Context : The detection of GeV photons from gamma-ray bursts ( GRBs ) has important consequences for the interpretation and modelling of these most-energetic cosmological explosions .
The full exploitation of the high-energy measurements relies , however , on the accurate knowledge of the distance to the events .

Aims : Here we report on the discovery of the afterglow and subsequent redshift determination of GRB 080916C , the first GRB detected by the Fermi Gamma-Ray Space Telescope with high significance detection of photons at energies > 0.1 GeV .

Methods : Observations were done with 7-channel imager GROND at the 2.2m MPI/ESO telescope , the SIRIUS instrument at the Nagoya-SAAO 1.4 m telescope in South Africa , and the GMOS instrument at Gemini-S .

Results : The afterglow photometric redshift of z = 4.35 \pm 0.15 , based on simultaneous 7-filter observations with the Gamma-Ray Optical and Near-infrared Detector ( GROND ) , places GRB 080916C among the top 5 % most distant GRBs , and makes it the most energetic GRB known to date .
The detection of GeV photons from such a distant event is rather surprising .
The observed gamma-ray variability in the prompt emission together with the redshift suggests a lower limit for the Lorentz factor of the ultra-relativistic ejecta of \Gamma > 1090 .
This value rivals any previous measurements of \Gamma in GRBs and strengthens the extreme nature of GRB 080916C .

Conclusions :