AAS 207th Meeting, 8-12 January 2006
Session 151 GRB: Theory, X-rays, and Catalogs
Oral, Wednesday, 10:00-11:30am, January 11, 2006, Wilson A/B

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[151.03] The Importance of Off-Axis Beaming in Gamma-Ray Burst Jet Models

T. Q. Donaghy (Univ. of Chicago)

Gamma-Ray Bursts (GRBs) are widely thought to originate from collimated jets of material moving at relativistic velocities. Emission from such a jet should be visible even when viewed from outside the angle of collimation. We summarize current work on the relativistic beaming transformation of the burst quantities Eiso and Epeak as a function of viewing angle. These formulae serve as input for our Monte Carlo population synthesis method, with which we investigate the importance of off-axis beaming in the context of several variable opening-angle uniform jet models. Such models should produce large populations of observable off-axis bursts that lie away from the Epeak ~ Eiso1/2 relation, but are not seen in current datasets. We investigate the effect of the bulk \gamma value upon this off-axis population, as well as the effect of including an \Omegajet-E\gamma correlation to jointly fit the Epeak ~ Eiso1/2 and Epeak ~ E\gamma\beta relations. We find that an inverse correlation between \Gamma and \Omegajet has the effect of greatly reducing the visibility of off-axis events. We also find that the X-Ray Flashes (XRFs) seen by HETE-2 and BeppoSAX cannot be easily explained as classical GRBs viewed off-axis.

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