TITLE: GCN CIRCULAR NUMBER: 44690 SUBJECT: EP260518a: Einstein Probe detection of an sub-threshold X-ray transient with counterpart AT 2026ndp DATE: 26/05/22 08:38:05 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> R. D. Liang, W. X. Li, D. Y. Li, C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report the detection of a sub-threshold X-ray transient, EP260518a, associated with the optical transient AT 2026ndp. EP260518a was detected by the Wide-field X-ray Telescope (WXT) aboard the Einstein Probe (EP) mission at R.A. = 299.034 deg, Dec. = -48.618 deg (J2000), with an uncertainty radius of 3 arcmin, at 2026-05-18 13:44:21 UTC. The measured 0.5–4 keV flux was approximately 2.5 x 10^-11 erg cm^-2 s^-1. Due to the low signal-to-noise ratio, no significant flare-like feature is apparent in the X-ray light curve. A candidate optical counterpart, AT 2026ndp, was identified through real-time cross-matching. The transient was discovered by ATLAS at 2026-05-19 07:44:02.400 UTC, approximately 17 hours after the WXT detection, with a c-band magnitude of 18.73 +/- 0.10. There is a faint point-like source in Legacy Survey dr10, which is 1.8 arcsec away. Follow-up observations were performed with the Follow-up X-ray Telescope (FXT) aboard EP, starting at 2026-05-21 08:23:08 UTC (~66 hr after the WXT detection). An uncatalogued X-ray source was detected within the WXT error region at R.A. = 299.0407 deg, Dec. = -48.6362 deg (J2000), with an uncertainty of 10 arcsec. The FXT position is offset by 4.7 arcsec from the optical counterpart. The FXT spectrum can be fitted with an absorbed power-law model with a photon index of 2.5 (+0.8/-0.7) and hydrogen column density of 2.3 (+2.0/-1.7) x 10^21 cm^-2. The estimated average 0.3–10 keV flux is around 4.6 (+1.9/-1.2) x 10^-13 erg cm^-2 s^-1. 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/44690. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...