EP240703a: EP-WXT detection of a fast X-ray transient

We report on the detection of a fast X-ray transient, designated EP240703a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-07-03T00:38:40 (UTC). The position of the source is R.A. = 273.803 deg, DEC = -9.681 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The corresponding Galactic coordinates are l = 20.174, b = 3.521. The transient lasted for approximately 300 seconds and had a peak absorbed flux of ~5 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a column density of 1.5(+0.7/-0.6) x 10^22 cm^-2 and a photon index of 2.0(+1.0/-0.9). The derived average unabsorbed 0.5-4 keV flux is 5.9(+8.6/-2.6) x 10^-9 erg/s/cm^2. If the column density is fixed at the Galactic value of 4.5 x 10^21 cm^-2, the derived photon index is 0.4(+/-0.3) and the average unabsorbed 0.5-4 keV flux is 2.5(+0.4/-0.3) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. No previously known X-ray sources at a similar flux level are found within the 3 arcmin region around the source position. We have triggered a Swift target of opportunity observation. Further follow-up observations are encouraged to identify the nature of this X-ray transient. The above observation was made with the WXT instrument during the commissioning phase of EP. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.