Explaining GRB prompt emission with sub-photospheric dissipation and Comptonization

TL;DR

This paper investigates how sub-photospheric Comptonization and episodic energy injections in relativistic jets can produce GRB spectra consistent with observations, using a detailed Monte Carlo radiative transfer simulation.

Contribution

It introduces a realistic photon-electron ratio and demonstrates the necessity of multiple energy injection events to replicate observed GRB spectra within the photospheric emission model.

Findings

Critical number of energy injections needed (~10-100)

Relativistic electron acceleration to gamma ~10-100

Photon spectrum shape independent of initial conditions

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 $N_{\gamma}/N_{e} \sim 10^5$ 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 $N_{rh,cr} \sim {\rm few}\ 10{\rm s}-100$ in the jet in addition to the electron-proton Coulomb coupling in order to inject sufficient energy $E_{inj,cr} \sim 2500-4000\ m_e c^2$ 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 $\gamma_{e,in} \sim {\rm few}\ 10{\rm s}-100$ in a jet with bulk Lorentz factor $\Gamma \sim 30-100$, starting out from moderate optical depths $\tau_{in} \sim 20-40$. 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.