Black-hole spin-up by coherent accretion

Coherent prograde gas accretion increases the angular momentum of a black hole. For a zero-torque thin disc, the rest mass

d m_{0}

entering from the innermost stable circular orbit changes the hole by

d M = E_{ISCO} d m_{0}

and

dJ = ℓ_{ISCO} d m_{0}

. Its dimensionless spin obeys

d a_{*} / d lo g M = c ℓ_{ISCO} / (GM E_{ISCO}) - 2 a_{*}

. Starting from zero spin and neglecting captured radiation, the formal extremal endpoint has

M_{f} = 6 M_{i}

and requires rest-mass supply approximately

1.8464 M_{i}

. Photon capture leads instead to the Thorne spin limit.

Table of contents
- Thorne spin limit Black-hole spin-up by coherent accretion

Thorne spin limit ( $a_{*} ≃ 0.998$ )

 0  0

The Thorne spin limit is the equilibrium spin produced by competition between coherent thin-disc matter accretion and capture of disc photons by a Kerr black hole. Preferential capture of counterrotating photon angular momentum prevents the standard radiatively efficient model from reaching