Magnetic Coupling of a Rotating Black Hole with the Surrounding Accretion Disc

The evolution characteristics and energy extraction of a rotating black hole are investigated by considering magnetic coupling with the surrounding accretion disc. It is found that both mass and spin of the black hole might be reduced by the joint effects of disc accretion and magnetic coupling, provided that the latter is stronger than the former. The efficiencies of the two energy mechanisms are calculated and compared to a variety of parameters. In addition, the validity of the laws of black hole thermodynamics is discussed.