Two‐Component Jet Models of Gamma‐Ray Burst Sources

Abstract

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 (opening half-angle {theta}{sub n}), highly relativistic (initial Lorentz factor {eta}{sub n} & 10{sup 2}) outflow, from which the {gamma}-ray emission originates, and a wider ({theta}{sub w} {approx} E{sub n}, as expected for the decoupled neutron and proton components, respectively, in an initially neutron-rich, hydromagnetically accelerated jet model, then the narrow component only dominates the early afterglow and the wide component takes over after its nominalmore » deceleration time t{sub dec,w} (typically {approx} 0.1-1 days). For characteristic parameter values t{sub dec,w} is comparable to the jet break time t{sub jet,n} of the narrow component. In this case the emergence of the wide component at t{sub dec,w} may mask the jet break in the narrow component at t{sub jet,n} if E{sub w} & E{sub n}, which in turn may lead to an overestimate of the {gamma}-ray energy emitted by the source and hence of the required {gamma}-ray emission efficiency. We apply this scheme also to X-ray flash sources, which we interpret as GRB jets viewed at an angle {theta}{sub obs} > {theta}{sub n}. Finally, we argue that a neutron-rich hydromagnetic outflow may naturally give rise to repeated brightening episodes in the afterglow lightcurve as observed in GRB 021004 and GRB 030329.« less