TITLE: GCN CIRCULAR NUMBER: 43178 SUBJECT: EP251221a: Einstein Probe detection of a fast X-ray transient DATE: 25/12/22 04:07:09 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> C. Y. Dai (NJU), J. H. Wu (GU), W. F. Wen (SZTU), H. Q. Cheng, W. D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251221a. The transient triggered EP-WXT (ID: 01709250307) at 2025-12-21T21:39:06 (UTC). The WXT position of the source is R.A. = 104.531 deg, DEC = 23.374 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The analysis of the WXT data shows that the event lasted for ~100 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 8.1 x 10^20 cm^-2 and a photon index of 0.45 (-0.34/+0.36). The derived average unabsorbed 0.5-4 keV flux is 7.28 (-2.23/+3.30) x 10^(-10) erg/s/cm^2 (All errors are 1 sigma uncertainties.). 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/43178. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...