Testing the Predictions of the Universal Structured Gamma-Ray Burst Jet Model

The two leading models for the structure of GRB jets are the uniform jetmodel and the universal structured jet (USJ) model. In the latter, all GRB jetsare intrinsically identical and the energy per solid angle drops as the inversesquare of the angle from the jet axis. The simplicity of the USJ model gives ita strong predictive power, including a specific prediction for the observed GRBdistribution as a function of both the redshift $z$ and the viewing angle$\theta$. We show that the current sample of GRBs with known $z$ and estimated$\theta$ does not agree with the predictions of the USJ model. This can be bestseen for a relatively narrow range in $z$, in which the USJ model predicts thatmost GRBs should be near the upper end of the observed range in $\theta$, whilein the observed sample most GRBs are near the lower end of that range. Sincethe current sample is very inhomogeneous (i.e. involves many differentdetectors), it should be taken with care and cannot be used to rule out the USJmodel. Nevertheless, this sample strongly disfavors the USJ model. Comparingthe prediction for the observed GRB distribution both in $\theta$ and in $z$,with a larger and more homogeneous GRB sample, like the one expected fromSwift, would either clearly rule out the USJ model, or alternatively, provide astrong support for it. The test presented here is general, and can be used totest any model that predicts both a luminosity function and a luminosity-anglerelation.Comment: 11 pages, 1 figure; accepted for publication in ApJ