Black Hole Spin Evolution

Abstract

We consider a subset of the physical processes that determine the spin j =\na/M of astrophysical black holes. These include: (1) Initial conditions. Recent\nmodels suggest that the collapse of supermassive stars are likely to produce\nblack holes with j ~ 0.7. (2) Major mergers. The outcome of a nearly equal mass\nblack hole-black hole merger is not yet known, but we review the current best\nguesses and analytic bounds. (3) Minor mergers. We recover the result of\nBlandford & Hughes that accretion of small companions with isotropically\ndistributed orbital angular momenta results in spindown, with j ~ M^{-7/3}. (4)\nAccretion. We present new results from fully relativistic magnetohydrodynamic\naccretion simulations. These show that, at least for one sequence of flow\nmodels, spin equilibrium (dj/dt = 0) is reached for j ~ 0.9, far less than the\ncanonical value 0.998 of Thorne that was derived in the absence of MHD effects.\nThis equilibrium value may not apply to all accretion flows, particularly thin\ndisks. Nevertheless, it opens the possibility that black holes that have grown\nprimarily through accretion are not maximally rotating.\n