GRB 260618A

General information

GRB 260618A was detected by Fermi/GBM at 06:45:34 UT on 18 June 2026 and reported as a likely long GRB. An Einstein Probe transient, EP260618a, lies within the GBM error circle and has X-ray and optical detections consistent with a possible afterglow.

Wavelength coverage

• Gamma-ray: Fermi/GBM triggered on the burst; the GBM light curve has T90 ~ 140 s and the 10-1000 keV fluence is (5.9 +/- 0.2)E-06 erg/cm^2.
• X-ray: EP-WXT detected an uncatalogued 0.5-4 keV transient with flux 6.9(-1.7/+2.1)e-11 erg cm^-2 s^-1; EP-FXT later detected a consistent uncatalogued X-ray source with 0.5-10 keV flux 1.28(+0.13/-0.10)e-12 erg s^-1 cm^-2.
• Optical/NIR: COLIBRÍ detected an uncatalogued source at r = 21.90 +/- 0.04 and z = 21.49 +/- 0.08 AB; LCO confirmed it at r = 21.93 +/- 0.16 AB; JinShan reported a later revised brightness of r ~ 22.4 mag. No radio, millimeter, ultraviolet, neutrino, or gravitational-wave observations are mentioned.

Lightcurve and spectrum

• GBM: variable emission episode with T90 about 140 s in 50-300 keV.
• GBM spectrum: best fit by a cutoff power law from T0-17 to T0+150 s, with power-law index -1.4 +/- 0.1 and Epeak = 43 +/- 2 keV.
• GBM peak flux: 1-s peak photon flux 2.7 +/- 0.3 ph/s/cm^2 starting at T0+25 s in 10-1000 keV.
• WXT: no significant variability detected during the 2.7 ks exposure; absorbed power-law fit with photon index 1.4(-0.4/+0.5).
• FXT: absorbed power-law fit with Galactic NH fixed and photon index 2.09 +/- 0.13.
• Optical: LCO found little or no fading relative to the COLIBRÍ observation about 6.5 hr later.

Redshift

What’s special vs typical

• The event has a coherent gamma-ray, X-ray, and optical counterpart picture from several facilities, making it better characterized than a single-instrument GBM-only alert.
• The reported optical behavior includes little or no fading between two early observations, which was explicitly noted by the observers.