The two leading models for the structure of gamma-ray burst (GRB) jets are the uniform jet model and the universal structured jet (USJ) model. In the latter, all GRB jets are intrinsically identical and the energy per solid angle drops as the inverse square of the angle from the jet axis. The simplicity of the USJ model gives it a strong predictive power, including a specific prediction for the observed GRB distribution as a function of both the redshift z and the viewing angle θ. We show that the current sample of GRBs with known z and estimated θ does not agree with the predictions of the USJ model. This can be best seen for a relatively narrow range in z, in which the USJ model predicts that most GRBs should be near the upper end of the observed range in 6, while in the observed sample most GRBs are near the lower end of that range. Since the current sample is very inhomogeneous (i.e., involves many different detectors), it should be taken with care and cannot be used to rule out the USJ model. Nevertheless, this sample strongly disfavors the USJ model. Comparing the prediction for the observed GRB distribution both in θ and in z, with a larger and more homogeneous GRB sample, like the one expected from Swift, would either clearly rule out the USJ model, or alternatively, provide a strong support for it. The test presented here is general and can be used to test any model that predicts both a luminosity function and a luminosity-angle relation.
Bibliographical noteFunding Information:
We are grateful to Rosalba Perna for her comments about the selection effects related to the viewing angle, which greatly improved this Letter. We thank Tsvi Piran for illuminating discussions and helpful remarks. E. N. is supported by the Horowitz Foundation and through the generosity of the Dan David Prize Scholarship 2003. E. N. thanks the Institute for Advanced Study for its warm hospitality and great working atmosphere during the course of this work. J. G. is supported by the W. M. Keck Foundation and by NSF grant PHY-0070928. D. G. acknowledges the Research Training Network “Gamma-Ray Bursts: An Enigma and a Tool” and NSF grant AST 03-07502 for supporting this work.
- Gamma rays: bursts
- ISM: jets and outflows
- Radiation mechanisms: nonthermal