GRMHD Simulations of Disk/Jet Systems: Application to the Inner Engines   of Collapsars
  We have carried out 2D and 3D general relativistic magnetohydrodynamic simulations of jets launched self-consistently from accretion disks orbiting Kerr black holes and applied the results to the inner engine of the collapsar model of gamma-ray bursts. The accretion flow launches energetic jets in the axial funnel region of the disk/jet system, as well as a substantial coronal wind. The jets feature knot-like structures of extremely hot, ultra-relativistic gas; the gas in these knots begins at moderate velocities near the inner engine, and is accelerated to ultra-relativistic velocities (Lorentz factors of 50, and higher) by the Lorentz force in the axial funnel. The increase in jet velocity takes place in an acceleration zone extending to at least a few hundred gravitational radii from the inner engine. The overall energetics of the jets are strongly spin-dependent, with high-spin black holes producing the highest energy and mass fluxes. In addition, with high-spin black holes, the ultra-relativistic outflow is cylindrically collimated within a few hundred gravitational radii of the black hole, whereas in the zero- spin case the jet retains a constant opening angle of approximately 16 degrees. The simulations also show that the coronal wind, though considerably slower and colder than the jets, also carries a significant amount of mass and energy. When simulation data is scaled to the physical dimensions of a collapsar the jets operate for a period ranging from 0.1 to 1.4 seconds, until the accretion disk is depleted, delivering $10^{48}$ to $10^{49}$ erg. 
Jean-Pierre De Villiers, Jan Staff, Rachid Ouyed