TITLE: GCN CIRCULAR NUMBER: 44214 SUBJECT: EP260403a: refined analysis of the EP-WXT and EP-FXT observations DATE: 26/04/04 06:09:17 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> C.-Y. Dai (NJU), R.-Z. Li (YNAO), C.-L. Guo, and W.-D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260403a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Guo et al., GCN 44204). Optical follow-up of EP260403a revealed a fading counterpart, first identified by COLIBRÍ (Antier et al., GCN 44206) and subsequently confirmed by LCO (Pérez-Fournon et al., GCN 44207; Quirola-Vasquez et al., GCN 44209). The burst beginning at T0 = 2026-04-03 10:27:45 (UTC). The WXT observation lasted for approximately 50 seconds, and was interrupted due to the autonomous follow-up observation. The WXT light curve exhibits a single-peaked structure. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at the Galactic value of 5.6×10^20 cm^-2 and a photon index of 0.3 (-0.3/+0.3). The derived average unabsorbed flux in the 0.5-4 keV band is 1.6 (-0.3/+0.3) × 10^-9 erg s^-1 cm^-2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-04-03 10:33:22 (UTC, T0+337 s). The exposure time of this observation is 3.6 ks. On-ground analysis of the FXT data found an uncatalogued fading source at R.A., Dec. = 219.1556, -25.5045 (J2000) with an uncertainty of about 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 model, with the hydrogen column density fixed at the Galactic value and a photon index of 1.94 (-0.02/+0.02). The derived average unabsorbed flux in the 0.5-10 keV band is 6.4 (-0.1/+0.1)×10^-11 erg s^-1 cm^-2. A follow-up observation with the EP/FXT was planned, and further information will be updated when the telemetry data are received. 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/44214. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...