TITLE: GCN CIRCULAR NUMBER: 41809 SUBJECT: EP250911a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/09/12 05:02:03 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Y. F. Liang (PMO), Q. Y. Wu, W. D. Zhang(NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP250911a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Liang et al., GCN 41788), and followed by Swift/XRT (Evans et al., GCN 41802) and several optical telescopes (Quirola-Vasquez et al., GCN 41790 , Angulo et al., GCN 41793, Lai et al., GCN 41801, Moskvitin et al., GCN 41805, Ma et al., GCN 41807). The refined analysis of the WXT data shows that the event started at T0=2025-09-11T08:57:55 (UTC) and lasted for 34 s with the peak flux of 9.3 x 10^(-9) erg/s/cm^2, before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 4.79 x 10^20 cm^-2 and a photon index of 0.77 (-/+0.95). The derived average unabsorbed 0.5-4 keV flux is 3.4 (-1.5/+3.1) x 10^(-9) erg/s/cm^2. The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2025-09-11T09:03:21 (UTC), about six minutes after T0. The exposure time of this observation is 3856 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 357.5194, 29.2999 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 4.79 x 10^20 cm^-2 and a photon index of 2.02 (-/+0.22). The derived average unabsorbed 0.5-10 keV flux is 7.5 (-1.1/+1.4) x 10^(-13) erg/s/cm^2. 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/41809. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...