Prompt optical emission from residual collisions in gamma-ray burst outflows

The prompt gamma-ray emission in gamma-ray bursts is believed to be produced by internal shocks within a relativistic unsteady outflow. The recent detection of prompt optical emission accompanying the prompt gamma-ray emission appears to be inconsistent with this model, because the outflowing plasma is expected to be highly optically thick to optical photons. We show here that fluctuations in flow properties on short, similar to 1 ms, timescales, which drive the gamma-ray-producing collisions at small radii, are expected to lead to "residual" collisions at much larger radii, where the optical depth to optical photons is low. The late residual collisions naturally account for the relatively bright optical emission. The apparent simultaneity of gamma-ray and optical emission is due to the highly relativistic speed with which the plasma expands. Residual collisions may also account for the X-ray emission during the early "steep decline" phase, where the radius is inferred to be larger than the gamma-ray emission radius. Finally, we point out that inverse Compton emission from residual collisions at large radii is expected to contribute significantly to the emission at high energy and may therefore "smear" the pair-production spectral cutoff.