1999ApJ...513L...5D

We have developed a computer model to calculate gamma-ray burst (GRB) light curves and efficiencies from the interaction of a single, thin blast wave with clouds in the external medium. Large-amplitude, short-timescale variability occurs when the clouds have radii r{Lt}R/Γ, where R is the mean distance of a cloud from the GRB source and Γ is the blast-wave Lorentz factor. Efficiencies ≳10% require a large number of small clouds, each with sufficiently large column densities to extract most of the available blast-wave energy in the region of interaction. The number and duration of pulses in the simulated GRB light curves are compared with the respective properties found in GRB light curves. If GRB sources are surrounded by clouds with such properties, then short-timescale variability of GRBs can be obtained in the external shock model.