General information
EP260806a is an Einstein Probe X-ray transient detected by WXT on 2026-08-06T23:02:28 UTC and followed automatically by FXT, with an optical counterpart subsequently identified as ZTF26abmjjnm / AT 2026yeh. The refined FXT position is RA=12.3066 deg, Dec=35.5603 deg with a 10 arcsec radius uncertainty, and the counterpart is repeatedly detected in optical follow-up.
Wavelength coverage
- X-ray: EP/WXT detected the transient and EP/FXT detected an uncatalogued source consistent with the WXT position; WXT measured an unabsorbed 0.5-4 keV flux of 1.65(-0.58/+5.7) x 10^-10 erg s^-1 cm^-2, while FXT measured an unabsorbed 0.5-10 keV flux of 2.13 +/- 0.03 x 10^-10 erg s^-1 cm^-2.
- Optical: LCO, GTC, Liverpool Telescope, COLIBRI, ZTF, MAO/AZT-22, WFST, NUTTELA-TAO, Zeiss-1000, and BTA reported detections in filters including g, r, z, y, R/Rc, and i.
- Radio: GTC reported that the counterpart and galaxy core are consistent with catalogued LOTSS and VLASS radio sources.
Bands with no observations mentioned: gamma-ray, ultraviolet, infrared, millimeter/submillimeter, gravitational-wave, and neutrino.
Lightcurve and spectrum
- WXT observed for about 290 s and reported no obvious variability or flaring activity during that observation.
- FXT observed from 374 s after the first WXT observation for 3950 s and reported that the source flux declined during the FXT observation.
- GTC image subtraction measured r=19.05 +/- 0.01 mag at 4.32 hr after trigger, implying a 0.27 +/- 0.04 mag brightening relative to the LCO observation.
- Liverpool Telescope measured r=19.22 +/- 0.03 and z=19.53 +/- 0.04, describing the color as blue.
- ZTF measured g=18.69 +/- 0.09 and r=18.93 +/- 0.09 at T0+12 hr, with non-detections about 4 days earlier at g,r > 19.9 AB.
- SAO RAS reported detections over multiple epochs and significant g-r color evolution from -0.24 +/- 0.13 at about 0.5 days to -1.23 +/- 0.16 at about 3.2 days after trigger.
- WXT and FXT spectra were fitted with absorbed power laws; FXT gave photon index 2.02 +/- 0.04 and intrinsic column density 1.92 +/- 0.13 x 10^21 cm^-2.
Redshift
GTC/OSIRIS+ spectroscopy detected [OII], [OIII], HeI, HII, [NII], [SII], and CaII absorption features at a common redshift z=0.089, suggested as the redshift of EP260806a. LCO earlier reported that the brightened known galaxy has a redshift of about 0.136 in REGALADE, so the provided circulars contain a redshift discrepancy.
What’s special vs typical
EP260806a is notable for rapid X-ray discovery plus automatic X-ray follow-up, a clear optical counterpart with extensive multi-epoch follow-up, spectroscopy, high inferred X-ray luminosity of 3.8 x 10^45 erg s^-1 at z=0.089, and color/light-curve behavior discussed as consistent with an AGN outburst while a fast-rising TDE is also considered.