XRISM/Xtend Transient Search (XTS) detected an X-ray flare possibly from a spectroscopic binary

ATel #16592; T. Yoneyama (Chuo U.), 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.), M. Yoshimoto (Osaka U.)
on 17 Apr 2024; 13:50 UT
Credential Certification: Yohko Tsuboi (tsuboi@phys.chuo-u.ac.jp)

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XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source candidate XRISM J1911+0509 on 2024-04-12 TT. The source position is determined to be (R.A., Dec.) = (287.737 deg, 5.152 deg) with systematic error of ~ 50 arcsec. The possible X-ray candidates are 2E 1908.4+0503, 4XMM J191056.2+050903, 2RXP J191055.5+050905, and 2RXP J191056.6+050909. Each candidate is located ~ 32, 11, 21, and 5 arcsec apart from the position of XRISM J1911+0509, respectively. The possible optical candidate is UCAC4 476-091023, which is a spectroscopic binary (~ 10 arcsec apart).

Due to a bad time interval of XRISM, the flare peak was not detected, but only the decay phase was detected. Only a rough constraint, 2024-04-12 10:08 – 11:15 TT is given for the flare peak time. The flare exponentially decays with an e-folding time of 8.4 (+3.2/-2.3) × 10³ sec (90% confidence level), which is derived by fitting the 0.4 – 2.0 keV light curve with a constant + exponential model in the QDP software package.

The spectrum in the flare phase is reproduced with a thermal plasma (APEC) model with absorption of NH < 1 × 10²² cm^-2 (90% upper limit) and temperature of kT = 0.4 (+1.0/-0.2) keV (90% confidence level). Due to the poor photon statistics, we obtain only the 90% upper limit for the model flux of < 3 × 10^-12 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.