General information
EP260616a is a fast X-ray transient detected by Einstein Probe/WXT starting at 2026-06-16T07:05:16 UTC, with reported temporal and positional consistency with Fermi/GBM GRB 260616B. EP-FXT later detected an uncatalogued X-ray source within the WXT error circle at RA 256.0839 deg, Dec -32.3007 deg.
Wavelength coverage
- X-ray: EP-WXT detected the initial transient; the average 0.5-4 keV spectrum is fit by an absorbed power law with photon index 1.8 ± 0.3, average unabsorbed flux 4.1 ± 0.5 x 10^-10 erg/s/cm^2, and peak flux around 3.4 x 10^-9 erg/s/cm^2. EP-FXT followed up about 25.5 hours later for 6.2 ks and detected an uncatalogued source with 0.5-10 keV unabsorbed flux about 8.7(+2.2/-1.7) x 10^-13 erg/s/cm^2.
- Gamma-ray: the WXT transient is reported as consistent in time and position with GRB 260616B detected by Fermi/GBM.
- Optical: COLIBRÍ observed from 22.86 to 24.20 hours after trigger in r and z and found no new source at the WXT position, with 5-sigma limits r > 22.76 AB and z > 22.20 AB.
No radio, infrared, ultraviolet, neutrino, gravitational-wave, or TeV observations are mentioned.
Lightcurve and spectrum
- Start time: 2026-06-16T07:05:16 UTC.
- WXT spectrum: absorbed power law with photon index 1.8(+0.3/-0.3), Galactic NH fixed at 3.4 x 10^21 cm^-2.
- WXT flux: average unabsorbed 0.5-4 keV flux 4.1 ± 0.5 x 10^-10 erg/s/cm^2; peak flux around 3.4 x 10^-9 erg/s/cm^2.
- FXT follow-up: about 25.5 hours after WXT detection, 6.2 ks exposure.
- FXT spectrum: absorbed power law with photon index 1.62(+0.27/-0.26), Galactic NH fixed at 3.4 x 10^21 cm^-2.
- FXT flux: unabsorbed 0.5-10 keV flux approximately 8.7(+2.2/-1.7) x 10^-13 erg/s/cm^2.
- Optical limits: no new source in stacked COLIBRÍ imaging to r > 22.76 AB and z > 22.20 AB.
Redshift
What’s special vs typical
- The event is notable for a soft X-ray transient detected by EP-WXT that is positionally and temporally consistent with a Fermi/GBM GRB, plus an uncatalogued EP-FXT X-ray source found within the WXT error circle about a day later.
- The lack of an optical counterpart to moderately deep COLIBRÍ limits is a potentially interesting feature, though no redshift or extinction-corrected interpretation is provided.