Extracting energy from accretion into a Kerr black hole

The highest efficiency of converting rest mass into energy by accreting mat ter into a Kerr black hole is similar to 31% (Thorne 1974). We propose a ne w process in which periods of accretion from a thin disk, and the associate d spin up of the black hole, alternate with the periods of no accretion and magnetic transfer of energy from the black hole to the disk. These cycles can repeat indefinitely, at feast in principle, with the black hole mast; i ncreasing by similar to 66% per cycle and up to similar to 43% of accreted rest mass radiated away by the disk.