A note on circular geodesics in the equatorial plane of an extreme Kerr-Newman black hole

Abstract

We examine the behaviour of circular geodesics describing orbits of neutral test particles around an extreme Kerr-Newman black hole. It is well known that the radial Boyer-Lindquist coordinates of the prograde photon orbit $r=r_{\rm ph}$, marginally bound orbit $r=r_{\rm mb}$ and innermost stable orbit $r=r_{\rm ms}$ of the extreme Kerr black hole all coincide with the event horizon's value $r=r_+$. We find that for the extreme Kerr-Newman black hole with mass $M$, angular momentum $J$ and electric charge $Q=\pm\sqrt{M^2-J^2/M^2}$ ($|J|\le M^2$) the coordinate equalities $r_{\rm ph}=r_+$, $r_{\rm mb}=r_+$ and $r_{\rm ms}=r_+$ hold if and only if $|J|$ is greater than or equal to $M^2/2$, $M^2/\sqrt{3}$ and $M^2/\sqrt{2}$, respectively.