General information
EP260601a is an X-ray transient detected by Einstein Probe/WXT on 2026-06-01, with a 3 arcmin error radius and an initial flare followed by tail emission. Two optical follow-up circulars report an uncatalogued fading counterpart within the WXT localization, supporting that this is a real astrophysical transient.
Wavelength coverage
- X-ray: Einstein Probe/WXT detected the transient, with the burst starting at 2026-06-01T19:12:41 UTC and triggering at 19:13:22 UTC. The light curve showed an initial flare lasting about 100 s followed by tail emission; the flare 0.5-4 keV unabsorbed flux was 5.2 (-1.7/+3.6) x 10^-08 erg cm^-2 s^-1, and the time-averaged 0.5-4 keV unabsorbed flux was 2.4 (-0.2/+0.3) x 10^-10 erg cm^-2 s^-1.
- Optical: JinShan detected an uncatalogued source at RA = 17:10:38.24, Dec = -01:38:42.4 with r ~ 18.4 at 0.73 hr post-trigger. COLIBRÍ later detected the same counterpart at r = 21.93 +/- 0.15 and z = 21.07 +/- 0.12 at 13.38-13.39 hr post-trigger, and explicitly reported fading.
- Near-infrared: No near-infrared observations were reported.
- Radio: No radio observations were reported.
- Gamma-ray: No gamma-ray observations were reported.
- UV: No UV observations were reported.
- Other bands with no observations mentioned are near-infrared, radio, gamma-ray, and UV.
Lightcurve and spectrum
- The X-ray light curve showed an initial flare lasting approximately 100 s, followed by tail emission.
- The time-averaged WXT 0.5-4 keV spectrum over the detected emission is fit by an absorbed power law with best-fit hydrogen column density 6.7 (-1.5/+1.6) x 10^21 cm^-2 and photon index 1.2 (-0.3/+0.3).
- The optical counterpart faded from r ~ 18.4 at 0.73 hr post-trigger to r = 21.93 +/- 0.15 by 13.38-13.39 hr post-trigger.
- At the later epoch, the counterpart had z = 21.07 +/- 0.12.
Redshift
What’s special vs typical
The notable feature in the provided circulars is the combination of an EP soft X-ray transient with a clearly fading optical counterpart independently detected by two teams. However, the event remains only moderately special in this text because no redshift, firm source class, or broader multi-wavelength/multi-messenger association is yet reported.