Simulations and Analytic Models of Relativistic Magnetized Jets

Alexander Tchekhovskoy (CfA)

Long-duration gamma-ray bursts (GRBs) occur when the core of a progenitor star collapses, forms a magnetized rotating compact object and powers a pair of magnetized relativistic jets. These jets emerge from the star with Lorentz factors \gamma of a few hundred and opening angles \theta \lesssim 0.1 radians. Achromatic breaks in GRB afterglow light curves indicate that \gamma\theta \gg 1. However, magnetohydrodynamic (MHD) simulations of collimated jets generally give \gamma\theta \lesssim 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 \theta ~ 0.04 rad, giving \gamma\theta ~ 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.