Gamma-ray bursts (GRBs) power relativistic jets with Lorentz factors gamma of a few hundred and opening angles heta <~ 0.1 radians. Achromatic breaks in GRB afterglow light curves indicate that gamma heta >> 1. However, magnetohydrodynamic (MHD) simulations of collimated jets generally give gamma heta <~ 1, indicating a conflict between these models and observations. In this talk we present a new class of MHD jet simulations in which the simulated jet is confined by a GRB progenitor star, modeled as a rigid confining wall that extends out to a certain radius. Beyond this radius, the jet becomes deconfined. We find that the onset of deconfinement outside the star causes a burst of acceleration with negligible change in the opening angle. In our fiducial model with a stellar radius equal to 10^4.5 times that of the central compact object, the jet achieves an asymptotic gamma ~ 500 far outside the star and an asymptotic heta ~ 0.04 rad, giving gamma heta ~ 20. These values are consistent with observations of typical long-duration GRBs, and explain the occurrence of jet breaks within the context of MHD jets.
View poster as pdf.
Author entry (protected)