When a Kerr black hole is connected to a disk rotating around it by a magnetic field, the rotational energy of the Kerr black hole provides an energy source for the radiation of the disk in addition to disk accretion. This figure shows the radiation flux profile of a thin non-accretion disk magnetically coupled to a Kerr black hole of mass MH and specific angular momentum a = 0.99 MH (in the geometrized units G = c = 1; red line), compared to that of a stand accretion disk around the same black hole (blue line). The horizontal axis is the radius from the center of the black hole, the vertical axis is the radiation flux measured by an observer corotating with the disk. For the non-accretion case (red line) the magnetic field is assumed to touch the disk at the inner boundary (rms, i.e. the marginally stable orbit). The radial radiation profile of the non-accretion disk is very different from that of a standard accretion disk: the emissivity index (i.e. the slope of the radiation flux curve) is significantly bigger, most radiation comes from a region closer to the center of disk. [For details see L. -X. Li, ApJ 567, 463 (2002).]
By accretion onto a Kerr black hole, up to 31% of rest mass can be converted into energy. By alternating accretion and the magnetic coupling, an indefinite cyclic process can be constructed with which the efficiency in converting rest mass into energy can be increased to 43% [L. -X. Li and B. Paczynski, ApJ 534, L197 (2000).]
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