The last orbit of binary black holes

TL;DR

This paper models the last orbit and merger of equal-mass binary black holes using a new numerical technique, revealing detailed gravitational waveforms and energy radiation during the final stages of coalescence.

Contribution

It introduces a new fully numerical evolution method for binary black holes without excision, providing detailed insights into the last orbit and merger process.

Findings

Binary completes about 1.33 orbits before merger.

Binary radiates 3.2% of its mass and 24% of angular momentum.

Waveform transitions from oscillatory to plunge and quasinormal ringing.

Abstract

We have used our new technique for fully numerical evolutions of orbiting black-hole binaries without excision to model the last orbit and merger of an equal-mass black-hole system. We track the trajectories of the individual apparent horizons and find that the binary completed approximately one and a third orbits before forming a common horizon. Upon calculating the complete gravitational radiation waveform, horizon mass, and spin, we find that the binary radiated 3.2% of its mass and 24% of its angular momentum. The early part of the waveform, after a relatively short initial burst of spurious radiation, is oscillatory with increasing amplitude and frequency, as expected from orbital motion. The waveform then transitions to a typical `plunge' waveform; i.e. a rapid rise in amplitude followed by quasinormal ringing. The plunge part of the waveform is remarkably similar to the waveform from the previously studied `ISCO' configuration. We anticipate that the plunge waveform, when starting from quasicircular orbits, has a generic shape that is essentially independent of the initial separation of the binary.