TITLE: GCN CIRCULAR NUMBER: 40434 SUBJECT: EP250511a: Einstein Probe follow-up obsevation and refined analysis DATE: 25/05/12 06:36:55 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> A. Li (BNU), Y. L. Wang (NAO, CAS), T. Y. Lian (NAO, CAS), C. C. Jin (NAO, CAS) behalf of the Einstein Probe (EP) team: The X-ray transient EP250511a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Lian et al., GCN 40429), and followed up by several telescopes (Perez-Garcia et al, GCN 40430; Schneider et al, GCN 40431; Lipunov et al, GCN 40432; Li et al, GCN 40433). Refined analysis of the WXT data shows that the event started at T0=2025-05-11T07:59:59 (UTC) and lasted for about 50s. The peak flux is about 3.8 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 fixed Galactic hydrogen column density of 2.7 x 10^20 cm^-2 and a photon index of 2.17 (+/-0.71). The derived average unabsorbed 0.5-4 keV flux is 2.3 (-0.8/+0.6) x 10^(-10) 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 2025-05-11T08:03:06 (UTC). Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 202.1113, DEC = -2.7023 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The averaged 0.5-10 keV FXT spectrum can be fitted by an absorbed power law model with a fixed Galactic hydrogen column density of 2.7 x 10^20 cm^-2 and a photon index of 2.98 (-0.59/+0.62). The derived average unabsorbed 0.5-10 keV flux is 1.7 (-0.7/+0.9) x 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. The contact TA of EP250511a is A. Li. Please contact him via email anli@mail.bnu.edu.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/40434. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...