General information
EP260727a is an Einstein Probe fast X-ray transient detected by WXT, with an uncatalogued FXT source at a position consistent with a repeatedly detected optical counterpart. The main counterpart position reported by NOT is R.A. = 346.46104 deg, Dec. = 3.19407 deg, with no NEO at that position according to MPC.
Wavelength coverage
- X-ray: EP/WXT triggered on the transient and EP/FXT detected an uncatalogued source consistent with the optical counterpart position; refined WXT and FXT spectral fits and fluxes were reported.
- Optical: NOT, Liverpool Telescope, OHP/T193, OSN, NUTTELA-TAO, and SAO RAS reported detections or limits in r, z, g', i', Ic, and Rc bands.
- Archival optical/near-optical imaging: Legacy Survey DR10, Pan-STARRS, and HSC archival information is used for comparison with the underlying source.
No observations are mentioned for gamma-ray, radio, neutrino, or gravitational-wave bands.
Lightcurve and spectrum
- WXT refined start time was T0 = 2026-07-27T20:36:47Z, and the event lasted about 70 s before interruption by autonomous follow-up.
- WXT 0.5-4 keV spectrum was fit with an absorbed power-law model at fixed z = 1.03, with photon index 0.5 (-0.6/+1.2), intrinsic NH = 7.1 (-7.1/+240.0) x 10^20 cm^-2, and unabsorbed flux 1.8 (-0.5/+0.8) x 10^-9 erg/s/cm^2.
- FXT observed about 50 min after T0 for 2568 s; the 0.5-10 keV spectrum was fit with photon index 2.0 (-0.6/+1.1), intrinsic NH = 0.15 (-0.15/+0.87) x 10^22 cm^-2, and unabsorbed flux 4.4 (-2.7/+0.2) x 10^-13 erg/s/cm^2.
- NOT detected the optical source at r = 21.98 +/- 0.08 mag at 4.18 hr post-trigger.
- NUTTELA-TAO started at 1.05 hr after trigger and did not detect the NOT counterpart to 5-sigma limits g' > 20.3 and i' > 19.75.
- Liverpool Telescope detected z = 21.66 +/- 0.17 at about 5.9 hr and reported r > 22.85; this r limit was not deep enough to assess fading relative to NOT.
- OHP/T193 measured the main candidate at r = 22.82 +/- 0.28 and reported a second candidate at r = 20.49 +/- 0.06.
- OSN measured the main counterpart at Ic = 21.76 +/- 0.26 and did not detect the OHP second candidate to Ic = 21.9.
- SAO RAS measured the main optical transient at Rc = 21.64 +/- 0.17 at t-T0 = 3.1932 hr.
Redshift
A photometric redshift z_phot = 1.03 +/- 0.24 is reported for the faint source at the optical counterpart position. The refined EP WXT spectral analysis fixed the redshift to z = 1.03 when deriving spectral parameters and luminosity.
What’s special vs typical
The event is more notable than a routine single-alert transient because it combines an EP fast X-ray trigger, FXT confirmation, multiple optical counterpart detections, archival brightening evidence, and a photometric redshift. It remains below exceptional multi-messenger status in the supplied text because no confirmed gravitational-wave, neutrino, or gamma-ray association is reported.