 Kerr black hole

A Kerr black hole is the rotating, stationary, axisymmetric vacuum spacetime described by the Kerr metric. In geometrized units $G=c=1$, let $M>0$ be its mass and $J=Ma$ its angular momentum. Its radial horizon polynomial isA Kerr black hole requires $|a|\le M$; equality is the extremal black hole limit. The Schwarzschild black hole is recovered at $a=0$. The outer Killing horizon rotates with angular velocity $a/(r_{+}^2+a^2)$. The exterior Kerr ergoregion is distinct from that horizon and permits rotational black-hole energy extraction.^[ref]