The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C
Cenko, S. B.; Greiner, J.; Berger, E.; Sari, R.; McBreen, S.; Cucchiara, A.; Klose, S.; Ajello, M.; Kawai, N.; von Kienlin, A.; Rau, A.; Krühler, T.; Filgas, R.; Clemens, C.; Rossi, A.; Zhang, X. -L.; Fox, D. B.; Lichti, G. G.; Küpcü Yoldaş, A.; Yoldaş, A.; Afonso, P.; Nagayama, T.; Szokoly, G.; Löw, S.; Sato, S.
Germany, United States, Japan, Ireland, Hungary
Abstract
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 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 the 7-channel “Gamma-Ray Optical and Near-infrared Detector” (GROND) at the 2.2 m 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 ± 0.15, based on simultaneous 7-filter observations with 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 unexpected because of the predicted opacity due to interaction with the extragalactic background light. 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 Γ > 1090. This value rivals any previous measurements of Γ in GRBs and strengthens the extreme nature of GRB 080916C.