TITLE: GCN CIRCULAR NUMBER: 43272 SUBJECT: EP251230a / GRB 251230A: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/12/30 14:21:51 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> B.T. Wang (YNAO, CAS), J.P. Chen (SYSU), D. Zhu, K.J. Zhang (YNU), and Z.X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP251230a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Wang et al., GCN 43259) and Swift/BAT as GRB 251230A (Lanava et al., GCN 43252, Osborne et al., GCN 43263), and was followed by several optical telescopes (Li et al., GCN 43255, Lipunov et al., GCN 43256, Busmann et al., GCN 43258, Masi et al., GCN 43260; Ghosh et al., GCN 43262; Ma et al., GCN 43265, Starling et al., GCN 43267, He et al., GCN 43269, Tak et al., GCN 43270, Breeveld et al., GCN 43271). The refined analysis of the WXT data shows that the event started at T0=2025-12-30T01:21:31 (UTC) and lasted for 72 s with a single pulse. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 1.21(-0.48/+0.59) x 10^22 cm^-2 and a photon index of 2.16 (-0.82/+0.89). The derived average unabsorbed 0.5-4 keV flux is 6.56 (-2.40, +2.80) x 10^(-9) erg/s/cm^2. The autonomous observation by the Follow-up X-ray Telescope (FXT) on board EP was performed at 2025-12-30T01:24:36 (UTC), about 3 minutes after T0. The exposure time of this observation is 1859 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 179.8974, 11.4382 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 2.0 x 10^20 cm^-2 and a photon index of 1.83 (-0.04/+0.04). The derived average unabsorbed 0.5-10 keV flux is 4.02 (-0.13/+0.13) x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 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/43272. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...