Loaded magnetohydrodynamic flows in Kerr spacetime

The effect of mass and energy loading on the efficiency at which energy can be extracted magnetically from a Kerr black hole is explored, using a semianalytic, ideal magnetohydrodynamics model that incorporates plasma injection on magnetic field lines. We find a critical load below which the specific energy of the plasma inflowing into the black hole is negative, and above which it is positive, and identify two types of flows with distinct properties; at subcritical loads a magnetic outflow is launched from the ergosphere, owing to extraction of the black hole spin energy, as originally proposed by Blandford and Znajek. At supercritical loads the structure of the flow depends on the details of the injection process. In cases where the injected plasma is relativistically hot, a pressure-driven, double transmagnetosonic flow is launched from a stagnation point located outside the ergosphere, between the inner and outer light cylinders. Some fraction of the energy deposited in the magnetosphere is then absorbed by the black hole and the rest emerges at infinity in the form of a relativistic outflow. When the injected plasma is cold an outflow may not form at all. We discuss the implications of our results to gamma ray bursts and active galactic nuclei.