Two-component jet models of gamma-ray burst sources

Recent observational and theoretical studies have raised the possibility that the collimated outflows in gamma-ray burst (GRB) sources have two distinct components: a narrow, highly relativistic outflow, from which the γ-ray emission originates, and a wider, moderately relativistic surrounding flow. Using a simple synchrotron emission model, we calculate the R-band afterglow lightcurves expected in this scenario and derive algebraic expressions for the flux ratios of the emission from the two jet components at the main transition times in the lightcurve. We apply this scheme to the interpretation of the afterglow lightcurves in GRB and X-ray flash sources and show that it may significantly alleviate the radiative efficiency requirements in the internal shock model of GRBs.