General information
EP260626a is an Einstein Probe fast X-ray transient detected by EP-WXT on 2026-06-26, with no known X-ray source in the original WXT error circle. EP-FXT later detected an uncatalogued X-ray source within the WXT localization at R.A. 277.4479 deg, Dec. 21.2498 deg, with a 10 arcsec 90% confidence uncertainty.
Wavelength coverage
- X-ray: EP-WXT detected a significant flare in 0.5-4 keV, and EP-FXT detected an uncatalogued source 10.8 hours later in 0.5-10 keV.
- Optical: Liverpool Telescope and COLIBRÍ observed in r/z bands and reported only upper limits, with no new source detected in the WXT region or at the reported X-ray position.
No observations are mentioned in gamma-ray, UV, infrared, radio, gravitational-wave, or neutrino bands.
Lightcurve and spectrum
- The WXT flare started before the beginning of the observation and ended approximately 100 seconds after the observation start.
- The average WXT 0.5-4 keV spectrum is fitted by an absorbed power-law model with Galactic NH fixed at 2.04e21 cm^-2 and photon index 1.93(-0.4,+0.42), with unabsorbed flux 8.02(-1.7,+2.1)e-11 erg cm^-2 s^-1.
- EP-FXT observed about 10.8 hours after the WXT detection for 6.1 ks and measured an absorbed power-law spectrum with photon index 2.13(+0.6,-0.6) and unabsorbed 0.5-10 keV flux about 9.3(+4.5,-2.1)e-14 erg cm^-2 s^-1.
- Optical non-detections include Liverpool limits of r' > 22.89 and z' > 22.37 at about 21 hours, COLIBRÍ limits of r > 23.87 and z > 23.15 at 26.5-27.9 hours, and further Liverpool limits of r > 23.44 and z > 22.94 at about 20.6 hours.
Redshift
What’s special vs typical
- The event is notable as a fast X-ray transient with no known prior X-ray source in the WXT error circle.
- The FXT detection of an uncatalogued X-ray source provides a refined X-ray position, while repeated optical follow-up found no counterpart to relatively deep r/z-band limits.