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XRISM/Xtend Transient Search (XTS) detected the second X-ray flare from UCAC2 15735923

ATel #16731; K. Hayashi, K. Fukushima, 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. Yoneyama (Chuo U.), M. Yoshimoto (Osaka U.)
on 23 Jul 2024; 09:15 UT
Credential Certification: Tomokage Yoneyama (tyoneyama263@g.chuo-u.ac.jp)

Subjects: X-ray, Star

XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J1138-3742, whose plausible counterpart is UCAC2 15735923, on 2024-07-21. This is the second flare of this source during the XRISM observation of NGC3783 (ATel #16728) after ∼ 59 hour of quiescence. The source position is determined to be (R.A., Dec.) = (174.415 deg, -37.707 deg), with a systematic error of ∼ 40 arcsec. As mentioned below, the second flare is the similar luminosity to the first one.

The flare peaked at 2024-07-21 21:03 – 21:20 TT. Due to a bad time interval of XRISM, the flare decay is not observed. Only a rough constraint, < 2.4 ks, is given for the decay time.

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 = 0.87 (+0.39/-0.44) keV (90% confidence level) and kT₂ = 3.0 (+7.3/-1.1) keV, respectively. Then, the model flux is calculated as 7.5 (+1.9/-1.5) × 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 1.5 (+0.3/-0.4) × 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.