The Case for Anisotropic Afterglow Efficiency within Gamma-Ray Burst Jets

Early X-ray afterglows recently detected by {\it Swift} frequently show a phase of very shallow flux decay lasting from a few hundred seconds up to $\sim 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 the decaying tail of the prompt emission and the delayed onset of the afterglow emission observed from viewing angles slightly outside the edge of the jet, as predicted previously. This would imply that a significant fraction of viewers have a very small external shock energy along their line of sight and a very high $\gamma $-ray to kinetic energy ratio. The early flat phase in the afterglow light curve implies, according to this or other interpretations, a very large $\gamma $-ray efficiency, typically $\gtrsim 90%$, which is very difficult to produce by internal shocks.