Explaining GRB prompt emission with sub-photospheric dissipation and Comptonization

Abstract

Abstract 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 (MCRaT) 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 Nγ/Ne ∼ 105 consistent with the observed radiative efficiency of a few percent. We show that it is necessary to have a critical number of episodic energy injection events Nrh, cr ∼ few 10s − 100 in the jet in addition to the electron-proton Coulomb coupling in order to inject sufficient energy Einj, cr ∼ 2500 − 4000 mec2 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 γe, in ∼ few 10s − 100 in a jet with bulk Lorentz factor Γ ∼ 30 − 100, starting out from moderate optical depths τin ∼ 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.