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

Abstract

Ultra-wide triple black-holes (TBHs; with an outer orbit $>10^3$ AU) in the field can be considerably perturbed by flyby encounters with field stars by the excitation of the 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 turn the TBHs unstable and follow chaotic evolution. This leads to a binary-single resonant encounter between the outer 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 escapes and is ejected. The compact remnant binary may still inspiral through GW-emission, although on longer timescales. A significant number of these would lead to a delayed GW-merger in less than a Hubble time. We find a volumetric merger rate of $\sim 3-10\mathrm{Gp}{\mathrm{c}}^{-3}\mathrm{y}{\mathrm{r}}^{-1}$ contributed by the (former) prompt-merger TBH channel and $\sim 100-250\mathrm{Gp}{\mathrm{c}}^{-3}\mathrm{y}{\mathrm{r}}^{-1}$ 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 {\rm eccentric} volumetric merger rate to be $\sim 1-10\mathrm{Gp}{\mathrm{c}}^{-3}\mathrm{y}{\mathrm{r}}^{-1}$ from both channels. We expect these mergers to show no significant spin-orbit alignment, and uniform delay time distribution.

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