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

Abstract

XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source, XRISM J1911+0509, on 2025-03-27 TT. The source position is determined to be (R.A., Dec.) = (287.733, 5.150), with a systematic error of ∼ 40 arcsec. A plausible counterpart is an X-ray source 2E 1908.4+0503, which may correspond to a spectroscopic binary UCAC4 476-091023 at 259 pc. UCAC4 476-091023 is located ∼ 5 arcsec apart from the position of XRISM J1911+0509. All statistical uncertainties in this report will be provided at a 90% confidence level unless stated otherwise. The flare started around 2025-03-27 at 20:00 TT, peaked on 2025-03-27 at 20:54 TT, and then, exponentially decayed in 2 × 10³ sec, which is derived by fitting the 0.4 – 2.0 keV light curve with a constant + burst model in the QDP software package. In order to estimate the source flux, we fit the spectrum in the flare phase with an absorbed APEC model with a temperature of kT = 1.3 (+0.4/-0.2) keV and a hydrogen column density N<sub>H<sub> < 4 × 10²¹ cm⁻². Then, the model flux is calculated as (6.5 +1.5/- 1.3) × 10⁻¹³ erg s⁻¹ cm⁻² (0.4 – 10.0 keV). A systematic error of roughly 20% should be added to the statistical error. Corresponding luminosity is (5.2 +1.2/-1.1) D²₂₅₉ ₚ꜀ × 10³⁰ erg s⁻¹. 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 and their corresponding counterparts in the same field of view. <sub> : subscript