Einstein Probe detected of a fast X-ray transient EP240315a

We report on the detection of a fast X-ray transient EP240315a at 2024-03-15T20:10:44 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 141.644 deg, DEC = -9.547 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the source shows a multi-peak profile. The transient event lasts for ~1600 seconds and has a peak flux of ~3e-9 erg/s/cm^2 in the 0.5-4 keV band. The averaged spectrum can be fitted by an absorbed power-law with NH = 1.5(-0.9/+1.0) x 10^21 cm^-2 and a photon index of 1.7(-0.4/+0.4). The derived average unabsorbed 0.5 - 4.0 keV flux is 5.3(-0.7/+1.0) x 10^-10 erg/s/cm^2. However, we note that the derived source parameters may be subject to larger uncertainties than those quoted here since in-orbit calibration of the instrument is still in progress. No previously known bright X-ray sources have been found within the 3 arcmin region around the source position, with only one optical counterpart of white dwarf candidate located at the distance of ~380 pc. Based on the shape and timescale of the observed flare light curve, we tend to consider that the source is not a stellar flare, although this cannot be ruled out. A Swift target of opportunity observation has been proposed, and further follow-up observations are strongly encouraged to identify the nature of this X-ray flare. 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.