The case for anisotropic afterglow efficiency within gamma-ray burst jets

Early X-ray afterglows recently detected by Swift frequently show a phase of very shallow flux decay lasting from ∼10^2.5 up to ∼10 ⁴ s, followed by a steeper, more familiar decay. We suggest that the flat early part of the light curve may be a combination of (1) the decaying tail of the prompt emission and (2) the delayed onset of the afterglow emission observed from viewing angles slightly outside the edge of that part of the jet that generates prominent afterglow emission, as predicted previously. This would imply that a significant fraction of viewers get very little external shock energy along their line of sight and, therefore, see a very high γ-ray-to-kinetic energy ratio at early times. The early flat phase in the afterglow light curve implies, for standard afterglow theory, a very large γ-ray efficiency, typically ≳90%, which is very difficult to extract from baryons by internal shocks.