Black hole-neutron star mergers from triples

Abstract

Mergers of black hole (BH) and neutron star (NS) binaries are expected to be observed by gravitational wave observatories in the coming years. Until now, LIGO has only set an upper limit on this merger rate. BH-NS binaries are expected to merge in isolation, as their formation is suppressed in star clusters by the mass segregation and the strong heating by BHs in the cluster core. Another viable scenario to BH-NS mergers is in triple systems. In this paper, we carry out a systematic statistical study of the dynamical evolution of triples comprised of an inner BH-NS binary by means of high-precision $N$-body simulations, including Post-Newtonian (PN) terms up to 2.5PN order. We start from the main sequence massive stars and model the supernovae (SN) events that lead to the formation of BHs and NSs. We adopt different prescriptions for the natal velocity kicks imparted during the SN processes and illustrate that large kicks lead to more compact and massive triples that merge on shorter timescales. We also show that BH-NS merging in triples have a significant eccentricity in the LIGO band, typically much larger than BH-NS merging in isolated binaries. Finally, we estimate a rate of ${\Gamma }_{\mathrm{BH}-\mathrm{NS}}\approx 1.0\times 10^{-3}-3.5\times 10^{-2}{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$, for non-zero velocity kicks, and ${\Gamma }_{\mathrm{BH}-\mathrm{NS}}=19{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$, for no natal kicks. Our rate estimate overlaps with the expected BH-NS rate in isolated binaries and within the LIGO upper limit.

8 pages, 5 figures, 1 table, accepted by MNRAS

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• Black hole mergers from quadruples
• On the formation of GW190814
• Modelling Neutron Star-Black Hole Binaries: Future Pulsar Surveys and Gravitational Wave Detectors
• Multiple Stellar Evolution: a population synthesis algorithm to model the stellar, binary, and dynamical evolution of multiple-star systems
• Effective spin distribution of black hole mergers in triples
• Was GW190412 born from a hierarchical 3+1 quadruple configuration?
• Algebraic and machine learning approach to hierarchical triple-star stability
• Demographics of neutron stars in young massive and open clusters
• Massive binary star mergers in galactic nuclei: implications for blue stragglers, binary S-stars and gravitational waves
• Properties of von Zeipel-Lidov-Kozai oscillations in triple systems at the quadrupole order: relaxing the test particle approximation
• Impact of natal kicks on merger rates and spin-orbit misalignments of black hole -- neutron star mergers
• Companion-driven evolution of massive stellar binaries
• Secular dynamics of hierarchical multiple systems composed of nested binaries, with an arbitrary number of bodies and arbitrary hierarchical structure. III. Suborbital effects: hybrid integration techniques and orbit-averaging corrections
• Amplification of superkicks in black-hole binaries through orbital eccentricity
• Constraining neutron star radii in black hole-neutron star mergers from their electromagnetic counterparts
• Unexpected LIGO events and the Mirror World
• Formation of the First Two Black Hole-Neutron Star Mergers (GW200115 and GW200105) from Isolated Binary Evolution
• Black Holes LIGO/Virgo Domination and Single-lined Binaries with a Black Hole Candidate Component
• Host galaxies and electromagnetic counterparts to binary neutron star mergers across the cosmic time: Detectability of GW170817-like events