The evolution and efficiency of energy release of magnetized black-hole accretion disks

On the basis of the equivalent circuit model, we investigate two different mechanisms of extracting energy of rotation and angular momentum from a black hole by magnetic field, namely, the Blandford-Znajek (BZ) process and the magnetic coupling (MC) process. The contributions to the efficiency of energy release via pure accretion process, BZ process and MC process are compared in detail by studying the evolutionary characteristics of the spin parameter of the black hole at the center of the magnetized accretion disk. It is shown that the MC process is an important new mechanism of extracting energy from the rotating black hole and its efficiency of energy release is almost as high as that of the BZ process. The efficiency of energy release via pure accretion process is higher than those of BZ process and magnetic coupling process. However, when the rotation of a black hole approaches that of an extreme Kerr black hole,the efficiency of energy release is mainly due to the contributions of BZ process and MC process.