Modeling of the Gamma Ray Burst photospheric emission: Monte Carlo simulation of the GRB prompt emission, numerical results and discussion

Abstract

We have carried out a detailed study of the Gamma-Ray Burst (GRB) photospheric emission model predicting a quasi-blackbody spectrum slightly broader than a Planck function. This model was suggested within the relativistic fireball dynamics for interpreting a still not well understood thermal component in the GRB prompt emission, recently observed by the GBM (Gamma-ray Burst Monitor) on board the Fermi space telescope. We propose a Monte Carlo (M C) code for elucidating the observed spectrum, the outflow dynamics and its geometry for a basic and a structured plasma jets whose parameters are implemented. The code involves a simulation part describing the photon propagation assuming an unpolarized, non-dissipative relativistic outflow and a data analysis part for exploring main photospheric emission properties such as the energy, arrival time and observed flux of the simulated seed photons and the photospheric radius. Computing the latter two observables by numerical integration, we obtained values very concordant with the M C simulated results. Fitting Band functions to the photon spectra generated by this method, we derived best-fit values of the photon indices matching well those featuring the observed spectra for most typical GRBs, but corresponding to fit functions inconciliable with blackbody spectral shapes. Various derived results are reported, compared to previous ones and discussed. They show to be very sensitive to the structure of the Lorentz factor that plays a crucial role in determining the presence and strength of geometrical effects. The latter manifest themselves by large broadenings of the simulated spectra featured by multiple peak energies consistently with GRB observations. They are assumed, with multiple Compton scattering, to produce bumps pointed out at very low photon energies. The interpretation of GRB observations is further performed via a Band spectral analysis using the RMFIT software. Finally, developments of this work are put into perspective.