{
  "submittedHow": "web",
  "circularId": 45779,
  "eventId": "EP260927a",
  "submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "subject": "EP260927a: Einstein Probe detection of a fast X-ray transient",
  "format": "text/plain",
  "eventType": [
    "X-ray Transient"
  ],
  "body": "C. L. Guo, B. B. Zhang, K. Ao, S. Q. Jiang, H. Sun (NAO, CAS), D. F. Hu (PMO, CAS), A. D. Zhu(HUST), J. Y. Liu(PMO,CAS), on behalf of the Einstein Probe (EP) team: \n\nWe report the discovery of a new fast X-ray transient EP260927a, identified in the EP-WXT telemetry data at RA = 31.833 deg, Dec =-27.169 deg (J2000), with an uncertainty of 2.6 arcmin in radius (90%, C.L statistical and systematic). The transient started about 2026-09-27T14:02:35 (UTC) and lasted for about 800 s. The 0.5-4 keV flux estimated from the WXT data is approximately 2.91x10^-11 erg/s/cm^2. We note the presence of a historical X-ray source 1eRASS J020729.2-271052, with flux density 7.79e-13 erg/s/cm^2, located 2.2 arcmin away from the center of the WXT error circle.This source is likely associated with Gaia DR3 5117556925932530176 and may be a QSO candidate according to GAIA classification.\n\nWe performed a follow-up observation of EP260927a with the Follow-up X-ray Telescope on board EP to confirm the transient nature of the WXT detection. The observation started at 2026-09-28T03:53:50 (UTC). The effective exposure time is around 6 ks. On-ground analysis of the FXT data found two sources within the WXT error circle. The first source is an uncatalogued source, detected at RA = 31.8105 deg, Dec =-27.1855 deg (J2000), with an uncertainty of 10 arcsec in radius (90%, C.L statistical and systematic). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.47 × 10^20 cm^-2, and a photon index of 2.28(-0.78/+0.87). The derived average unabsorbed 0.5-10 keV flux is 5.6(+4.9/-2.0)× 10^(-14) erg/s/cm^2. \n\nThe position of the second source is consistent with the historical X-ray source 1eRASS J020729.2-271052. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.47 × 10^20 cm^-2, and a photon index of 2.7(-0.3/+0.3). The derived average unabsorbed 0.5-10 keV flux is 2.5(+0.4/-0.4)× 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above spectral parameters.\n\nWe tend to suggest that the first FXT source is the X-ray counterpart of EP260927a. Multi-wavelength follow-up observations are encouraged.\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).\n",
  "createdOn": 1790654414469
}