Energy injection in gamma-ray burst afterglow models

We extend the standard fireball model, widely used to interpret gamma-ray burst (GRB) afterglow light curves, to include energy injections and apply the model to the afterglow light curves of GRB 990510, GRB 000301C, and GRB 010222. We show that discrete energy injections can cause temporal variations in the optical light curves, and we present fits to the light curves of GRB 000301C as an example. A continuous injection may be required to interpret other bursts, such as GRB 010222. The extended model accounts reasonably well for the observations in all bands ranging from X-rays to radio wavelengths. In some cases the radio light curves indicate that additional model ingredients may be needed.