Spinning black holes with a separable Hamilton-Jacobi equation from a modified Newman-Janis algorithm

TL;DR

This paper demonstrates that rotating black hole spacetimes derived via a modified Newman-Janis algorithm always allow for the separability of the Hamilton-Jacobi equation for null geodesics, simplifying analysis of light propagation.

Contribution

It shows that the spacetimes from the MNJA admit Hamilton-Jacobi separability for null geodesics, regardless of ambiguity, and extends this to light in plasma under specific conditions.

Findings

Hamilton-Jacobi equation is separable for null geodesics in MNJA spacetimes.

Separability of light propagation in plasma depends on plasma frequency constraints.

Computed black hole shadows and lensing for MNJA-derived black holes.

Abstract

Obtaining solutions of the Einstein field equations describing spinning compact bodies is typically challenging. The Newman-Janis algorithm provides a procedure to obtain rotating spacetimes from a static, spherically symmetric, seed metric. It is not guaranteed, however, that the resulting rotating spacetime solves the same field equations as the seed. Moreover, the former may not be circular, and thus expressible in Boyer-Lindquist-like coordinates. Amongst the variations of the original procedure, a modified Newman-Janis algorithm (MNJA) has been proposed that, by construction, originates a circular, spinning spacetime, expressible in Boyer-Lindquist-like coordinates. As a down side, the procedure introduces an ambiguity, that requires extra assumptions on the matter content of the model. In this paper we observe that the rotating spacetimes obtained through the MNJA always admit separability of the Hamilton-Jacobi equation for the case of null geodesics, in which case, moreover, the aforementioned ambiguity has no impact, since it amounts to an overall metric conformal factor. We also show that the Hamilton-Jacobi equation for light rays propagating in a plasma admits separability if the plasma frequency obeys a certain constraint. As an illustration, we compute the shadow and lensing of some spinning black holes obtained by the MNJA.