EP240820a: EP detection of a fast X-ray transient

We report on the detection of a fast X-ray transient designated EP240820a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-08-20T00:54:47(UTC). The WXT position of EP240820a is R.A.= 16.221 deg, DEC = -34.698 deg (J2000) with an uncertainty of 2.1 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the transient observed by the WXT lasts around 250 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.2(-0.7, +0.8) (with a column density fixed at the Galactic one of 1.62 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.2(-0.5, +0.7) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP. The observation began at 2024-08-20T09:48:47 (UTC), and the exposure time is about 1.8 ks. FXT-B detected an uncataloged weak souce at R.A. = 16.2459 , DEC = -34.6941 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). Given the limited counts, the FXT spectrum is fitted using an absorbed power law and the parameters are fixed to those obtained by the EP-WXT spectral fitting. The derived flux is about 2.0 x 10^(-13) erg/s/cm^2. Further follow-up observations are encouraged to identify the nature of this X-ray transient. 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).