Suppression of the Early Optical Afterglow of Gamma-Ray Bursts

Recent observations of gamma-ray bursts (GRBs) are providing prompt few-arcminute gamma-ray localizations, rapid few-arcsecond X-ray positions, and rapid and extensive follow-up in the X-ray, UV, optical, and radio bands. Thirteen of these bursts include extraordinary optical upper limits at very early epochs after the burst, in marked contrast to the bright optical flashes previously believed to be the norm. Although host extinction can explain the properties of some bursts, and the natural range of burst energies and distances can explain some others, comparison of our optical, X-ray, and gamma-ray data sets reveals that these considerations alone cannot explain the full diversity of the burst population. Instead, one or more mechanisms must act to suppress the optical flash and provide a significantly enhanced efficiency of the prompt gamma-ray emission for some bursts. One possibility is that a fraction of the burst population is powered by Poynting flux-dominated outflows, resulting in a very inefficient transfer of magnetic energy to the ambient medium; if so, then the diversity of the GRB population would extend to the very heart of their highly-relativistic engines