcsec. The plausible counterpart is UCAC2 15735923, a star with a distance of -128 pc. UCAC2 15735923 is located – 8 arcsec apart from the position of XRISM J1138-3742.
Due to a bad time interval of XRISM, the intrinsic flare peak may not be detected, but a rough constraint, 2024-07-19 09:33 – 10:11 TT is given for the flare peak time. The flare exponentially decayed with an e-folding time of (6.8 +/- 1.7) × 103 sec (90% confidence level), which is derived by fitting the 0.4 – 10.0 keV light curve with a constant + burst model in the QDP software package.
In order to estimate the source flux, we fit the spectrum in the flare peak phase with unabsorbed two APEC models with temperatures of kT1 = 1.2 (+0.7/-0.9) keV (90% confidence level) and kT2 = 4.3 (+5.4/-1.2) keV, respectively. Then, the model flux is calculated as 8.2 (+0.2/-1.2) × 10-12 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 1.6 (+/-0.2) × 10 31 erg s-1 by assuming the distance to UCAC2 15735923 of 128 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.
