XRISM/Xtend Transient Search (XTS) detected an X-ray flare possibly from 4XMM J190821.5+065854
ATel #16558; M. Yoshimoto (Osaka U.), K. Hayashi, Y. Kanemaru, S. Ogawa, T. Yoshida (JAXA), K. Akasu (Chuo U.), M. Audard (U. de Geneve), E. Behar (Technion), S. Inoue (Kyoto U.), T. Kohmura (TUS), Y. Maeda (JAXA), M. Mizumoto (UTEF), N. Nemoto (Chuo U.), 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.)
on 28 Mar 2024; 15:28 UT
Credential Certification: Yohko Tsuboi (tsuboi@phys.chuo-u.ac.jp)
Subjects: X-ray, Transient, Variables
Referred to by ATel #: 16561
XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source candidate XRISM J1908+0659 on 2024-03-27 TT. The source position is determined to be (R.A., Dec.) = (287.089 deg, 6.976 deg) with a systematic uncertainty of 40 arcsec. Within 40 arcsec, only one X-ray source 4XMM J190821.5+065854 is located ~ 21 arcsec apart, while 20 optical sources are found in the Gaia DR3 catalog.
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, 2024-03-27 03:05 – 03:55 is given for the flare peak time. The flare exponentially decays with an e-folding time of (3.0 +3.3/-1.3) × 103 sec (90% C.L.), 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 is represented either by a power-law (PL) model with a photon index of Γ=1.8 (+0.8/-0.7) or a thermal plasma (APEC) model with kT > 1.9 keV (90% C.L.). The flux is (5.3 +3.0/-1.9) × 10-13 erg/s/cm2 for PL model and (4.6 +3.4/-1.9) × 10-13 erg/s/cm2 for APEC model, respectively (0.4 – 10 keV; 90% C.L.). A systematic error of roughly 20% should be added to the statistical error. The flux of the possible X-ray counterpart 4XMM J190821.5+065854 was 2.1 × 10-14 erg/s/cm2 (0.2 – 12 keV), which is significantly lower than that XRISM observed.
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 uncertainty for the source position from the separations between the detected sources with the corresponding counterparts in the same field of view.
