The Emergence of Structure in the Binary Black Hole Mass Distribution

Abstract

We use the gravitational wave signals from binary black hole merger events observed by LIGO and Virgo to reconstruct the underlying mass and spin distributions of the population of merging black holes. We reconstruct the population using the mixture model framework VAMANA (Tiwari 2020) using observations in GWTC-2 occurring during the first two observing runs and the first half of the third run (O1, O2, and O3a). Our analysis identifies a structure in the chirp mass distribution of the observed population. Specifically, we identify peaks in the chirp mass distribution at 8, 14, 26, and 45M and a complementary structure in the component mass distribution with an excess of black holes at masses of 9, 16, 30, and 57M. Intriguingly, the location of subsequent peaks are separated by a factor of around two and there is a lack of mergers with chirp masses of 10--12M. We speculate that these features could be footprints of the hierarchical merger scenario. In simplest terms, these features can be explained by a mass-gap near 13M causing black-holes pile-up near the first peak combined with the scenario in which lower mass black-holes hierarchically merge to produce higher mass black-holes. However, if we accept this scenario we have to attribute the existence of a mass gap, lack of cross-generation merger peaks, and lack of high spins in most of the observations to unknown physics. Currently, the results are limited in measurement accuracy due to small numbers of observations, but if confirmed by the aid of future gravitational wave observations these features could have far-reaching implications

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