Emission Spectra from Internal Shocks in Gamma-Ray Burst Sources

Abstract

Unsteady activity of gamma-ray burst sources leads to internal shocks in their emergent relativistic wind. We study the emission spectra from such shocks, assuming that they produce a power-law distribution of relativistic electrons and possess strong magnetic fields. The synchrotron radiation emitted by the accelerated electrons is Compton upscattered multiple times by the same electrons. A substantial fraction of the scattered photons acquire high energies and produce e(+)e(-) pairs. The pairs transfer back their kinetic energy to the radiation through Compton scattering. The generic spectral signature from pair creation and multiple Compton scattering is highly sensitive to the radius at which the shock dissipation takes place and to the Lorentz factor of the wind. The entire emission spectrum extends over a wide range of photon energies, from the optical regime up to TeV energies. For reasonable values of the wind parameters, the calculated spectrum is found to be in good qualitative agreement with the observed burst spectra.