The modification of photon trapping orbits as a diagnostic of non-Kerr spacetimes

TL;DR

This paper investigates how modifications to photon trapping orbits can serve as indicators of non-Kerr spacetimes, revealing that deviations from Kerr metrics alter the existence and nature of spherical photon orbits.

Contribution

It derives a general condition for the existence of spherical orbits and analyzes how non-Kerr metrics modify photon trapping orbits, linking these changes to metric separability.

Findings

Separable Kerr-like spacetimes support spherical photon orbits.

Non-separable, non-Kerr spacetimes lack spherical orbits, replacing them with spheroidal ones.

Deviations from Kerr lead to the disappearance of low-inclination spheroidal orbits.

Abstract

Photon circular orbits, an extreme case of light deflection, are among the hallmarks of black holes and are known to play a central role in a variety of phenomena related to these extreme objects. The very existence of such orbits when motion is not confined in the equatorial plane, i.e. spherical orbits, is indeed a special property of the separable Kerr metric and may not occur, for instance, in the spacetime of other more speculative ultracompact objects. In this paper we consider a general stationary-axisymmetric spacetime and examine under what circumstances spherical or more general, variable-radius, `spheroidal' non-equatorial photon orbits may exist with the ultimate goal of using the modifications -- or even loss -- of photon trapping orbits as a telltale of non-Kerr physics. In addressing this issue, we first derive a general necessary condition for the existence of spherical/spheroidal orbits and then go on to study photon trapping orbits in a variety of known non-Kerr metrics (Johannsen, Johanssen-Psaltis, and Hartle-Thorne). The first of these is an example of a separable spacetime which supports Kerr-like spherical photon orbits. A more detailed analysis reveals a deeper connection between the presence of spherical orbits and the separability of a metric (that is, the existence of a third integral of motion). Specifically, a spacetime that does not admit spherical orbits in any coordinates is necessarily non-separable. The other two spacetimes considered here exhibit a clear non-Kerr behaviour by having spherical photon orbits replaced by spheroidal ones. More importantly, subject to the degree of deviation from Kerr, equatorial photon rings give place to non-equatorial ones with an accompanying loss of low-inclination spheroidal orbits. The implications of these results for the electromagnetic and gravitational wave signature of non-Kerr objects are briefly discussed.