TITLE: GCN CIRCULAR NUMBER: 43802 SUBJECT: EP260221a: EP-FXT follow-up observation DATE: 26/02/22 05:03:51 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Tian-Yu Liu, X. X. Sun, M.-H. Zhang, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260221a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 43791). The optical upper limits were reported by COLIBRI telescope, GOTO and the 2.16m Xinglong Telescope (GCN 43793, GCN 43794, GCN 43798). The refined analysis of the WXT data shows that the event started at T0=2026-02-21T11:39:48 (UTC) and lasted for 550 seconds. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 3.5 x 10^20 cm^-2 and a photon index of 2.9 (-0.3/+0.4). The derived average unabsorbed 0.5-4 keV flux is 3.1 (-0.7/+0.5) x 10^(-10)erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source at 2026-02-21T12:44:34 (UTC), about 1 hour 5 minutes after T0. The exposure time of this observation is 5997s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 178.7560, DEC = -20.9819 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 3.5 x 10^20 cm^-2 and a photon index of 1.36 (-0.22/+0.22). The derived average unabsorbed 0.5-10 keV flux is 6.9 (-1.6/+2.0) x 10^(-13)erg/s/cm^2. 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/43802. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...