TITLE: GCN CIRCULAR NUMBER: 45301 SUBJECT: EP260731a: refined analysis of the EP-WXT and autonomous EP-FXT observations DATE: 26/08/07 03:41:59 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> K.R. Ni (CCNU), Y.F. Liang (PMO, CAS), H.Y. Liu, and H.W. Pan (NAO, CAS), on behalf of the Einstein Probe (EP) team: The X-ray transient EP260731a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Ni et al., GCN 45261), and followed up by NOT/ALFOSC and COLIBRÍ/DDRAGO (Liu et al., GCN 45263; de Ugarte Postigo et al., GCN 45264). We report the refined analysis of the EP-WXT and autonomous EP-FXT observations. The refined analysis of the WXT data shows that the event started at T0 = 2026-07-31T00:53:50 (UTC) and lasted for approximately 35 s. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 3.10 x 10^20 cm^-2 and a photon index of 0.89 (-0.50/+0.52). The derived average unabsorbed 0.5-4 keV flux is 2.30 (-0.70/+0.94) x 10^-9 erg s^-1 cm^-2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously starting at 2026-07-31T00:56:34 (UTC, T0+164 s). The exposure time of this observation was 2982 s. On-ground analysis of the FXT data found an uncatalogued X-ray source at R.A. = 344.4246 deg, DEC = -11.7626 deg (J2000), with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a free hydrogen column density of 1.83 (-1.83/+2.06) x 10^21 cm^-2 and a photon index of 2.34 (-0.68/+0.75). The derived average unabsorbed 0.5-10 keV flux is 1.15 (-0.21/+0.33) x 10^-12 erg s^-1 cm^-2. The uncertainties are at the 90% confidence level for the above spectral 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/45301. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...