Speaker: Vlasios Vasileiou, LUPM, Universite de Montpellier, Montpellier, France
Abstract: During its first three years of operations, since August 2008, the Large Area Telescope on board the Fermi spacecraft (Fermi-LAT) has detected more than 30 GRBs in the ~30 MeV - 300 GeV energy range, and has set upper limits on the E>100 MeV emission on hundreds of GRBs occurring in its field of view. These results revealed new and puzzling features in the temporal and spectral behavior of GRBs, such as a delayed onset of the high-energy (E>100 MeV) emission with respect to the lower-energy (keV/MeV) signal detected by the Gamma-Ray Burst Monitor (Fermi-GBM), the presence of an extra spectral component in addition to the Band function, an extended emission lasting significantly longer than the lower-energy keV/MeV prompt emission, and the presence of spectral cutoffs at tens-of-MeV energies. We will present these discoveries and their theoretical implications, focusing both on individual cases and also in the context of a systematic and unbiased study performed towards the first Fermi-LAT GRB catalog. In addition, constraints on any energy dependence of the speed of light placed with GRB observations will be presented.