TITLE: GCN CIRCULAR NUMBER: 43583 SUBJECT: EP260131a: refined analysis of the EP-WXT and autonomous EP-FXT observations DATE: 26/01/31 13:33:19 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Q. Y. Wu (NAO, CAS), T. Wu , H. C. Ding (AHNU) , H. Sun (NAO,CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260131a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Ding et al., GCN 43574). The refined analysis of the WXT data shows that the event started at T0=2026-01-31T03:28:00.15 (UTC), and lasted for approximately 340 seconds, after which the observation was interrupted due to Earth occultation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 3.36 × 10^20 cm^-2, an intrinsic hydrogen column density of 5 (-3/+3) × 10^21 cm^-2, and a photon index of 1.9 (-0.8/+0.9). The derived average unabsorbed 0.5-4 keV flux is 6.3 (-1.5/+3.6) × 10^(-10) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP autonomously observed this source a few minutes after the on board trigger. The source was initially occulated the Earth and entered the FXT field of view since 2026-01-31T04:14:01 (UTC, T0+2.7 ks). The effective exposure time of the observation is around 3 ks. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 149.9017, DEC = -3.3074 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position and also the optical counterpart (Li et al., GCN 43577; Álvarez et al., GCN 43578; Malesani et al., GCN 43581; Schneider et al., GCN 43582). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 3.36 × 10^20 cm^-2, an intrinsic hydrogen column density of 1.5 (-1.1/+1.3) × 10^21 cm^-2, and a photon index of 2.2 (-0.4/+0.4). The derived average unabsorbed 0.5-10 keV flux is 4.4 (-0.6/+0.6) × 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. Further FXT follow-up observations have been arranged. 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/43583. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...