Measuring binary black hole orbital-plane spin orientations

TL;DR

This paper proposes a new method for measuring the orbital-plane spin orientations of binary black holes near merger, significantly improving measurement accuracy for current and future gravitational wave detections.

Contribution

It introduces measuring spins at a fixed time or frequency near merger, enhancing the accuracy of orbital-plane spin angle measurements over traditional reference-stage methods.

Findings

Improved measurement of $oldsymbol{\phi_1}$ and $oldsymbol{\phi_2}$ for several GW events.

Better measurement of $oldsymbol{\Delta oldsymbol{\phi}}$ near merger for louder signals.

Numerical relativity surrogate models are essential for reliable measurements at moderate SNRs.

Abstract

Binary black hole spins are among the key observables for gravitational wave astronomy. Among the spin parameters, their orientations within the orbital plane, $\phi_1$, $\phi_2$ and $\Delta \phi=\phi_1-\phi_2$, are critical for understanding the prevalence of the spin-orbit resonances and merger recoils in binary black holes. Unfortunately, these angles are particularly hard to measure using current detectors, LIGO and Virgo. Because the spin directions are not constant for precessing binaries, the traditional approach is to measure the spin components at some reference stage in the waveform evolution, typically the point at which the frequency of the detected signal reaches 20 Hz. However, we find that this is a poor choice for the orbital-plane spin angle measurements. Instead, we propose measuring the spins at a fixed dimensionless time or frequency near the merger. This leads to significantly improved measurements for $\phi_1$ and $\phi_2$ for several gravitational wave events. Furthermore, using numerical relativity injections, we demonstrate that $\Delta \phi$ will also be better measured near the merger for louder signals expected in the future. Finally, we show that numerical relativity surrogate models are key for reliably measuring the orbital-plane spin orientations, even at moderate signal-to-noise ratios like $\sim 30-45$.