TITLE: GCN CIRCULAR NUMBER: 40669 SUBJECT: EP250610a: refined analysis of the EP-WXT and EP-FXT observations DATE: 25/06/10 14:16:44 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> T.Y. Lian, H. Q. Cheng (NAO, CAS), Y. J. Zhang (THU) and C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team: The fast X-ray transient EP250610a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 40660), and followed up by several optical and X-ray telescopes (Schneider et al., GCN 40661, Brivio et al., GCN 40665, Evans et al., GCN 40662). Refined analysis of the WXT data shows that the event started at T0=2025-06-10T04:23:24 (UTC) and lasted for about 140s before the interruption by the autonomous follow-up observation. The peak flux (0.5-4 keV) is estimated to be 2.2 x 10^-9 erg/s/cm^2. The averaged 0.5-4 keV spectrum can be fitted with an absorbed power-law with a Galactic hydrogen column density of 9.5 x 10^19 cm^-2 and a photon index of 0.7 (-/+0.8). The derived average unabsorbed 0.5-4 keV flux is 5.2 (-2.3, +4.2) x 10^-10 erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2025-06-10T04:26:32, about 3 minutes after T0, with an exposure time of 3946 seconds. Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 248.3567, DEC = 38.5362 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is spatially consistent with the candidate X-ray counterpart detected by Swift/XRT (Source 1 in Evans et al., GCN 40662). The averaged 0.5-10 keV FXT spectrum can be fitted with an absorbed power-law with a Galactic hydrogen column density of 9.5 x 10^19 cm^-2 and a photon index of 2.34 (-0.12, +0.12). The derived unabsorbed 0.5-10 keV flux is 1.87 (-0.17, +0.19) x 10^-12 erg/s/cm^2. The uncertainties are at the 90% confidence level for all 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). The contact TAs of this source are Tianying Lian and Huaqing Cheng, please contact them via the email tylian@nao.cas.cn and hqcheng@nao.cas.cn if needed. View this GCN Circular online at https://gcn.nasa.gov/circulars/40669. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...