TITLE: GCN CIRCULAR NUMBER: 44251 SUBJECT: EP260409a: refined analysis of the EP-WXT and autonomous EP-FXT observations DATE: 26/04/10 06:06:36 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), S. Q. Jiang, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260409a was detected by the Wide-field X-ray Telescople (WXT) on board the Einstein Probe (EP) mission (Jiang et al., GCN 44240). The refined analysis of the WXT data shows that the event started at T0=2026-04-09T00:16:43 (UTC). WXT coverage of the transient lasted for approximately 100 seconds after trigger, and was interrupted due to 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 2.74 × 10^(20) cm^-2, and a photon index of 0.6 +/- 0.7. The derived average unabsorbed 0.5-4 keV flux is 3.2 (-1.2, +1.7) × 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 FXT observation started at 2026-04-09T00:22:08 (UTC, T0+325 s). The effective exposure time of the observation is around 2 ks. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 172.9096, DEC = -9.3457 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectra can be fitted with an absorbed power-law with a fixed Galactic hydrogen column density of 2.74 × 10^(20) cm^-2, and a photon index of 1.9 +/- 0.4. The derived average unabsorbed 0.5-10 keV flux is 6.4 (-1.8, +2.8) × 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. The optical follow-up observations were performed by Lipunov et al. (GCN 44241), Noémie Globus et al. (GCN 44243), Li et al. (GCN 44244). 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/44251. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...