XRISM/Xtend Transient Search (XTS) detected a possible stellar flare from 1RXS J113700.0-651617

ATel #16773; 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 15 Aug 2024; 02:32 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 J1137-6516 on 2024-08-13 TT. The source position is determined to be (R.A., Dec.) = (174.244, -65.273), with a systematic error of – 40 arcsec. The plausible counterpart is 1RXS J113700.0-651617, a star with a distance of – 155 pc. 1RXS J113700.0-651617 is located – 12 arcsec apart from the position of XRISM J1137-6516.

Due to a bad time interval of XRISM, the intrinsic flare peak may not be detected, but a rough constraint, 2024-08-13 20:41 – 21:19 TT is given for the flare peak time. The flare exponentially decayed with an e-folding time of ∼ 2 × 10⁴ sec, which is derived by fitting the 0.4 – 10.0 keV light curve with a constant + exponential model in the QDP software package. The confidence range is not constrained due to low photon statistics.

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 = 1.4 (+0.3/-0.2) keV (90% confidence level). Then, the model flux is calculated as 1.2 (+/-0.2) × 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 3.5 (+/-0.6) × 10 ³⁰ erg s^-1 by assuming the distance to 1RXS J113700.0-651617 of 155 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.