Binary Vision: The Mass Distribution of Merging Binary Black Holes via Iterative Density Estimation

Abstract

Binary black hole (BBH) systems detected via gravitational-wave emission are a recently opened astrophysicalfrontier with many unknowns and uncertainties. Accurate reconstruction of the binary distribution with as fewassumptions as possible is desirable for inference of formation channels and environments. Most populationanalyses have, though, assumed a power law in binary mass ratio q, and/or assumed a universal q distributionregardless of primary mass. Methods based on kernel density estimation allow us to dispense with suchassumptions and directly estimate the joint binary mass distribution. We deploy a self-consistent iterative methodto estimate this full BBH mass distribution, finding local maxima in primary mass consistent with previousinvestigations and a secondary mass distribution with a partly independent structure, inconsistent both with apower law and with a constant function of q. We find a weaker preference for near-equal-mass binaries than inmost previous investigations; instead, the secondary mass has its own “spectral lines” at slightly lower values thanthe primary, and we observe an anticorrelation between primary and secondary masses around the ∼10 Me peak.