= Horizon angular velocity
{title2=$\chi=k+\Omega_Hm$}
In an axisymmetric stationary spacetime, the <horizon angular velocity> is the constant coefficient in the null horizon generator $\chi=k+\Omega_Hm$, where $k$ is the stationary Killing vector normalized at infinity and $m$ generates axial orbits with period $2\pi$. It measures rotation relative to the stationary frame at infinity. For a <Kerr black hole>, $\Omega_H=a/(r_+^2+a^2)$. It enters the rotating physical-process first law through the horizon flux of $E-\Omega_HJ$.
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