Black Hole Spin Evolution

We consider a subset of the physical processes that determine the spin j =a/M of astrophysical black holes. These include: (1) Initial conditions. Recentmodels suggest that the collapse of supermassive stars are likely to produceblack holes with j ~ 0.7. (2) Major mergers. The outcome of a nearly equal massblack hole-black hole merger is not yet known, but we review the current bestguesses and analytic bounds. (3) Minor mergers. We recover the result ofBlandford & Hughes that accretion of small companions with isotropicallydistributed orbital angular momenta results in spindown, with j ~ M^{-7/3}. (4)Accretion. We present new results from fully relativistic magnetohydrodynamicaccretion simulations. These show that, at least for one sequence of flowmodels, spin equilibrium (dj/dt = 0) is reached for j ~ 0.9, far less than thecanonical value 0.998 of Thorne that was derived in the absence of MHD effects.This equilibrium value may not apply to all accretion flows, particularly thindisks. Nevertheless, it opens the possibility that black holes that have grownprimarily through accretion are not maximally rotating.Comment: 22 pp, 4 figures, accepted to Ap