The flat decay phase in the early X-ray afterglows of Swift GRBs

Many Swift GRBs show an early phase of shallow decay in their X-ray afterglows, lasting from t ∼ 10^2.5 s to ∼ 10⁴ s after the GRB, where the flux decays as ∼ t^-0.2 - t ^-0.8. This is perhaps the most mysterious of the new features discovered by Swift in the early X-ray afterglow, since it is still not clear what causes it. I discuss different possible explanations for this surprising new discovery, as well as their potential implications for the gamma-ray efficiency, the afterglow kinetic energy, and perhaps even for the physics of collisionless relativistic shocks.