{
  "createdOn": 1779439085164,
  "submittedHow": "web",
  "bibcode": "2026GCN.44690....1L",
  "submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "body": "R. D. Liang, W. X. Li, D. Y. Li, C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:\nWe report the detection of a sub-threshold X-ray transient, EP260518a, associated with the optical transient AT 2026ndp.\n\nEP260518a 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.\nA 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. \n\nFollow-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.\n\nThe 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.\n\nLaunched 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).",
  "eventId": "EP260518a",
  "circularId": 44690,
  "format": "text/plain",
  "subject": "EP260518a: Einstein Probe detection of an sub-threshold X-ray transient with counterpart AT 2026ndp"
}