Maximal spin and energy conversion efficiency in a symbiotic system of
  black hole, disk and jet
  We study a combined model of black hole - accretion disk - magnetosphere -
jet symbiosis, applicable for supermassive black holes. We quantify the mass
and spin evolution and we analyze how the limiting value of the spin parameter
and the conversion efficiency of accreted mass into radiation depend on the
interplay of electromagnetic radiation reaction, magnetosphere characteristics
and truncation radius of radiation. The dominant effect comes from the closed
magnetic field line region, which reduces the spin limit to values ~0.89
(instead ~0.99 in its absence). Therefore observations on black hole spins
could favour or disfavour the existence of the closed magnetic field line
region (or its coupling to the disk). We also find that the suppression of
radiation from the innermost part of the accretion disk, inferred from
observations, and a collimated jet both increase the spin limit and the energy
conversion efficiency.
