Extracting energy from black holes: The relative importance of the Blandford-Znajek mechanism

We critically assess the role of the Blandford-Znajek mechanism in the powering of outflows from accretion-disk-fed black holes. We argue that there is no reason to suppose that the magnetic field threading the central spinning black hole differs significantly in strength from that threading the central regions of the disk. In this case, we show that the electromagnetic output from the inner disk regions is expected in general to dominate over that from the hole. Thus the spin (or not) of the hole is probably irrelevant to the expected electromagnetic power output from the system. We also point out that the strength of the poloidal field in the center of a standard accretion disk has been generally overestimated, and we discuss scenarios that might give rise to more significant central poloidal fields.