Spectral Implications of Variability in GRB Fireballs

Abstract

Cosmological $γ$-ray bursts originate from relativistic winds. Temporal fluctuations in the wind velocity can give rise to internal shocks which dissipate a significant fraction of the wind kinetic energy. Part of the energy dissipated is transferred to the electrons through Fermi acceleration. If the post shock fluid is strongly magnetized, the relativistic electrons cool initially through synchrotron emission, and later through Compton scattering. The upsacttered radiation triggers a cascade of $e^{+}e^{-}$-pairs. We compute the final spectrum for a wide range of parameter values for the emission region. We show that the spectral diversity observed by BATSE can be naturally explained by emission from internal shocks, which are associated with the observed source variability.