Large Scale Magnetic Field Configurations Of Black-hole Accretion Disc And Mechanisms Of Extracting Energy

A model for magnetic extraction of energy from a rotating black hole surrounded by an accretion disk is proposed based on the coexistence of the Blandford-Znajek (BZ) process and magnetic coupling (MC) process (BZMC). In addition, we discuss the very origin of the magnetic field configuration of the MC process and the screw instability of the magnetic field. The main results are summarized as follows.Firstly, two types of mechanisms of extracting energy from a rotating black hole by magnetic field, the BZ process and MC process, are investigated. The unified analytical expressions for the BZ and MC powers and torques are derived by an improved equivalent circuit based on the black hole theory"Membrane Paradigm", and the condition for the coexistence of the BZ process and the MC process are discussed. Furthermore, the MC process can be used to explain a very steep emissivity in the inner region of the disc, which is consistent with the recent observations of the nearby Seyfert I galaxy MCG-6-30-15, and HFQPO.Secondly, the black hole is immerged in the plasma, which might be instable. The magnetic field configurations with both poloidal and toroidal components can be screw-unstable. We derive a criteria for the screw instability in the coexistence of the BZ and MC mechanisms. The results show the state of the coexistence always accompanies the screw instability, and the ratio of extracting energy by the BZ process and MC process is changed as the result of the screw instability, respectively. Furthermore, we studied the"inner engine"for GRB/supernovae by this model.Thirdly, the poloidal magnetic field, which connect the black hole and the disk, are generated by a single toroidal electric current flowing in an accretion disc around a black hole. We derive a criterion for the screw instability of the magnetic field. It turns out that the screw instability is related to two parameters: the BH spin and the radius of the disc. The occurrence of screw instability is depicted in a parameter space of the BH spin and the radius of the disc. Finally, We discuss the effects of the screw instability on the magnetic field configuration and on extracting energy. We find that the screw instability restricts the configuration of the magnetic field and decreases the energy extracted in the MC processes.