TITLE: GCN CIRCULAR NUMBER: 41737 SUBJECT: EP250905a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/09/06 11:24:15 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Y. Wang (PMO), Y.H.Cheng (SWIFAR,YNU), R. D. Liang, W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team: The X-ray transient EP250905a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (GCN 41725), with several multi-wavelength follow-up observations (GCN 41727, GCN 41732, GCN 41734). The refined WXT data shows that the flare started at about 2025-09-05 11:07:08 UTC (T0). WXT detected the source from T0 to before the slew with a peak flux of 1.4 x 10^(-9) erg/s/cm^2. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 7.0 x 10^20 cm^-2 and a photon index of 1.7 (+/-0.4). The derived average unabsorbed 0.5-4 keV flux is around 5.7 (-1.3/+1.6) x 10^(-10) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 405 s after T0. On-ground analysis of the FXT data found an fading source at R.A. = 4.4504 deg, DEC = 37.4697 deg (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 Galactic hydrogen column density of 7.0 x 10^20 cm^-2 and a photon index of 2.3(+/-0.4). The derived average unabsorbed 0.5-10 keV flux is around 4.5(-1.0/+1.5) 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/41737. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...