TITLE: GCN CIRCULAR NUMBER: 39028 SUBJECT: EP250125a: a new X-ray transient detected by Einstein Probe DATE: 25/01/25 10:27:35 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Q.-Y. Wu (NAOC, CAS), J. Yang (NJU), X.-Y. Zhou (PRIC) and C.-C. Jin (NAOC, CAS) report on behalf of the Einstein Probe team: We report on the detection of an X-ray transient, designated EP250125a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, which triggered the on-board processing unit at 2025-01-25T02:37:31 (UTC) (trigger ID: 01709130802). The transient event started at 2025-01-25T02:36:19 (UTC) and lasted for 74 s before the observation was interrupted by the autonomous follow-up observation. The WXT position of the source is R.A. = 175.364 deg, DEC = -21.708 deg (J2000) with an uncertainty of 2.3 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 equivalent hydrogen column density of 4.15 x 10^20 cm^-2 and a photon index of 0.8 (-0.5/+0.5). The derived average unabsorbed 0.5-4 keV flux is 1.8 (-0.5/+0.7) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. An autonomous observation was performed by the Follow-up X-ray Telescope (FXT) about two minutes later. An uncatalogued source was detected at R.A. = 175.3639, DEC = -21.7138 (J2000) with an uncertainty of about 20 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the optical conterpart reported by Levan et al., GCN 39027. 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 4.15 x 10^20 cm^-2 and a photon index of 2.0 (-0.1/+0.1). The derived average unabsorbed 0.5-10 keV flux is 5.4 (-0.6/+0.6) x 10^(-12) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. More information on this source will be updated when the full telemetry data is received. Further follow-up observations are encouraged to identify the nature of this X-ray transient. 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/39028. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...