TITLE: GCN CIRCULAR NUMBER: 40554 SUBJECT: EP250526a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/05/27 12:44:31 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Jiahua Wu (GZHU), D. Y. Li (NAO, CAS), B.-T. Wang (YNAO, CAS), W. F. Wen (SZTU) and Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team: The X-ray transient EP250526a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Li et al., GCN 40550), and followed up by several optical telescopes (Lipunov et al, GCN 40551, Angulo et al., GCN 40552), while no optical counterpart has been detected yet. Refined analysis of the WXT data shows that the event started at T0=2025-05-26T19:24:55 (UTC) and lasted for about 175 s (T90). The peak flux (0.5-4 keV) is estimated to be 4 x 10^-9 erg/s/cm^2 (T0+39 s). The averaged 0.5-4 keV spectrum of T90 can be fitted with an absorbed power-law with a Galactic hydrogen column density of 7 x 10^20 cm^-2 and a photon index of 1.31 (-/+0.42). The derived average unabsorbed 0.5-4 keV flux is 1.75 (-0.37, +0.51) x 10^-9 erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2025-05-26T19:31:10, about 375 seconds after T0, with an exposure time of 2270 seconds. The averaged 0.5-10 keV spectrum can be fitted with an absorbed power-law with a photon index of 2.59 (-0.43, +0.44). The derived unabsorbed 0.5-10 keV flux is 7.0 (-1.4, +2.0) x 10^-13 erg/s/cm^2. Another FXT observation was conducted at 2025-05-27T04:18:56, about 9 hours after T0. The X-ray spectrum can be fitted with an absorbed power-law with a photon index of 2.37 (-0.93, +0.91), and the derived unabsorbed 0.5-10 keV flux is 1.2 (-0.5, +1.2) x 10^-13 erg/s/cm^2. The absorption was fixed at the Galactic value during the spectral fitting. 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). The contact TA of this source is Jiahua Wu, please contact him via the email jhwu@e.gzhu.edu.cn if needed. View this GCN Circular online at https://gcn.nasa.gov/circulars/40554. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...