XRISM/Xtend Transient Search (XTS) detected an outburst from a white dwarf candidate

XRISM/Xtend Transient Search (XTS) detected an X-ray outburst from an X-ray source XRISM J1826-3651 on 2025-04-12 TT. The source position is determined to be (R.A., Dec.) = (276.543, -36.849), with a systematic error of ~40 arcsec. A plausible counterpart is a white dwarf candidate Gaia DR3 6728071049507752320, which is located ~16 arcsec apart from the position of XRISM J1826-3651. The XRISM observation was started at 2025-04-12T18:07:19 TT, where the flux was estimated to be 8 × 10-14 erg s-1 cm-2 (0.4 – 10.0 keV). The source flux was increased by an order of magnitude (8 × 10-13 erg s-1 cm-2) by 2025-04-12 at ~22:52 TT. The outburst was continuing at the end of the observation, 2025-04-13T02:55:13 TT. Corresponding luminosities were 3 × D1.7 kpc2 × 1031 erg s-1 and 3 × D1.7 kpc2 × 1032 erg s-1, respectively by assuming the distance to XRISM J1826-3651 of D1.7 kpc. 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.