{
  "format": "text/plain",
  "circularId": 45826,
  "subject": "EP261006a: Einstein Probe detection of an X-ray transient",
  "body": "Q.H. Lao (NAO, CAS), L.N. Xu (USTC), H.Y. Liu (NAO, CAS), W.F. Wen (SZTU), X.H. Tang (THU) and C.C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n\nWe report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP261006a. The transient triggered EP-WXT (ID: 01709320985) at 2026-10-06T17:49:57 (UTC) . The WXT position of the source is R.A. = 21.806 deg, DEC = 12.9 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The exposure time of WXT observation (ID: 01709320985) is 384 s, and no significant variability was detected during the observation.\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 = 3.84 x 10^20 cm^-2 at the source position. The best-fit photon index is 1.73 (-0.66/+0.77), and the derived average unabsorbed 0.5-4 keV flux is 1.46 (-0.53/+0.78) 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 2026-10-06T17:53:48(UTC), 231s after the trigger. With the telemetry data received, the refined on-ground analysis gives an X-ray source position of RA=21.7913 deg, DEC=12.8907 deg (J2000) , with an uncertainty of about 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows an overall declining 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, with the hydrogen column density fixed at Galactic NH = 3.84 x 10^20 cm^-2. The best-fit photon index is 1.64 (-0.01/+0.01). The derived average unabsorbed 0.5-10 keV flux is 1.17 (-0.02/+0.02) x 10^-10 erg s^-1 cm^-2.\n\nWithin the FXT error circle, there is a galaxy, Gaia DR3 2587044864659595392 (Hughes et al. 2022) with a photometric redshift of 0.184 (Duncan et al. 2022). If this transient is located within this galaxy, its estimated 0.5-10 keV luminosity is 1.12×10^46 erg s^-1.\n\nThe uncertainties are at the 90% confidence level for the above spectral parameters.\n\nSubsequent follow-up observations were reported by Novotny et al. (GCN 45821) and Shi et al. (GCN 45823). Further multi-wavelength follow-up observations are encouraged to explore the nature of EP261006a.\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",
  "submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
  "eventType": [
    "X-ray Transient"
  ],
  "createdOn": 1791357207042,
  "eventId": "EP261006a",
  "submittedHow": "web"
}