Simulations of gamma-ray bursts from external shocks: Time variability and spectral correlations

We compute burst spectra and time structures arising from synchrotron and inverse Compton scattering by nonthermal electrons accelerated in shocks that form during the interaction between a thin ultrarelativistic fireball and a stationary external medium. We investigate the effect of varying the most important model parameters on the resulting burst spectra, and we present a set of correlations among the spectral and temporal features of the bursts. The spectral hardness, various spectral-temporal correlations, and the spectral evolution of the simulated bursts are compared to those of observed bursts for a representative set of model parameters. Multipulse structures are simulated using a variable magnetic field and anisotropic emission, and the most important spectral and temporal properties of these pulses are compared with observations.