XRISM/Xtend Transient Search (XTS) detected a flux increase of XRISM J1236-6432

XRISM/Xtend Transient Search (XTS) detected a gradual increase in the flux of the X-ray source XRISM J1236-6432 from 2025-6-12 to 2025-6-13 TT. The source position is determined to be (R.A., Dec.) = (188.884, -64.528), with a systematic error of ? 40 arcsec. A plausible counterpart is the 4XMM J123530.3-643125, which is located ? 20 arcsec apart from the position of XRISM J1236-6432. All statistical uncertainties in this report are provided as a 90% confidence level unless stated otherwise. In the duration of ? 80 ks, the flux increased several times. The average flux is calculated as 5 × 10-13 erg s-1 cm-2 (0.4 – 10.0 keV), using an absorbed power-law model. This result is one order of magnitude greater than the catalog flux of the 4XMM J123530.3-643125. The photon index and column density are derived as 1 and < 3 × 1020 cm-2, respectively. 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.