Resonances in Extreme Mass-Ratio Inspirals: Asymptotic and Hyperasymptotic Analysis

TL;DR

This paper analyzes the effects of resonances in extreme mass-ratio inspirals on gravitational wave signals, using asymptotic and hyperasymptotic methods to understand the late-time behavior of the system during resonance passage.

Contribution

It introduces a detailed asymptotic analysis of resonance effects in inspirals, distinguishing between weak and strong resonances and their impact on gravitational wave phasing.

Findings

Weak resonances cause a resonant memory in frequency evolution.

Strong resonances lead to linearly decaying solutions.

Transition between resonance types exhibits a square-root singularity.

Abstract

An expected source of gravitational waves for future detectors in space are the inspirals of small compact objects into much more massive black holes. These sources have the potential to provide a wealth of information about astronomy and fundamental physics. On short timescales the orbit of the small object is approximately geodesic. Generic geodesics for a Kerr black hole spacetime have a complete set of integrals and can be characterized by three frequencies of the motion. Over the course of an inspiral, a typical system will pass through resonances where two of these frequencies become commensurate. The effect of the resonance will be to alter significantly the rate of inspiral for the duration of the resonance. Understanding the impact of these resonances on gravitational wave phasing is important to detect and exploit these signals for astrophysics and fundamental physics. Two differential equations that might describe the passage of an inspiral through such a resonance are investigated. These differ depending on whether it is the phase or the frequency components of a Fourier expansion of the motion that are taken to be continuous through the resonance. Asymptotic and hyperasymptotic analysis are used to find the late-time analytic behavior of the solution for a system that has passed through a resonance. Linearly growing (weak resonances) or linearly decaying (strong resonances) solutions are found depending on the strength of the resonance. In the weak-resonance case, frequency resonances leave an imprint (a resonant memory) on the gravitational frequency evolution. The transition between weak and strong resonances is characterized by a square-root singularity, and as one approaches this transition from above, the solutions to the frequency resonance equation bunch up into families exponentially fast.