EP240816b: EP-WXT detection and EP-FXT follow-up observation of a fast X-ray transient

We report on the detection of a fast X-ray transient, designated EP240816b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2024-08-16T01:44:27 (UTC) and triggered the WXT on-board processing unit (trigger ID: 01709034033). The light curve lasts for more than 50 seconds. The WXT position of the source is R.A. = 16.013 deg, DEC = 15.398 deg (J2000) with an uncertainty of 3.33 arcmin in radius (90% C.L. statistical and systematic). The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.6(+0.7/-0.7) (with a frozen column density value of 5.2 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.3(+0.7/-0.5) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. The follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was triggered automatically at 2024-08-16T01:47:34 (UTC). An uncatalogued X-ray source is detected at R.A. = 16.0161 deg, DEC = 15.4151 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The average FXT spectrum in 0.5-10 keV band can be fitted by an absorbed power law with a photon index of 1.69(+0.23/-0.22) (with a freely fitted column density value of 9.5(+6.9/-6.7) x 10^20 cm^-2), giving an average unabsorbed flux in 0.5-10 keV of 4.3(+0.6/-0.5) x 10^-12 erg/s/cm^2. No previously known bright X-ray sources are found within the error circle around the source position. 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).