TITLE: GCN CIRCULAR NUMBER: 44155 SUBJECT: EP260329b: refined analysis of the EP-WXT and EP-FXT observations DATE: 26/03/30 13:25:23 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> The fast X-ray transient EP260329b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Song et al., GCN 44142) , and followed by several optical telescopes (Li et al., GCN 44139, Zhu et al., GCN 44140, Eyles-Ferris et al., GCN 44144, Magnani et al., GCN 44146, Aryan et al., GCN 44147, Lipunov et al., GCN 44150). The refined analysis of the WXT data shows that the event started at T0=2026-03-29T10:35:25 (UTC) and lasted for 35 s with a single pulse, before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.51 x 10^20 cm^-2 and a photon index of 1.42 +/- 0.65. The derived average unabsorbed 0.5-4 keV flux is 1.07 (-0.37/+0.56) x 10^(-9) erg/s/cm^2. The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-03-29T10:38:50.16 (UTC), about 3 minutes after T0. The exposure time of this observation is 4388 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 181.3131, 4.3505 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT detected a sharp pulse lasting about 200 s. About 400 s after the end of the pulse, the light curve started to rise again. The rising phase was slower than that of the initial pulse and lasted for roughly 800 s, until the observation was interrupted due to Earth occultation. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 1.51 x 10^20 cm^-2 and a photon index of 1.91 +/-0.04. The derived average unabsorbed 0.5-10 keV flux is 1.54 +/-0.05 x 10^(-11) erg/s/cm^2. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/44155. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...