Extracting energy from a black hole through its disk

When some magnetic field lines connect a Kerr black hole with a disk rotati ng around it, energy and angular momentum are transferred between them. If the black hole rotates faster than the disk, ca/GM(H) > 0.36 for a thin Kep lerian disk, then energy and angular momentum are extracted from the black hole and transferred to the disk (M-H is the mass and aM(H), is the angular momentum of the black hole). This way, the energy originating in the black hole may be radiated away by the disk. The total amount of energy that can be extracted from the black hole spun down from ca/GM(H) = 0.998 to ca/GM( H) = 0.36 by a thin Keplerian disk is approximate to 0.15M(H)c(2). This is larger than approximate to 0.09M(H)c(2), which can be extracted by the Blan dford-Znajek mechanism.