Short Timescale Variability in the External Shock Model of Gamma Ray Bursts

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, spiky variability occurs when the clouds have radii r << R/Gamma, where R is the mean distance of a cloud from the GRB source and Gamma 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. For such efficiencies, we find that erratic variability persists in the simulated GRB light curves. If GRB sources are surrounded by clouds with such properties, then short timescale variability of GRBs is possible in the external shock model.