Explaining GRB prompt emission with sub-photospheric dissipation and Comptonization

Even though the observed spectra for GRB prompt emission is well constrained, no single radiation mechanism can robustly explain its distinct non-thermal nature. Here, we explore the radiation mechanism with the photospheric emission model using our Monte Carlo Radiative Transfer code. We study the sub-photospheric Comptonization of fast cooled synchrotron photons while the Maxwellian electrons and mono-energetic protons are accelerated to relativistic energies by repeated dissipation events. Unlike previous simulations, we implement a realistic photon to electron number ratio N-gamma/N-e similar to 105 consistent with the observed radiative efficiency of a few per cent. We show that it is necessary to have a critical number of episodic energy injection events N-rh,N-cr similar to few tens to hundreds in the jet in addition to the electron-proton Coulomb coupling in order to inject sufficient energy E-inj,E-cr similar to 2500-4000 m(e)c(2) per electron and produce an output photon spectrum consistent with observations. The observed GRB spectrum can be generated when the electrons are repeatedly accelerated to highly relativistic energies gamma(e,in) similar to few tens to hundreds in a jet with bulk Lorentz factor Gamma similar to 30-100, starting out from moderate optical depths tau(in) similar to 20-40. The shape of the photon spectrum is independent of the initial photon energy distribution and baryonic energy content of the jet and hence independent of the emission mechanism, as expected for photospheric emission.