{
  "circularId": 45803,
  "createdOn": 1791020753967,
  "eventId": "EP261003a",
  "eventType": [
    "X-ray Transient"
  ],
  "subject": "EP261003a: refined analysis of the EP-WXT and autonomous EP-FXT observations",
  "submittedHow": "web",
  "submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "format": "text/plain",
  "body": "\nQ.H. Lao, H.Y. Liu (NAO, CAS), Y.H. Jiang (NJU), Y.H. Cheng (YNU) and Y. Liu (NAO, CAS), on behalf of the Einstein Probe (EP) team:\n\nThe fast X-ray transient EP261003a triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at 2026-10-03T00:14:43 (UTC) (Lao et al. GCN 45798). Subsequent follow-up observations were reported by Segura et al. (GCN 45796) and Globus et al. (GCN 45801). The refined analysis of the WXT data shows that the event started at T0 = 2026-10-03T 00:12:15 (UTC), approximately 150 s before the WXT trigger. \n\nThe average WXT 0.5-4 keV spectrum can be fitted with an absorbed power-law model, with the hydrogen column density fixed at Galactic NH = 4.15 x 10^21 cm^-2 at the source position. The best-fit photon index is 1.52 (-0.54/+0.58), and the derived average unabsorbed 0.5-4 keV flux is 8.14 (-2.39/+3.37) x 10^-10 erg s^-1 cm^-2.\n\nThe Follow-up X-ray Telescope (FXT) on board EP observed the source autonomously following the WXT trigger at 2026-10-03T00:17:27(UTC), approximately 160 s after the trigger. With the telemetry data received, the refined on-ground analysis gives an X-ray source position of RA=307.5337 deg, DEC=-12.9423 deg (J2000) , with an uncertainty of about 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows a rapid declining trend during the first 700s of 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, with the hydrogen column density fixed at Galactic NH = 4.15 x 10^21 cm^-2 at the source position. The best-fit photon index is 1.69 (-0.22/+0.23). The derived average unabsorbed 0.5-10 keV flux is 7.06 (-1.30/+1.59) x 10^-12 erg s^-1 cm^-2.\n\nThe uncertainties are at the 90% confidence level for the above spectral parameters.\n\nFurther multi-wavelength follow-up observations are encouraged to explore the nature of EP261003a.\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"
}