{
  "eventId": "EP260920b",
  "createdOn": 1789971052246,
  "body": "G. J. Yang (NAO, CAS), K. R. Ni (CCNU), S. Q. Zheng, Jin-Rui Liu (BNU), Tian-Yu Liu, B. B. Zhang and Z.-X. Ling (NAO, CAS), on behalf of the Einstein Probe (EP) team:\n\nThe fast X-ray transient EP260920b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-09-20T14:07:19 (UTC) (Ni et al., GCN 45664). Subsequent follow-up observations were reported by Buckley et al. (GCN 45665), Liu et al. (GCN 45668) and García-García et al. (GCN 45669). The refined analysis of the WXT data shows that the event started at T0 = 2026-09-20T14:06:32 (UTC). The WXT light curve shows a main outburst lasting approximately 500 s. The outburst comprises two broad pulses, with the second brighter than the first, and ends with a rapid decay.\n\nThe WXT spectrum extracted over the burst interval in the 0.5–4 keV band is well fitted by an absorbed power-law model. The best-fit hydrogen column density is N_H = 2.5 (+0.7/-0.7)×10^21 cm^-2, and the photon index is 1.72 (+0.24/-0.23). The corresponding average unabsorbed flux in the 0.5–4 keV band is 2.1 (+0.2/-0.2)×10^-10 erg s^-1 cm^-2.\n\nThe Follow-up X-ray Telescope (FXT) on board EP observed the source field at 2026-09-20T20:14:12, approximately 6 hours after the T0. The refined analysis shows that an uncatalogued X-ray source was detected at R.A. = 325.5875 deg, Dec. = -28.0402 deg (J2000), with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows a decline trend during the observation.\n\nWe jointly fitted the FXT-A and FXT-B spectra in the 0.5-10 keV band with an absorbed power-law model. The best-fit hydrogen column density is NH = 8.5 (-7.6/+8.0) x 10^20 cm^-2, and the photon index is 1.96 (-0.24/+0.25). The derived average unabsorbed 0.5-10 keV flux is 1.3 (+0.1/-0.1) × 10^-12 erg s^-1 cm^-2. The uncertainties are at the 1sigma confidence level for the above spectral parameters.\n\nMore observations with EP-FXT have been scheduled. Further 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",
  "circularId": 45670,
  "eventType": [
    "X-ray Transient"
  ],
  "subject": "EP260920b: refined analysis of the EP-WXT and EP-FXT observations",
  "submittedHow": "web",
  "submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "format": "text/plain"
}