GRB prompt emission from internal shocks: Spectral slope in the low gamma-ray range, high-energy photons and optical prompt emission

We compute the emission from internal shocks using a new approach, where a detailed model for the dynamics of shock waves is coupled to a radiative code that solves the time evolution of shock-accelerated electrons and emitted photons in the comoving frame of the shocked material, taking into account all relevant radiative processes. Using this model, we discuss which process (synchrotron vs inverse Compton) is most likely dominating the observed low gamma-ray emission and consider a possible mechanism to reconcile the synchrotron process with the observed spectral slope (a ̃ -1) in this energy range. We then focus on the high-energy emission (Fermi-LAT range) and show that very different spectral shapes can be expected. We describe physical diagnostics to constrain the properties of the relativistic outflow, the physical conditions in the shocked medium and the dominant radiative process.