XRISM/Xtend Transient Search (XTS) detected an X-ray flare from the flare star HD 120476

Abstract

XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J1349+2659 on 2025-01-21 TT. The source position is determined to be (R.A., Dec.) = (207.270, 26.975), with a systematic error of ∼ 40 arcsec. A plausible counterpart is a binary star system HD 120476, which is composed by K4V and K6V. The source corresponds to an X-ray source EXO 1346.7+2713. HD 120476 is located ∼ 20 arcsec apart from the position of XRISM J1349+2659. All statistical uncertainties in this report will be provided as a 90% confidence level unless stated otherwise. This is the first report of the X-ray flare from this source.

The flare started at 2025-01-21 at ∼ 16:37 TT, and the flux is still increasing at the end of the observation at 2025-01-21 at 18:15.

In order to estimate the source flux, we fit the spectrum in the flare peak phase with an absorbed APEC model with a temperature of kT = 0.2 keV and hydrogen column density NH = 6 × 10²¹ cm⁻². Then, the model flux is calculated as 7 × 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 7 × D₁₃.₅ₚ꜀² × 10²⁸ erg s⁻¹ by assuming the distance to XRISM J1349+2659 of D₁₃.₅ₚ꜀.

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.