Null geodesics in extremal Kerr-Newman black holes

TL;DR

This paper analyzes null geodesics in extremal Kerr-Newman black holes, revealing unique stable light orbits, deriving analytical solutions, and exploring how extremality influences light deflection and orbit structure.

Contribution

It provides the first detailed analysis of null geodesics in extremal Kerr-Newman black holes, including analytical expressions and the effects of extremality on light ring structures.

Findings

Existence of stable spherical light orbits at the horizon.

Analytical solutions for light orbits reaching infinity.

Enhanced light deflection divergence near extremal horizons.

Abstract

We study the null geodesics in the extremal Kerr-Newman exterior. We clarify the roots of the radial potential and obtain the parameter space of the azimuthal angular momentum and the Carter constant of the light rays for varieties of the orbits. It is known that one of the unique features of extremal black holes for the null geodesics is the existence of the stable double root at the horizon, giving rise to the stable spherical motion. For the black hole's spin $a<M/2$, the stable double root is isolated from the unstable one. However, for $ a\ge M/2$, the unstable and stable double roots merge at the triple root so that the unstable double root in some parameter region can lie at the horizon, giving a very different shape to the light ring. We then find the analytical expressions of light orbits, which can reach spatial infinity for both nonequatorial and equatorial motions. In particular, for the orbits starting from the near horizon of the extremal Kerr-Newman black holes with the parameters for the unstable double and triple roots, the solutions are remarkably simple in terms of elementary functions. It is also found that the analytical solutions of the equatorial motion can shed light on the deflection of the light by black holes. Varying the azimuthal angular momentum, as either the double or triple root at the horizon is approached from the turning point, the stronger power-law divergence in the deflection angle is found in comparison with the typical logarithmic divergence in nonextremal black holes in the strong deflection limit. This could be another interesting effect of light deflection by extremal black holes.