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arXiv · 2507.19582

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

Manuel O. Mariano·Carlos A. R. Herdeiro
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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.

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Manuel O. Mariano, Carlos A. R. Herdeiro. 2026-01-12. There and back again: Outspiraling motion in non-Kerr compact objects. https://doi.org/10.1103/9713-l7bx

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