General information
EP260723a is an Einstein Probe fast X-ray transient detected by EP-WXT on 2026-07-23T18:56:01 UTC, lasting about 100 s. A consistent optical counterpart was reported by Liverpool Telescope, confirmed by NOT and COLIBRI, and later an uncatalogued EP-FXT X-ray source was found consistent with the optical position.
Wavelength coverage
- X-ray: EP-WXT detected the initial transient with peak 0.5-4 keV flux of ~1.4 x 10^-9 erg cm^-2 s^-1; EP-FXT later found a weak first excess and then an uncatalogued source with average unabsorbed 0.5-10 keV flux of 3.1(+3.0/-1.4) x 10^-14 erg cm^-2 s^-1.
- Optical: Liverpool Telescope detected r = 21.86 +/- 0.11 and g > 22.25 at ~10.8 hr; NOT detected r = 22.10 +/- 0.05 at 10.18 hr; COLIBRI measured r = 22.25 +/- 0.09 and z = 21.57 +/- 0.14 at 12.96-14.68 hr; Liverpool later measured r = 22.17 +/- 0.08 and z = 21.48 +/- 0.13 at ~32.5 hr.
- Catalog/host context: A Legacy Survey source at the candidate position is reported with r = 23.62 +/- 0.15 and photometric redshift z = 0.91 +/- 0.13.
No radio, infrared, gamma-ray, gravitational-wave, or neutrino observations are mentioned.
Lightcurve and spectrum
- The X-ray transient lasted about 100 s.
- The WXT 0.5-4 keV spectrum is fit by an absorbed power law with fixed Galactic NH = 2.0 x 10^20 cm^-2 and photon index 1.4(-0.8/+0.9).
- The first EP-FXT follow-up found only a weak X-ray excess with S/N approximately 2.2 at the optical counterpart position, with unclear association.
- The second EP-FXT follow-up spectrum is fit by an absorbed power law with fixed Galactic NH = 2.0 x 10^20 cm^-2 and photon index 2.96(+1.53/-1.21).
- Liverpool Telescope observations at ~32.5 hr found magnitudes consistent with earlier COLIBRI measurements, suggesting an extended optical plateau or rebrightening.
Redshift
A Legacy Survey source consistent with the optical candidate position has a reported photometric redshift of z = 0.91 +/- 0.13 and was suggested as a possible host galaxy.
What’s special vs typical
The event is more notable than a routine single-alert EP transient because it has a fast X-ray trigger, repeated optical counterpart confirmations, a late EP-FXT X-ray source at a consistent position, and reported optical behavior consistent with an hours-long plateau or rebrightening.