TITLE: GCN CIRCULAR NUMBER: 42415 SUBJECT: EP251023b/GRB 251023B : Einstein Probe detection of a fast X-ray transient DATE: 25/10/24 05:39:45 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Y. Wu (NJU), D. F. Hu (PMO, CAS), H. N. Yang, T. Zhao, W. D. Zhang (NAO, CAS), on behalf of the Einstein Probe (EP) team: We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP251023b. The transient triggered EP-WXT (ID: 01709247298) at 2025-10-23T08:48:47 (UTC). The refined analysis of the WXT data shows that the event started at T0=2025-10-23T08:46:06 (UTC) and lasted for ~160 s. The WXT position of the source is R.A. = 73.502 deg, Dec. = 12.648 deg (J2000) with an uncertainty of 2.2 arcmin in radius (90% C.L. statistical and systematic). The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 2.69 x 10^(21) cm^-2 and a photon index of 1.66（-/+0.25）. The derived average unabsorbed 0.5-4 keV flux is 3.9 (-/+0.5) x 10^(-9) erg/s/cm^2. We performed a Target-of-Opportunity observation with the Follow-up X-ray Telescope on board EP. The observation began at 2025-10-23T14:40:06 (UTC).An uncatalogued X-ray source was detected within the WXT error circle at R.A. = 73.5021 deg, Dec. = 12.6574 deg (J2000), with a positional uncertainty of 10 arcsec in radius (90% confidence level, including both statistical and systematic uncertainties). The FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with the same fixed Galactic hydrogen column density value and a photon index of 2.16 (-/+0.21). The derived average unabsorbed 0.5-10 keV flux is 2.03 (-0.29/+0.34) x 10^(-12) erg/s/cm^2. The source is spatially consistent with the counterpart reported (Kang et al. GCN 42392, Evans et al. GCN 42401, Nakahira et al. GCN 42402, Mandarakas et al. GCN 42404, Cheung et al. GCN 42407). 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/42415. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...