Ordering the chaos: stellar black hole mergers from non-hierarchical triples

Manuel Arca-Sedda, Gongjie Li, Bence Kocsis

Published 2018-05-16Version 1

We investigate the evolution of triple, non-hierarchical, black hole (BH) systems making use of 29,000 3-body simulations. Varying the mutual orbital inclination, the three BH masses and the inner and outer eccentricities, we show that retrograde, nearly planar configurations leads to a significant shrinkage of the inner binary. We found an universal trend of triple systems, that they tend to evolve toward prograde configurations, independently on the initial conditions. Moreover, we demonstrate that the orbital flip, driven by the torque exerted from the outer BH on the inner binary (BHB), leads in general to tighter inner orbits. In some cases, the resulting BHB undergoes coalescence within a Hubble time, releasing gravitational waves (GWs). Frequently, the inner BHB merger occurs after a component swap between one of its components and the outer BH. The mass spectrum of the BHBs that underwent the component exchange differs significantly from the case in which the BHB merge without any swap. A large fraction of merging BHBs with initial separation 1 AU enter the 0.001-0.1 Hz frequency band with large eccentricities, possibly being detected by the next generation of detectors. Mergers originating from initially tighter BHB (a ~ 0.01 AU), instead, have a large probability to have eccentricities above 0.7 in the 1 Hz band. Intriguingly, we found that the chirp mass of at least 3 of the GW sources detected by LIGO (GW150914, GW170814 and GW170104) lie in the most probable range of chirp masses for coalescing BHBs formed in a non-hierarchical triple.