Picky Partners: The Pairing of Component Masses in Binary Black Hole Mergers

Maya Fishbach, Daniel E. Holz

Published 2019-05-29Version 1

We examine the relationship between individual black hole (BH) masses in merging binary black hole (BBH) systems. Analyzing the ten BBH detections from LIGO/Virgo's first two observing runs, we find that the masses of the black holes comprising each binary are unlikely to be randomly drawn from the same underlying distribution. Instead, we find that the two BHs prefer to be of comparable mass. We show that it is $\sim 7$ times more likely that the component BHs in a given binary are always equal (to within 5\%) than that they are randomly paired. If we insist that component BHs are randomly drawn from the same underlying power-law distribution with slope $\gamma$, we find $\gamma = -0.8^{+1.0}_{-1.0}$ (median and 90\% credible interval). However, it is more likely that the probability of a merger between two BHs depends on their mass ratio, $q \leq 1$. If we assume a scaling of $q^\beta$, so that $\beta=0$ corresponds to random pairings, we find $\beta>0$ is favored at credibility $0.994$. If we additionally introduce a minimum mass ratio threshold, $q_\mathrm{min} < q < 1$, we find $\beta = 5.0^{+6.2}_{-7.5}$, $q_\mathrm{min} = 0.6^{+0.3}_{-0.4}$, and $\gamma = -1.4^{+0.9}_{-0.8}$. This implies that only 1\% of merging binaries have mass ratios less than $q_{1\%} = 0.66^{+0.24}_{-0.27}$, compared to $q_{1\%} = 0.17^{+0.07}_{-0.06}$ if the pairing is done at random. We conclude that merging black holes do not form random pairings; instead they are selective about their partners, preferring to mate with black holes of a similar mass. The details of these selective pairings provide insight into the underlying formation channels of merging binary black holes.