Physical processes shaping GRB X-ray afterglow lightcurves: theoretical implications from the Swift XRT observations

With the successful launch of the Swift Gamma-ray Burst Explorer, a rich trove of early X-ray afterglow data has been collected by its on-board X-Ray Telescope (XRT). Some interesting features are emerging, including a distinct rapidly decaying component preceding the conventional afterglow component in many sources, a shallow decay component before the more “normal” decay component observed in a good fraction of GRBs (e.g. GRB 050128, GRB 050315, GRB 050319, and GRB 050401), and X-ray flares in nearly half of the afterglows (e.g. GRB 050406, GRB 050502B, GRB 050607, and GRB 050724). These interesting early afterglow signatures reveal valuable and unprecedented information about GRBs, including the prompt emission afterglow transition, GRB emission site, central engine activity, forward-reverse shock physics, and the GRB immediate environment. In this paper, we systematically analyze the possible physical processes that shape the properties of the early X-ray afterglow lightcurves, and use the data to constrain various models. We suggest that the steep decay component is consistent with the tail emission of the prompt gamma-ray bursts and/or of the X-ray flares. This provides clear evidence that the prompt emission and afterglow emission are two distinct components, supporting the internal origin