Kavli Affiliate: Salvatore Vitale

| First 5 Authors: Thomas A. Callister, Carl-Johan Haster, Ken K. Y. Ng, Salvatore Vitale, Will M. Farr

| Summary:

Hierarchical analysis of the binary black hole (BBH) detections by the

Advanced LIGO and Virgo detectors has offered an increasingly clear picture of

their mass, spin, and redshift distributions. Fully understanding the formation

and evolution of BBH mergers will require not just the characterization of

these marginal distributions, though, but the discovery of any correlations

that exist between the properties of BBHs. Here, we hierarchically analyze the

ensemble of BBHs discovered by the LIGO and Virgo with a model that allows for

intrinsic correlations between their mass ratios $q$ and effective inspiral

spins $chi_mathrm{eff}$. At $98.7%$ credibility, we find that the mean of

the $chi_mathrm{eff}$ distribution varies as a function of $q$, such that

more unequal-mass BBHs exhibit systematically larger $chi_mathrm{eff}$. We

find Bayesian odds ratio of $10.5$ in favor of a model that allows for such a

correlation over one that does not. Finally, we use simulated signals to verify

that our results are robust against degeneracies in the measurements of $q$ and

$chi_mathrm{eff}$ for individual events. While many proposed astrophysical

formation channels predict some degree correlation between spins and mass

ratio, these predicted correlations typically act in an opposite sense to the

trend we observationally identify in the data.

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