TITLE: GCN CIRCULAR NUMBER: 38660 SUBJECT: EP241223a: EP-WXT detection of a fast X-ray transient DATE: 24/12/24 03:08:39 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> H. Zhou (PMO, CAS), B. -T. Wang (YNAO, CAS), H. Sun, C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team: We report on the detection of a fast X-ray transient, designated EP241223a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event was observed to start at 2024-12-23T07:21:23 (UTC). The WXT position of the source is R.A. = 74.804 deg, DEC = 7.110 deg (J2000) with an uncertainty of 3.3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for about 80 seconds. The peak flux in the 0.5-4 keV is around 2.4 x 10^-9 erg/s/cm^2. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 2.01(+1.02/-0.85) (with a galactic column density fixed at 1.22 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 9.0(+4.0/-4.0) x 10^-10 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. A follow-up observation with the Follow-up X-ray Telescope(FXT) on board EP has been scheduled. Further observations are encouraged to explore the origin of EP241223a. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/38660. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...