**Authors**: Michael Kesden, Guglielmo Lockhart, E. Sterl Phinney

**Date**: 4 May 2010

**Abstract**: Black holes of mass M must have a spin angular momentum S below the Kerr limit chi = S/Mˆ2 < 1, but whether astrophysical black holes can attain this limiting spin depends on their accretion history. Gas accretion from a thin disk limits the black-hole spin to chi_gas < 0.9980 +- 0.0002, as electromagnetic radiation from this disk with retrograde angular momentum is preferentially absorbed by the black hole. Extrapolation of numerical-relativity simulations of equal-mass binary black-hole mergers to maximum initial spins suggests these mergers yield a maximum spin chi_eq < 0.95. Here we show that for smaller mass ratios q = m/M << 1, the superradiant extraction of angular momentum from the larger black hole imposes a fundamental limit chi_lim < 0.9979 +- 0.0001 on the final black-hole spin even in the test-particle limit q -> 0 of binary black-hole mergers. The nearly equal values of chi_gas and chi_lim imply that measurement of supermassive black-hole spins cannot distinguish a black hole built by gas accretion from one assembled by the gravitational inspiral of a disk of compact stellar remnants. We also show how superradiant scattering alters the mass and spin predicted by models derived from extrapolating test-particle mergers to finite mass ratios.

1005.0627
(/preprints)

2010-05-06, 17:05
**[edit]**

© M. Vallisneri 2012 — last modified on 2010/01/29

*Tantum in modicis, quantum in maximis*