Magnetically Driven Jets in the Kerr Metric

Abstract

We compute a series of three-dimensional general relativistic magnetohydrodynamic simulations of accretion flows in the Kerr metric and investigate the properties of the resulting unbound outflows. The strength of the outflows increases sharply with increasing black hole rotation rate. Several generic features are observed. The mass in the outflow in concentrated in a hollow cone whose opening angle is largely determined by the effective potential for matter with specific angular momentum comparable to that of the innnermost stable circular orbit. The pressure in the accretion disk's corona provides the dominant force accelerating the matter outward. The principle element shaping the outflow is therefore the centrifugal barrier preventing accreting matter from coming close to the rotation axis. The funnel inside the centrifugal barrier contains very little matter and is dominated by electromagnetic fields that rotate at a rate tied closely to the rotation of the black hole, even when the black hole spins in a sense opposite to the rotation of the accretion flow. These fields carry an outward-going Poynting flux whose immediate energy source is the rotating spacetime of the Kerr black hole. When the spin parameter a/M of the black hole exceeds 0.9, the energy carried to infinity by these outflows can be comparable to the nominal radiative efficiency predicted in the Novikov-Thorne model. Similarly, the expelled angular momentum can be comparable to that accreted by the black hole. Both the electromagnetic and the matter outflows contribute significantly to the energy and angular momentum of the outflow.