Single- and Two-Component GRB Spectra in the Fermi GBM-LAT Energy Range

TL;DR

This paper explains Fermi GRB spectra using magnetically dominated jets with photospheric emission extended by inverse Compton scattering, accounting for observed spectral features and time delays.

Contribution

It introduces a model where GRB spectra are produced by magnetically dominated jets with dissipative photospheres and external shocks, explaining spectral shapes and delays.

Findings

Spectra can be modeled with photospheric emission and inverse Compton scattering.

Bulk Lorentz factors are estimated between 300-600.

Time delays between MeV and GeV emissions are naturally explained.

Abstract

Most Fermi GRB spectra appear as either a broken power law extending to GeV energies or as a broken power with a separate GeV power law component. Here we show that such spectra can be understood in terms of magnetically dominated relativistic jets where a dissipative photosphere produces the prompt MeV emission, which is extended into the GeV range by inverse Compton scattering in the external shock, with possible contributions from a reverse shock as well. The bulk Lorentz factors required in these models are in the range of 300-600, and the MeV-GeV time delays arise naturally. In some cases an optical flash and a sub-dominant thermal component are also present.