Black Hole and Neutron Star Binary Mergers in Triple Systems: II. Merger Eccentricity and Spin-Orbit Misalignment

Bin Liu; Dong Lai; Yi-Han Wang

doi:10.3847/1538-4357/ab2dfb

paper

Black Hole and Neutron Star Binary Mergers in Triple Systems: II. Merger Eccentricity and Spin-Orbit Misalignment

arXiv:1905.00427 · doi:10.3847/1538-4357/ab2dfb

Abstract

We study the dynamical signatures of black hole (BH) and neutron star (NS) binary mergers via Lidov-Kozai oscillations induced by tertiary companions in hierarchical triple systems. For each type of binaries (BH-BH and BH-NS), we explore a wide range of binary/triple parameters that lead to binary mergers, and determine the distributions of eccentricity ( $e_{m}$ ) and spin-orbit misalignment angle ($θ_\rm {sl}^\rm f$) when the binary enters the LIGO/VIRGO band. We use the double-averaged (over both orbits) and single-averaged (over the inner orbit) secular equations, as well as N-body integration, to evolve systems with different hierarchy levels, including the leading-order post-Newtonian effect, spin-orbit coupling and gravitational radiation. We find that for merging BH-BH binaries with comparable masses, about $7%$ have $e_{m} > 0.1$ and $0.7%$ have $e_{m} > 0.9$ . The majority of the mergers have significant eccentricities in the LISA band. The BH spin evolution and $θ_\rm {sl}^\rm f$ are correlated with the orbital evolution and $e_{m}$ . Mergers with $e_{m} ≲ 1 0^{- 3}$ have a distribution of $θ_\rm {sl}^\rm f$ that peaks around $9 0^{\circ}$ (and thus favoring a projected binary spin parameter $χ_{eff} \sim 0$ ), while mergers with larger $e_{m}$ have a more isotropic spin-orbit misalignments. For typical BH-NS binaries, strong octuple effects lead to more mergers with non-negligible $e_{m}$ (with $\sim 18%$ having $e_{m} > 0.1$ and $2.5%$ having $e_{m} > 0.9$ ), and the final BH spin axis tends to be randomly orientated. Measurements or constraints on eccentric mergers and $θ_\rm {sl}^\rm f$ from LIGO/VIRGO and LISA would provide useful diagnostics on the dynamical formation of merging BH or NS binaries in triples.

arXiv comment13 pages, 15 figures. Published in ApJ

Black Hole and Neutron Star Binary Mergers in Triple Systems: II. Merger Eccentricity and Spin-Orbit Misalignment

Abstract

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