Fermi/LAT Gamma Ray Burst emission models and jet properties
  The GeV emission of Gamma Ray Bursts, first detected by EGRET in an handful
of bursts, is now an established property of roughly the 10% of all bursts,
thanks to the Fermi/LAT observations. GRB 090510, a short burst, is
particularly interesting because the good timing allows to derive a severe
limit to theories of quantum gravity. With the dozen bursts detected in the
0.1-30 GeV band so far, we start to see some common properties: (i) the
duration is often longer than the duration of the softer emission detected by
the Gamma Burst Monitor (GBM) onboard Fermi; (ii) the spectrum is consistent
with F(v)~v^{-1} with no 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}; iv) the peak energy of the GBM emission exceeds 500 keV (rest
frame). These properties suggest a similar process for the origin of the GeV
flux. We propose that it is afterglow synchrotron emission shortly following
the start of the prompt phase. The steep decay slope suggests that the fireball
emits in the radiative regime, i.e. all dissipated energy is radiated away. The
large peak energy of the GBM flux suggests that electron-positron pairs might
play a crucial role. The rapid onset, but with some delay, of the GeV flux with
respect to the GBM one suggests that the bulk Lorentz factor Gamma of these
bursts is of the order of 1000. Therefore the relatively small fraction of
bursts detected at high energies might correspond to the fraction of bursts
having the largest Gamma. If the emission occurs in the radiative regime we can
start to understand why the observed X-ray and optical afterglow energetics are
much smaller than the energetics emitted during the prompt phase.
