Energy Injection Episodes in Gamma-Ray Bursts: The Light Curves and Polarization Properties of GRB 021004

Several GRB afterglow light curves deviate strongly from the power-law decay observed in most bursts. We show that these variations can be accounted for by including refreshed shocks in the standard fireball model previously used to interpret the overall afterglow behavior. As an example, we consider GRB 021004, which exhibited strong light-curve variations and has a reasonably well time-resolved polarimetry. We show that the light curves in the R band, X-rays, and radio can be accounted for by four energy injection episodes in addition to the initial event. The polarization variations are shown to be a consequence of the injections.