TITLE: GCN CIRCULAR NUMBER: 43554 SUBJECT: EP260128a: refined analysis of the EP-WXT and autonomous EP-FXT observations DATE: 26/01/29 10:01:54 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> H. Zhou (PMO, CAS), T. Wu, H.-C. Ding (AHNU), Z.-X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260128a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhou et al., GCN 43549). The refined analysis of the WXT data shows that the event started at T0=2026-01-28T 23:38:19(UTC), and lasted for approximately 100 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 5.84×10^20 cm^-2 and a photon index of 0.05 +/- 0.73. The derived average unabsorbed 0.5-4 keV flux is 6.98(-4.40/+11.93)×10^(-9) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-01-28T23:43:01(UTC, T0+282 s). The exposure time of this observation is 2102s. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 128.3253, DEC = -10.2750 (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 5.84×10^20 cm^-2 and a photon index of 1.99 +/-0.06. The derived average unabsorbed 0.5-10 keV flux is (2.76 +/- 0.15)×10^(-11) erg/s/cm^2. The best-fitted FXT spectra show no additional absorption is required. Further FXT follow-up observations have been arranged. Although a faint counterpart of EP260128a has been reported (Lipunov et al., GCN 43550; Quirola-Vasquez et al., GCN 43552; Eyles-Ferris et al., GCN 43553), further multi-band follow-up observations are encouraged to explore the nature of EP260128a. 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/43554. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...