Very Early X-ray Lightcurve Bump and Grb 011121

We show that the forward-reverse shocks model (in the ISM case), the late internal shocks model and the early energy injection model are able to power an early X-ray bump in GRB “afterglows”. However, the temporal behaviors of their decline are much different, which in turn provides us a simple but reliable tool to distinguish them. We suggest that the early X-ray bump observed in GRB 011121 could be understood in the model of re-activity of the central engine (i.e., the late internal shocks model or the early energy injection model). The bumps detected in XRF 050406 and GRB 050502b can only be interpreted by the late internal shocks model. If the mechanism is more or less universal, we believe that all of them are powered by the late internal shocks, which implies that the central engine operates for a much longer time than the duration of the prompt γ−ray emission. Subject headings: Gamma Rays: bursts−ISM: jets and outflows–radiation mechanisms: nonthermal