GCN 45214: EP260725a: EP-WXT detection of an X-ray transient

Y.J. Zhang (PKU), R. D. Liang, W. Chen, H. W. Pan (NAOC) on behalf of the Einstein Probe (EP) team:              

We report on the detection of an X-ray transient, designated EP260725a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2026-07-25 02:45:59 (UTC), with a peak flux of ~ 1 x 10^(-10) erg cm^-2 s^-1 in 0.5-4 keV. The position of the transient is R.A. = 337.069 deg, Dec. = 50.764 deg (J2000), with an uncertainty radius of 3 arcmin in radius (90% confidence level, including both statistical and systematic uncertainties). 

The average WXT 0.5-4 keV spectrum can be fitted by an absorbed power-law model with a fixed Galactic hydrogen column density of 7.0×10^20 cm^-2 and a photon index of 3.5 (-1.7/+2.2). The derived average unabsorbed 0.5-4 keV flux is 2.0(-0.8/+2.4) x 10^(-11) erg cm^-2 s^-1. The uncertainties are at the 90% confidence level for the above parameters.

EP-FXT performed a follow-up observation of EP260725a detected by EP-WXT. The observation started at 2026-07-25 17:40:12 UTC, about 15 hr after the WXT detection, with a total exposure time of around 3.0 ks. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 337.0718  deg, DEC = 50.7757 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).

Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the dynamic X-ray sky and perform rapid follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).