Degeneracy between mass and spin in black-hole-binary waveforms

TL;DR

This paper investigates the persistent degeneracy between mass and spin in black-hole binary gravitational waveforms across a broad mass range, impacting detection and parameter estimation in gravitational wave astronomy.

Contribution

It demonstrates that the mass-spin degeneracy persists beyond the inspiral phase into merger and ringdown, affecting parameter extraction in black-hole binary detections.

Findings

Degeneracy persists across a broad mass range.

At low masses, degeneracy is between mass ratio and total spin.

At higher masses, the degeneracy is less characterized by chirp mass.

Abstract

We explore the degeneracy between mass and spin in gravitational waveforms emitted by black-hole binary coalescences. We focus on spin-aligned waveforms and obtain our results using phenomenological models that were tuned to numerical-relativity simulations. A degeneracy is known for low-mass binaries (particularly neutron-star binaries), where gravitational-wave detectors are sensitive to only the inspiral phase, and the waveform can be modelled by post-Newtonian theory. Here, we consider black-hole binaries, where detectors will also be sensitive to the merger and ringdown, and demonstrate that the degeneracy persists across a broad mass range. At low masses, the degeneracy is between mass ratio and total spin, with chirp mass accurately determined. At higher masses, the degeneracy persists but is not so clearly characterised by constant chirp mass as the merger and ringdown become more significant. We consider the importance of this degeneracy both for performing searches (including searches where only non-spinning templates are used) and in parameter extraction from observed systems. We compare observational capabilities between the early (~2015) and final (2018 onwards) versions of the Advanced LIGO detector.