TITLE: GCN CIRCULAR NUMBER: 43333 SUBJECT: EP260105a: refined analysis of the EP-WXT and EP-FXT observations DATE: 26/01/05 14:30:43 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> H. Zhou (PMO, CAS), Z. C. Zou (NJU), D. Y. Li, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260105a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhou et al., GCN 43327). The refined analysis of the WXT data shows that the event started at T0=2026-01-05T05:47:50 (UTC) and lasted for 100 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 2.4 x 10^20 cm^-2 and a photon index of 0.89(-0.65/+0.67). The derived average unabsorbed 0.5-4 keV flux is (1.1+/-0.5) x 10^(-9) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-01-05T05:52:07 (UTC), about 4.3 minutes after T0. The exposure time of this observation is 2698s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 181.0148, DEC = 22.7092 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the WXT position. A significant decay was detected in the light curve during the initial phase of FXT observation. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.4 x 10^20 cm^-2 and a photon index of 2.07 (-0.56/+0.58). The derived average unabsorbed 0.5-10 keV flux is 4.2 (-1.7/+2.1) x 10^(-13) erg/s/cm^2. No optical counterpart of EP260105a has been detected so far (Lipunov et al., GCN 43328; Corcoran et al., GCN 43332) , and further mutil-band follow-up observations are encouraged to explore the nature of EP260105a. Another ToO observation with EP-FXT is being performed start from 2026-01-05T12:31:46 (UTC). Further information will be updated when the telemetry data is 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/43333. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...