GCN Circular 45826
Subject
EP261006a: Einstein Probe detection of an X-ray transient
Event
Date
2026-10-07T07:13:27Z (17 hours ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
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:
We 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.
The 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.
The 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.
We 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.
Within 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.
The uncertainties are at the 90% confidence level for the above spectral parameters.
Subsequent 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.
Launched 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).