A new spin on LIGO-Virgo binary black holes

TL;DR

This paper introduces a novel approach to measure the spins of black holes in binary systems by focusing on the objects with the highest and lowest spins, improving spin estimates especially for similar-mass binaries.

Contribution

It proposes a new parameterization for black hole spins that enhances measurement accuracy and applies it to LIGO-Virgo data, revealing significant spin features previously obscured.

Findings

The highest-spinning black hole is often constrained to have nonzero spin.

Most sources show the highest-spinning object near the Kerr limit.

Spin alignment with orbital angular momentum is excluded for most detected events.

Abstract

Gravitational waves from binary black holes have the potential to yield information on both of the intrinsic parameters that characterize the compact objects: their masses and spins. While the component masses are usually resolvable, the component spins have proven difficult to measure. This limitation stems in great part from our choice to inquire about the spins of the most and least massive objects in each binary, a question that becomes ill-defined when the masses are equal. In this paper we show that one can ask a different question of the data: what are the spins of the objects with the highest and lowest dimensionless spins in the binary? We show that this can significantly improve estimates of the individual spins, especially for binary systems with comparable masses. When applying this parameterization to the first 13 gravitational-wave events detected by the LIGO-Virgo collaboration (LVC), we find that the highest-spinning object is constrained to have nonzero spin for most sources and to have significant support at the Kerr limit for GW151226 and GW170729. A joint analysis of all the confident binary black hole detections by the LVC finds that, unlike with the traditional parametrization, the distribution of spin magnitude for the highest-spinning object has negligible support at zero spin. Regardless of the parameterization used, the configuration where all of the spins in the population are aligned with the orbital angular momentum is excluded from the 90% credible interval for the first ten events and from the 99% credible interval for all current confident detections.