Broad band simulation of Gamma Ray Bursts (GRB) prompt emission in presence of an external magnetic field

Abstract

The origin of prompt emission in GRBs is not yet well understood. The simplest and most popular model is Synchrotron, Self-Compton (SSC) emission produced by internal shocks inside an ultra-relativistic jet. However, recent observations of a delayed high energy component have encouraged alternative models. Here we use a recently developed formulation of relativistic shocks to simulate GRBs. It takes into account the evolution of quantities such as densities of colliding shells, electric and magnetic fields. We also extend the previous formulation by considering a precessing external magnetic field. These simulations are very realistic and present significant improvement with respect to previous phenomenological GRB simulations. They reproduce light curves of separated peaks of real GRBs and variety of observed spectral slopes at E > E_{peak}. The high energy emission can be explained by synchrotron emission and a subdominant contribution from inverse Compton. We also suggest an explanation for the extended tail emission observed by the Fermi-LAT instrument and relate it to the screening of the magnetic field and/or trapping of accelerated electrons in the electromagnetic energy structure of the plasma in the shock front. Spectral slopes of simulated bursts at E << E_{peak} are consistent with theoretical prediction and at E < E_{peak} can be flatter if the spectrum of electrons is roughly flat or has a shallow slope. The observed flat spectra at soft gamma-ray and hard x-ray bands is the evidence that there is a significant contribution at $E < E_{peak}$ from lower Lorentz factor wing of electrons distribution which have a roughly random acceleration rather than being thermal. This means that the state of matter in the jet at the time of ejection is most probably nonthermal and electrons have a shallow spectrum at low energies.(abbreviated)