Geodesics for the Painleve-Gullstrand form of Lense-Thirring spacetime

TL;DR

This paper studies geodesics in a novel Painleve-Gullstrand form of the Lense-Thirring spacetime, revealing integrability features, special cases with explicit solutions, and potential astrophysical applications as a black hole mimic.

Contribution

It introduces and analyzes the geodesic structure of a new Painleve-Gullstrand Lense-Thirring spacetime variant with unique geometric properties.

Findings

Geodesic equations involve ultra-elliptic integrals.

Complete integrability in special cases with elementary functions.

Potential astrophysical relevance as a black hole mimic.

Abstract

Recently, the current authors have formulated and extensively explored a rather novel Painleve-Gullstrand variant of the slow-rotation Lense-Thirring spacetime, a variant which has particularly elegant features -- including unit lapse, intrinsically flat spatial 3-slices, and a separable Klein-Gordon equation (wave operator). This spacetime also possesses a non-trivial Killing tensor, implying separability of the Hamilton-Jacobi equation, the existence of a Carter constant, and complete formal integrability of the the geodesic equations. Herein we investigate the geodesics in some detail, in the general situation demonstrating the occurrence of "ultra-elliptic" integrals. Only in certain special cases can the complete geodesic integrability be explicitly cast in terms of elementary functions. The model is potentially of astrophysical interest both in the asymptotic large-distance limit and as an example of a "black hole mimic", a controlled deformation of the Kerr spacetime that can be contrasted with ongoing astronomical observations.