XRISM/Xtend Transient Search (XTS) detected an X-ray flare from XRISM J0057+6021

Abstract

XRISM/Xtend Transient Search (XTS) detected an X-ray flare from an X-ray source XRISM J0057+6021 on 2024-12-29 TT. The source position is determined to be (R.A., Dec.) = (14.195, 60.356), with a systematic error of ∼ 40 arcsec. A plausible counterpart is a multiple star HD 5408 (B7Vn+B9VHgMn+A1V). HD 5408 is located ∼ 20 arcsec apart from the position of XRISM J0057+6021. All statistical uncertainties in this report will be provided as a 90% confidence level unless stated otherwise.

The flare started on 2024-12-29 at ∼ 11h TT. The flare exponentially decayed in 5 × 10³ sec. 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 = 0.5 keV and hydrogen column density NH = 6 × 10²¹ cm⁻². Then, the model peak flux is calculated as 1 × 10⁻¹² 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 3 × D₁₆₅ₚ꜀² × 10³⁰ erg s⁻¹ by assuming the distance to XRISM J0057+6021 of D₁₆₅ₚ꜀. 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.