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 powe ring of outflows from accretion-disk-fed black holes. We argue that there i s no reason to suppose that the magnetic field threading the central spinni ng black hole differs significantly in strength from that threading the cen tral regions of the disk. In this case, we show that the electromagnetic ou tput from the inner disk regions is expected in general to dominate over th at 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 poin t out that the strength of the poloidal field in the center of a standard a ccretion disk has been generally overestimated, and we discuss scenarios th at might give rise to more significant central poloidal fields.