XRISM/Xtend Transient Search (XTS) detected multiple stellar flares from WOH S 423

Abstract

XRISM/Xtend Transient Search (XTS) detected multiple X-ray flares from the X-ray source XRISM J0535-6613 on 2025-05-25 and 05-26 TT. The source position is (R.A., Dec.) = (83.827, -66.218), with a systematic error of ∼ 40 arcsec. The possible counterpart is a high proper motion star, WOH S 423 at 82 pc, which is located ∼ 30 arcsec apart from the position of XRISM J0535-6613. We report the larger one in this telegram. All statistical uncertainties in this report will be provided at a 90% confidence level unless stated otherwise.

The larger flare started on 2025-05-26 18:18 TT and rapidly peaked on 05-26 18:22 TT, about one day after a faint flare on 2025-05-25. Then, the flare exponentially decayed in ∼ 8 × 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 peak with two APEC models with temperatures of kT = 0.3 +/- 0.2 keV and 1.7 +/- 0.4 keV, which are subject to a weak absorption with hydrogen column density N<sub>H<sub> < 5 × 10²⁰ cm⁻². The model flux is calculated as (2.5 +/- 0.3) × 10⁻¹² erg s⁻¹ cm⁻² (0.4 – 10.0 keV), corresponding to a luminosity (2.0 +/- 0.2) D²₈₂ pc × 10³⁰ erg s⁻¹. 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 and their corresponding counterparts in the same field of view. <sub> : subscript