Can we measure individual black-hole spins from gravitational-wave observations?

TL;DR

This paper investigates whether gravitational-wave observations can accurately measure individual black-hole spins, concluding that typically only the larger black hole's spin is measurable with current detectors, due to degeneracies.

Contribution

The study analyzes the potential to measure individual black-hole spins across different binary configurations and finds limitations with current detector sensitivities.

Findings

The larger black hole's spin is generally measurable.

Degeneracies limit the measurement of the smaller black hole's spin.

Waveform models with a single effective spin are often sufficient.

Abstract

Measurements of black-hole spins from gravitational-wave observations of black-hole binaries with ground-based detectors are expected to be hampered by partial degeneracies in the gravitational-wave phasing: between the two component spins, and between the spins and the binary's mass ratio, at least for signals that are dominated by the binary's inspiral. Through the merger and ringdown, however, a different set of degeneracies apply. This suggests the possibility that, if the inspiral, merger and ringdown are all within the sensitive frequency band of a detector, we may be able to break these degeneracies and more accurately measure both spins. In this work we investigate our ability to measure individual spins for non-precessing binaries, for a range of configurations and signal strengths, and conclude that in general the spin of the larger black hole will be measurable (at best) with observations from Advanced LIGO and Virgo. This implies that in many applications waveform models parameterized by only one \emph{effective spin} will be sufficient. Our work does not consider precessing binaries or sub-dominant harmonics, although we provide some arguments why we expect that these will not qualitatively change our conclusions.