Decelerating Plasmoid Model for Gamma-Ray Burst Afterglows

The flaring and fading radio, optical, and X-ray afterglows from GRB 970508 are modeled by a highly relativistic plasma sphere which decelerates by sweeping up ambient gas. The afterglow emission is assumed to be synchrotron radiation emitted by nonthermal electrons in the magnetized plasmoid. The temporal behavior of the delayed emission is controlled by the evolution of the Doppler factor and by adiabatic expansion losses of the nonthermal electrons in the plasmoid. Model fits to the optical data of GRB 970508 are provided, and the relative delay of the radio peak to the optical peak is found to result from a decrease in the observed self-absorption frequency as the plasmoid expands and decelerates. A variety of afterglow behaviors occurs for different observing angles and plasmoid parameters. The degree of collimation inferred from our fit to GRB 970508 implies a space density of GRB sources which exceeds the estimates from scenarios involving coalescing compact objects. This model can be verified through observations of superluminal motion in the delayed radio emission. Subject headings: gamma rays: bursts — radiation mechanisms: nonthermal