Key points are not available for this paper at this time.
We present numerical simulations of axisymmetric, magnetically driven outflows that reproduce the inferred properties of ultra-relativistic gamma-ray burst (GRB) jets. These results extend our previous simulations (Komissarov et al. 2007) of outflows accelerated to moderately relativistic speeds, which we applied to jets of active galactic nuclei. In contrast to several recent investigations, which have employed the magnetodynamics approximation, our numerical scheme solves the full set of equations of special-relativistic, ideal MHD, which enables us to explicitly calculate the jet velocity and magnetic-to-kinetic energy conversion efficiency — key parameters of interest for astrophysical applications. We confirm that the magnetic acceleration scheme remains robust into the ultra-relativistic regime, as previously indicated by semi-analytic selfsimilar solutions. We find that all current-carrying outflows exhibit self-collimation and consequent acceleration near the rotation axis, but that unconfined outflows lose causal connectivity across the jet and therefore do not collimate or accelerate efficiently in their outer regions. We show that magnetically accelerated jets confined by
Komissarov et al. (Fri,) studied this question.