What GW170729's Exceptional Mass and Spin Tells Us about Its Family Tree

Abstract

Gravitational-wave observations give a unique insight into the formation and evolution of binary black holes. We use gravitational-wave measurements to address the question of whether GW170729's source, which is (probably) the most massive binary and the system with the highest effective inspiral spin, could contain a black hole which is a previous merger remnant. Using the inferred mass and spin of the system, and the empirically determined population of binary black holes, we compute the evidence for the binary being second-generation compared with first-generation. We find moderate evidence (a Bayes factor of ~6-7) that the mass and spin better match a second-generation merger, but folding in the expectation that only a small fraction of mergers are second-generation, we conclude that there is no strong evidence that GW170729 was the result of a second-generation merger. The results are sensitive to the assumed mass distribution, and future detections will provide more robust reconstructions of the binary black hole population.