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B. Zhang, P. Meszaros (Penn State University)
We consider a gamma-ray burst (GRB) anisotropic fireball with the energy per solid angle decreasing with the opening angle with an index of k, allowing the observer's viewing direction to be at an arbitrary angle with respect to the jet axis. This model can reproduce the key features predicted in the conventional on-axis uniform jet models, with the novelty that the achromatic breaking time in the broadband afterglow lightcurves corresponds to the epoch when the relativistic beaming angle is equal to the viewing angle, rather than to the jet half opening angle. If all the GRB fireballs have such a similar energy distribution form with 1.5 < k < 2, GRBs may have a quasi-universal beaming configuration, aside from a standard energy reservoir.