TITLE: GCN CIRCULAR NUMBER: 41611 SUBJECT: EP250830a/GRB 250830B: Einstein Probe detection of an X-ray transient and Swift/XRT follow-up DATE: 25/08/30 14:16:50 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> R.D. Liang (NAOC), K. R. Ni (CCNU), X. Tian(GXU), W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250830a. The transient triggered EP-WXT (ID: 01709200995) at 2025-08-30T03:57:09 (UTC), whcih is about 25 s after the trigger of GRB 250830B by MAXI (GCN 41602). The WXT position of the source is R.A. = 105.723 deg, DEC = -64.816 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic), spatially consistent with GRB 250830B. The 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model with Galactic hydrogen column density fixed to 1.1 x 10^21 cm^-2 and a photon index of 1.6 (+/- 0.2), and a peak flux of approximately 1 x 10^-9 erg/s/cm^2. We proposed a follow-up observation with the Swift XRT that was performed at 2025-08-30T07:00:13, approximately 3.05 h after the trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 105.73708 deg, DEC = -64.85286 deg (J2000) with an uncertainty of 5 arcsec in radius (90% C.L. statistical and systematic). The integrated 0.3-10 keV spectrum can be fitted with an absorbed powerlaw model with fixed identity Galactic hydrogen column density and a photon index of 0.9 (+/-0.5), and an averaged flux of 2.5 x 10^-12 erg/s/cm^2. We gratefully acknowledge the support of the Swift team. 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/41611. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...