EP240919a: EP-WXT detection of a fast X-ray transient

We report on the detection of a fast X-ray transient designated EP240919a, which was detected by EP-WXT at 2024-09-19T14:47:40 (UTC), and triggered the on-board processing unit at 2024-09-19T14:53:43 (trigger ID: 01709054141). An autonomous observation was performed by the Follow-up X-ray Telescope (FXT) about five minutes later. Afterwards, the FXT on-board processing unit was also triggered, showing that an uncatalogued source was detected at R.A. = 334.2797, DEC = -9.7362 (J2000) with an uncertainty of about 20 arcsec (radius, 90% C.L. statistical and systematic). The light curve of the transient observed by the WXT lasts more than 400 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed power-law with a photon index of 1.3(+0.3/-0.3) (with the column density fixed at the Galactic value of 5.4x10^20 cm^-2). The absorbed average flux in 0.5-4 keV is 7.0(+1.8/-1.5) x 10^(-10) erg/s/cm^2. The quoted uncertainties are at the 90% confidence level for the above parameters. 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 onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).