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 (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}< 3 {theta}{sub n}), moderately relativistic ({eta}{sub W} {approx} 10) 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. For viewing angles {theta}{sub obs} < {theta}{sub n} we find that the contribution of the wide component to the optical afterglow is negligible if its kinetic energy E{sub W} is significantly smaller than that of the narrow component, E{sub n}, as expected for the jet core and cocoon outflow components in the collapsar jet-breakout model. However, if E{sub w} > 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 nominal 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.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 839640
- Report Number(s):
- SLAC-PUB-10935; TRN: US0503480
- Country of Publication:
- United States
- Language:
- English
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