High rate of gravitational waves mergers from flyby perturbations of wide black hole triples in the field

Abstract

ABSTRACT Ultrawide triple black holes (TBHs; with an outer orbit >103 au) in the field can be considerably perturbed by flyby encounters with field stars through the excitation of their outer orbit eccentricities. We study the cumulative effect of such flybys, and show them to be conductive for the production of gravitational-wave (GW) sources. Flyby encounters with TBHs can destabilize them, leading to binary–single resonant encounters between the outer black hole (BH) and the inner binary. These encounters can result in either a prompt GW merger of two of the TBH components during the resonant phase, or the disruption of the TBH. In the latter case, a more compact binary is left behind, while the third BH is ejected. Such compact remnant binaries may still inspiral through GW emission, producing delayed GW mergers, with a significant fraction of these merging in less than a Hubble time. We find a volumetric merger rate of ∼3–10 Gpc−3 yr−1 contributed by the (former) prompt-merger TBH channel and ${\sim} 100\!-\!250\,{\rm {\rm Gpc^{-3}\,yr^{-1}}}$ contributed by the (latter) delayed-merger TBH channel. The prompt channel gives rise to eccentric mergers in the aLIGO band, while the majority of the delayed GW mergers are circularized when enter the aLIGO band. We find the total eccentric volumetric merger rate to be ∼1–10 Gpc−3 yr−1 from both channels. We expect these mergers to show no significant spin–orbit alignment, and uniform delay-time distribution.