There and back again: Outspiraling motion in non-Kerr compact objects

TL;DR

This paper investigates the possibility of outspiraling motion in non-Kerr compact objects, revealing conditions where inspiral can be followed by outspiral, which could serve as a signature of exotic spacetime features in gravitational wave signals.

Contribution

It derives conditions for outspiraling orbits in non-Kerr spacetimes and explores their gravitational wave signatures, providing a new potential observational test for exotic compact objects.

Findings

Outspiral motion can occur in certain non-Kerr spacetimes.

Conditions for outspiral depend on specific spacetime features.

Outspiral signatures could be detected in gravitational wave data.

Abstract

In Keplerian dynamics, a test body orbiting a point particle in circular motion has a monotonically increasing frequency, with decreasing radius. If a dissipative channel is introduced, such as gravitational wave (GW) emission, under the quadrupole approximation, the corresponding GW strain has an ever-increasing frequency with time. A similar statement holds for equatorial motion of a test particle on the Kerr manifold, except such inspiral is cut off at the ISCO, wherein stable circular orbits cease to exist and a plunge is expected. We analyze circular timelike orbits in generic spinning spacetimes and study the conditions in which exotic motion can occur, due to the presence of non-Kerr features. In particular, we derive conditions under which an inspiral towards a compact object is naturally followed by an outspiral motion, and give concrete examples as well as the corresponding GW phenomenology. This analysis serves both as a theoretical exploration of non-Kerrness as well as an example of a concrete smoking gun of exotic spacetimes.