General information
EP260221a is a fast X-ray transient detected by Einstein Probe/WXT on 2026-02-21, with an uncatalogued X-ray source confirmed by EP/FXT follow-up. Multiple optical facilities reported non-detections initially, followed by a fading optical counterpart candidate identified via image subtraction and supported by subsequent photometry.
Wavelength coverage
- X-ray: EP/WXT detection and EP/FXT follow-up with spectral fits and fluxes reported.
- Optical: COLIBRÍ (r,z) early limits and later subtraction detection; GOTO (L-band) limit; Xinglong (g, clear) limits; MASTER (clear) limits; Gemini-S (z) deep limit; Liverpool Telescope (i', z') detection at candidate position; Lulin/SLT (r) limit.
No observations were mentioned in gamma-ray, UV, IR, radio, neutrinos, or gravitational waves.
Lightcurve and spectrum
- WXT: event start T0=2026-02-21T11:39:48 UTC; duration 550 s; absorbed power-law with photon index 1.8 (-0.4/+0.2); unabsorbed flux (0.5–4 keV) 3.1 (-0.7/+0.5) × 10^(-10) erg/s/cm^2.
- FXT: observed at 2026-02-21T12:44:34 UTC (about 1 hr 5 min after T0); photon index 1.36 (-0.22/+0.22); unabsorbed flux (0.5–10 keV) 6.9 (-1.6/+2.0) × 10^(-13) erg/s/cm^2.
- Optical candidate (COLIBRÍ): r-band total magnitude (galaxy+candidate) 22.54±0.08 (epoch1) to 23.39±0.15 (epoch2), fading by 0.85±0.17 (5.0σ); z-band 22.03±0.17 to 22.69±0.16, fading by 0.66±0.23 (2.8σ).
- Optical photometry (Liverpool Telescope at candidate position): i’=22.21±0.12, z’=22.29±0.18.
Redshift
A coincident Legacy Survey galaxy at the candidate position is reported with photo-z = 1.22 ± 0.55.
What’s special vs typical
The optical counterpart candidate is identified through epoch-to-epoch image subtraction and appears to fade rapidly, with reported magnitudes brighter than the underlying catalogued host and consistent with a high photo-z host galaxy.