XRISM/Xtend Transient Search (XTS) detected a possible stellar flare from 4XMM J114021.0-651852/1eRASS J114021.5-651851

ATel #16779; S. Ogawa, K. Fukushima, K. Hayashi, Y. Kanemaru, 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 19 Aug 2024; 10:02 UT
Credential Certification: Tomokage Yoneyama (tyoneyama263@g.chuo-u.ac.jp)

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XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J1140-6519 on 2024-08-17 TT. The source position is determined to be (R.A., Dec.) = (175.081, -65.316), with a systematic error of ∼ 40 arcsec. The plausible X-ray counterparts are 4XMM J114021.0-651852 and 1eRASS J114021.5-651851, which are located ∼ 12 and 16 asec apart from the position of XRISM J1140-6519. 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 intrinsic flare peak may not be detected, but a rough constraint, 2024-08-17 02:54 – 03:59 TT is given for the flare peak time. The flare exponentially decayed with an e-folding time of ∼ 3.7 (+2.9/-2.3) × 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.

In order to estimate the source flux, we fit the spectrum in the flare peak phase with an unabsorbed APEC model with temperatures of kT = 1.2 (+0.3/-0.2) keV. Then, the model flux is calculated as 1.3 (+0.3/-0.3) × 10^-13 erg s^-1cm^-2 (0.4 – 10.0 keV). 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.