Spinning up black holes with super-critical accretion flows

TL;DR

This paper investigates how super-critical accretion flows can spin up black holes beyond previously established limits, showing that high accretion rates and low viscosity lead to higher equilibrium spins.

Contribution

It demonstrates that super-Eddington accretion can result in black hole spins exceeding the classical limit, with the equilibrium spin strongly dependent on the viscosity parameter.

Findings

Black holes can reach spin parameters above 0.9978 with super-Eddington accretion.

At rac{10}{Edd} and \u03b1=0.01, the spin reaches 0.9994.

Captured radiation has negligible impact on spin at high accretion rates.

Abstract

We study the process of spinning up black holes by accretion from slim disks in a wide range of accretion rates. We show that for super-Eddington accretion rates and low values of the viscosity parameter alpha (\lesssim 0.01) the limiting value of the dimensionless spin parameter a_* can reach values higher than a_*=0.9978 inferred by Thorne (1974) in his seminal study. For \mdot=10\mdot_{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.