TITLE: GCN CIRCULAR NUMBER: 39532 SUBJECT: EP250227a: Einstein Probe detected of a fast X-ray transient DATE: 25/02/28 15:52:07 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> W.F. Wen (SZTU), J. H. Wu (GZHU), H. Z. Wu (HUST), B. -T. Wang (YNAO, CAS), Z. X. Ling(NAO, CAS) report on behalf of the Einstein Probe team: We report on the detection of a fast X-ray transient EP250227a (trigger ID: 11916647689) at 2025-02-27 04:39:40 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The position of the source is R.A. = 204.133 deg, DEC = -13.820 deg (J2000) with an uncertainty of 2.7 arcmin (radius, 90% C.L. statistical and systematic). The transient event lasted for about 40 seconds. The 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with nH fixed at 1.87 x 10^20 cm^-2 and a photon index of -0.07(+2.18/-1.22). The unabsorbed 0.5-4.0 keV flux is 2.1 (+2.6/-1.2) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. We performed a follow-up target of opportunity observation with EP-FXT. The observation began at 2025-02-27 15:19:07 (UTC) with an exposure time of 3020 seconds, about 10.7 hours after the detection by EP-WXT. Within the WXT error cirlce, an uncatalogued X-ray source was detected at R.A. = 204.1444 deg, DEC = -13.8216 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The X-ray spectrum can be fitted with an absorbed powerlaw with the nH fixed at the Galactic value and a photon index of 1.2 (-0.8, +0.8). The derived unabsorbed 0.5-10 keV flux is 2.4 (-1.4, +5.2) x 10^-13 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/39532. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...