Spinning up black holes with super-critical accretion flows

We study the process of spinning up black holes by accretion from slim disks for a wide range of accretion rates. We show that for super-Eddington accretion rates and low values of the viscosity parameter alpha (less than or similar to 0.01) the limiting value of the dimensionless spin parameter a(*) can reach values higher than a(*) = 0.9978 inferred by Thorne in his seminal study. For (M) over dot = 10 (M) over dot(Edd) and alpha = 0.01, spin equilibrium is reached at a(*) = 0.9994. We show that the equilibrium spin value depends strongly on the assumed value of alpha. We also prove that for high accretion rates the impact of captured radiation on spin evolution is negligible.