TITLE: GCN CIRCULAR NUMBER: 40257 SUBJECT: EP250427a: Einstein Probe detection of a fast X-ray transient DATE: 25/04/27 09:34:46 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Y.Wang (PMO, CAS), D. Y. Li (NAO, CAS), Y. Q. Zhao (USTC, PRIC), J. H. Wu (GZHU), Y, Liu (NAO, CAS) 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 EP250427a (GCN Notice ID 01709135324). The WXT position of the source is R.A. = 277.281 deg, DEC = 7.570 deg (J2000) with an uncertainty of 2.0 arcmin in radius (90% C.L. statistical and systematic). A preliminary analysis of the EP-WXT data shows that the transient began at 2025-04-27T03:38:45(UTC) (before which the satellite was in the SAA region, and the real start time might be earlier than the value reported here) and lasted for about 180s, with a peak flux of 2 x 10^-8 erg/cm^2/s. The averaged WXT spectrum can be fitted by an absorbed power law model with a photon index of 1.70 (+0.36, -0.34) and a column density of 3.92 (+0.14, -0.13) x 10^21 cm^-2. The unabsorbed 0.5-4 keV flux is about 1.96 (+0.31, -0.23) x 10^-9 erg/cm^2/s. A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed about 5 hours after the WXT detection. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 277.2746 deg, DEC = 7.5639 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received. The contact TA of EP250427a is Y. Wang. Please contact him via email wangyun@pmo.ac.cn if needed. 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/40257. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...