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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.