TITLE: GCN CIRCULAR NUMBER: 43367 SUBJECT: EP260110a: refined analysis of the EP-WXT and EP-FXT observations DATE: 26/01/11 04:43:42 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> B. Zhang, J. P. Feng (USTC), Y. Wu (NJU), F. F. Song(YNAO, CAS), A. Li (BNU), C. Y. Wang (THU), G. Y. Zhao (SYSU), Y. Liu (NAOC, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260110a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 43363). The refined analysis of the WXT data shows that the event started at T0=2026-01-10T 11:56:58(UTC), and lasted for approximately 85 seconds, after which the WXT light curve 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 1.59×10^20 cm^-2 and a photon index of 0.83(-0.51/+0.56). The derived average unabsorbed 0.5-4 keV flux is 2.64(-0.88/+1.15)×10^(-10) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-01-10T12:00:52(UTC, T0+234 s). The exposure time of this observation is 4576s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 199.4321, DEC = 65.8490 (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 1.59×10^20 cm^-2 and a photon index of 2.02 (-0.14/+0.14). The derived average unabsorbed 0.5-10 keV flux is 1.52 (-0.19/+0.22)×10^(-12) erg/s/cm^2. We performed one target-of-opportunity (ToO) follow-up observations with FXT at 2026-01-10T16:23:31 (UTC, T0+15,993 s) with an exposure time of 3,032s. The average FXT 0.5-10 keV spectrum of the ToO observation can be fitted with an absorbed power law, with a fixed Galactic hydrogen column density of 1.59×10^20 cm^-2, and a photon index of 2.11 (-0.84/+1.00) . The derived average unabsorbed 0.5-10 keV flux is 1.15 (-0.57/+1.33)×10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. No optical counterpart of EP260110a has been detected so far (Perez-Garcia et al., GCN 43364; Kang et al., GCN 43365; Antier et al., GCN43366) , and further mutil-band follow-up observations are encouraged to explore the nature of EP260110a. 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/43367. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...