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

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 $q\equiv m_2\mathrm{/}{m}_1$ and effective inspiral spin ${\chi }_{\mathrm{e}\mathrm{f}\mathrm{f}}$ 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 $(q,{\chi }_{\mathrm{e}\mathrm{f}\mathrm{f}})$ 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 $(q,{\chi }_{\mathrm{e}\mathrm{f}\mathrm{f}})$ 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.