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Abstract
In this dissertation I investigate several scenarios for extracting energy from a black hole and consider their applications in astronomy, among them the Blandford-Znajek mechanism has ever been thought promising in powering many energetic astronomical phenomena ranging from gamma-ray bursts to extragalactic jets. I first demonstrate that, contrary to traditional thought, with quite general assumptions the original Blandford-Znajek mechanism cannot work efficiently. With a toy model, I show that for a Kerr black hole with a thin disk the energy extracted from the disk always dominates the energy extracted from the black hole. By considering the screw instability of the magnetic field, I show that a stringent limit on the distance from the load to the black hole exists. The latter reveals that the instability of magnetic fields has important effects on the Blandford-Znajek mechanism—a topic which has been neglected in all previous studies. I then turn to a variant of the Blandford-Znajek mechanism: the magnetic field lines threading the black hole horizon close on the disk rather than the remote load. I find that, the magnetic field connecting the black hole to the disk is very efficient in transferring energy and angular momentum between them. If the black hole rotates faster than the disk, the rotational energy of the black hole is extracted by the disk and ultimately radiated to infinity. This may greatly enhance the efficiency of the disk and produce interesting observational effects. Finally, I present a Tokamak model for gamma-ray bursts powered by the Blandford-Znajek mechanism, which demonstrates that in very specific cases the Blandford-Znajek mechanism may work efficiently for a Kerr black hole with a thick disk or torus.
