Universal Waveforms for Extreme Mass-Ratio Inspiral

TL;DR

This paper develops a universal method for calculating gravitational waveforms from extreme mass-ratio binary black hole mergers, accurately connecting inspiral and post-ISCO phases with minimal phase error.

Contribution

It introduces a universal waveform model that simplifies calculations by re-scaling, applicable to any binary merger in the extreme mass-ratio limit.

Findings

Waveforms accurately connect inspiral and post-ISCO phases.

Phase error remains below one radian for small mass ratios.

Model matches existing results from near the ISCO to merger.

Abstract

We engage with the challenge of calculating the waveforms of gravitational waves emitted by spinless binary black hole merger in extreme mass-ratio limit. We model the stellar-mass black hole as a test-particle, initially on a circular orbit, that undergoes adiabatic inspiral until it reaches the innermost stable circular orbit (ISCO), after which it follows a geodesic trajectory. We compute the gravitational waveforms emitted during both phases -- before and after the ISCO crossing -- and demonstrate how to accurately connect them. While the waveforms are calculated adiabatically up to the ISCO, the associated phase error near the ISCO scales as $\nu^{1/5}$ and remains below one radian for sufficiently small mass-ratios $\nu$. Our complete waveform is universal in the sense that all computationally expensive calculations are performed once, and its application to any binary merger can be obtained by appropriately re-scaling time, phase, and amplitude. We compare our results with existing models in the literature and show that our complete waveforms are accurate enough all the way from separations that are an order of one gravitational radii outside the ISCO, to the merger.