EP240426b: EP-WXT detection of a new fast X-ray transient

We report on the detection of a fast X-ray transient EP240426b at 2024-04-26T14:19:06 (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. = 173.787 deg, DEC = -40.741 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for ~300 seconds and has a peak flux of ~9.5 x 10^-10 erg/s/cm^2 in the 0.5-4 keV band. The averaged 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.8(-0.3/+0.3) (with the column density fixed at the Galactic value of 8.08 x 10^20 cm^-2), giving an unabsorbed flux of 3.6(-0.6/+0.6) x 10^-10 erg/s/cm2 in the 0.5-4 keV band. The 1-sigma uncertainties are given for the above parameters. Note: EP240426b is about 40 degrees away from the 90% credible region of the recent BH-NS gravitational-wave event S240422ed (GCN 36236), therefore it is an independent source unrelated to this event. No previously known X-ray sources at a similar flux level are found within the 3 arcmin region around the source position. Further multi-wavelength follow-up observations are 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.