TITLE: GCN CIRCULAR NUMBER: 41752 SUBJECT: EP250908b: EP-WXT detection of a fast X-ray transient and EP-FXT observation DATE: 25/09/09 02:42:35 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Y.-H. I. Yin (NJU), M. H. Zhang, M. J. Liu, X. X. Sun, T. Y. Liu, H. Y. Ren, H. Sun (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 EP250908b. The transient triggered EP-WXT (ID: 01709240653) at 2025-09-08T18:26:19 (UTC). The WXT position of the source is R.A. = 297.534 deg, DEC = -57.505 deg (J2000) with an uncertainty of 2.5 arcmin in radius (90% C.L. statistical and systematic). The WXT data shows that the event started at T0=2025-09-08T18:26:08 (UTC) and lasted for about 13 s before the interruption of the autonomous follow-up. The WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a hydrogen column density of 1.59 x 10^21 cm^-2 and a photon index of 1.14 (-0.45/+0.45). The derived peak unabsorbed 0.5-4 keV flux is 8.14 (-1.60/+2.00) x 10^-9 erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 164s after T0. On-ground analysis of the FXT data found a fading uncatalogued source at R.A. = 297.4784, DEC = -57.4938 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), within the WXT error circle. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a hydrogen column density of 1.59 (-0.76/+0.76) x 10^21 cm^-2 and a photon index of 2.02 (-0.23/+0.23). The derived average unabsorbed 0.5-10 keV flux is 1.10 (-0.09/+0.09) x 10^-12 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/41752. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...