Gamma-ray Burst Beaming: a Universal Configuration with a Standard Energy Reservoir?

We consider a gamma-ray burst (GRB) model based on an anisotropic fireball with an axisymmetric energy distribution of the form ǫ(θ) ∝ θ, and allow for the observer’s viewing direction being at an arbitrary angle θv with respect to the jet axis. This model can reproduce the key features expected from the conventional on-axis uniform jet models, with the novelty that the achromatic break time in the broadband afterglow lightcurves corresponds to the epoch when the relativistic beaming angle is equal to the viewing angle, θv, rather than to the jet half opening angle, θj . If all the GRB fireballs have such a similar energy distribution form with 1.5 < k< ∼2, GRBs may be modeled by a quasi-universal beaming configuration, and an approximately standard energy reservoir. Subject headings: gamma rays: bursts shock waves ISM: jets and outflows