TITLE: GCN CIRCULAR NUMBER: 40399 SUBJECT: EP250508a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/05/08 16:08:00 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> T.Zhao, R. D. Liang, H. Q. Cheng, W. D. Zhang (NAOC, CAS) on behalf of the Einstein Probe (EP) team: The X-ray transient EP250508a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Liang et al., GCN 40390), and followed up by several telescopes (Antonio et al, GCN 40391; Lipunov et al, GCN 40393; Brivio et al, GCN 40395). Refined analysis of the WXT data shows that the event started at T0=2025-05-08T06:02:28.850 (UTC) and lasted for about 60 s before the observation was interrupted by the autonomous follow-up observation. The peak flux is about 3.6 x 10^(-9) erg/s/cm^2. The averaged 0.5-4 keV spectrum can be fitted by an absorbed power law model with a Galactic hydrogen column density of 3.56 x 10^20 cm^-2 and a photon index of 0.91 (+/-0.54). The derived average unabsorbed 0.5-4 keV flux is 1.4 (-0.4/+0.5) x 10^(-9) erg/s/cm^2. The uncertainties are quoted at the 90% confidence level for the above parameters. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 3 min after T0, starting at 2025-05-08T06:05:21 (UTC). Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 178.2555, DEC = -19.9244 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). During the observation a decline in the source flux lasting for a few hundred seconds is observed from the beginning. The averaged 0.5-10 keV FXT spectrum can be fitted by an absorbed power law model with a Galactic hydrogen column density of 3.56 x 10^20 cm^-2 and a photon index of 2.30 (-0.45/+0.45). The derived average unabsorbed 0.5-10 keV flux is 2.0 (-0.5/+0.8) x 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. EP-FXT will continue monitoring this source. More follow-up observations are encouraged. The contact TA of EP250508a is R. D. Liang. Please contact him via email liangrd@bao.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/40399. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...