Evidence for a correlation between binary black hole mass ratio and black-hole spins

TL;DR

This study finds strong evidence of an anti-correlation between black hole mass ratio and spin in binary systems, offering insights into their formation processes and evolution mechanisms.

Contribution

It introduces an improved copula-based framework to robustly detect covariance between black hole parameters using gravitational wave data.

Findings

Evidence for anti-correlation with 99.7% credibility

Implications for high common-envelope efficiencies and accretion stages

Potential to explore tidal spin-up and active galactic nuclei properties

Abstract

The astrophysical origins of the binary black hole systems seen with gravitational waves are still not well understood. However, features in the distribution of black-hole masses, spins, redshifts, and eccentricities provide clues into how these systems form. Much has been learned by investigating these distributions one parameter at a time. However, we can extract additional information by studying the covariance between pairs of parameters. Previous work has shown preliminary support for an anti-correlation between mass ratio $q \equiv m_2/m_1$ and effective inspiral spin $\chi_\mathrm{eff}$ in the binary black hole population. In this study, we test for the existence of this anti-correlation using updated data from the third gravitational wave transient catalogue (GWTC-3) and improve our copula-based framework to employ a more robust model for black-hole spins. We find evidence for an anti-correlation in $(q, \chi_\mathrm{eff})$ with 99.7% credibility. This may imply high common-envelope efficiencies, stages of super-Eddington accretion, or a tendency for binary black hole systems to undergo mass-ratio reversal during isolated evolution. Covariance in $(q,\chi_\mathrm{eff})$ may also be used to investigate the physics of tidal spin-up as well as the properties of binary-black-hole-forming active galactic nuclei.