  Very early observations with the Swift satellite of gamma-ray burst (GRB)
afterglows reveal that the optical component is not detected in a large number
of cases. This is in contrast to the bright optical flashes previously
discovered in some GRBs (e.g. GRB 990123 and GRB 021211). Comparisons of the
X-ray afterglow flux to the optical afterglow flux and prompt gamma-ray fluence
is used to quantify the seemingly deficient optical, and in some cases X-ray,
light at these early epochs. This comparison reveals that some of these bursts
appear to have higher than normal gamma-ray efficiencies. We discuss possible
mechanisms and their feasibility for explaining the apparent lack of early
optical emission. The mechanisms considered include: foreground extinction,
circumburst absorption, Ly-alpha blanketing and absorption due to high
redshift, low density environments, rapid temporal decay, and intrinsic
weakness of the reverse shock. Of these, foreground extinction, circumburst
absorption, and high redshift provide the best explanations for most of the
non-detections in our sample. There is tentative evidence of suppression of the
strong reverse shock emission. This could be because of a
Poynting-flux-dominated flow or a pure non-relativistic hydrodynamical reverse
shock.
