GeV emission from Gamma Ray Bursts: a radiative fireball?
  We study the emission observed at energies greater than 100 MeV of 11 Gamma
Ray Bursts (GRBs) detected by the Fermi/Large Area Telescope (LAT) until
October 2009. The GeV emission has three main properties: (i) its duration is
often longer than the duration of the softer emission detected by the Gamma
Burst Monitor (GBM) onboard Fermi [this confirms earlier results from the
Energetic Gamma-Ray Experiment Telescope (EGRET)]; (ii) its spectrum is
consistent with F(v) propto v^(-1) and does not show strong spectral evolution;
(iii) for the brightest bursts, the flux detected by the LAT decays as a power
law with a typical slope: t^(-1.5). We argue that the observed >0.1 GeV flux
can be interpreted as afterglow emission shortly following the start of the
prompt phase emission as seen at smaller frequencies. The decay slope is what
expected if the fireball emission is produced in the radiative regime, i.e. all
dissipated energy is radiated away. We also argue that the detectability in the
GeV energy range depends on the bulk Lorentz factor Gamma of the bursts, being
strongly favoured in the case of large Gamma. This implies that the fraction of
bursts detected at high energies corresponds to the fraction of bursts having
the largest Gamma. The radiative interpretation can help to explain why the
observed X-ray and optical afterglow energetics are much smaller than the
energetics emitted during the prompt phase, despite the fact that the collision
with the external medium should be more efficient than internal shocks in
producing the radiation we see.
