Eccentric Black Hole Mergers in Dense Star Clusters: The Role of BinaryBinary Encounters
Abstract
We present the first systematic study of strong binarysingle and binarybinary black hole (BH) interactions with the inclusion of general relativity. By including general relativistic effects in the equations of motion during strong encounters, the dissipation of orbital energy from the emission of gravitational waves (GWs) can lead to captures and subsequent inspirals with appreciable eccentricities when entering the sensitive frequency ranges of the LIGO and Virgo GW detectors. It has been shown that binarysingle interactions significantly contribute to the rate of eccentric mergers, but no studies have looked exclusively into the contribution from binarybinary interactions. To this end, we perform binarybinary and binarysingle scattering experiments with general relativistic dynamics up through the 2.5 postNewtonian order included, both in a controlled setting to gauge the importance of nondissipative postNewtonian terms and derive scaling relations for the cross section of GW captures, as well as experiments tuned to the strong interactions from stateofthe art globular cluster (GC) models to assess the relative importance of the binarybinary channel in facilitating GW captures and the resultant eccentricity distributions of inspiral from channel. Although binarybinary interactions are 10100 times less frequent in GCs than binarysingle interactions, their longer lifetime and more complex dynamics leads to a higher probability for GW captures to occur during the encounter. We find that binarybinary interactions contribute 25%45% of the eccentric mergers that occur during strong BH encounters in GCs, regardless of the properties of the cluster environment. The inclusion of higher multiplicity encounters in dense star clusters therefore have major implications on the predicted rates of highly eccentric binaries potentially detectable by the LIGO/Virgo network. Because gravitational waveforms of eccentric inspirals are distinct from those generated by merging binaries that have circularized, measurements of eccentricity in such systems would highly constrain their formation scenario.
 Publication:

The Astrophysical Journal
 Pub Date:
 January 2019
 DOI:
 10.3847/15384357/aaf6ec
 arXiv:
 arXiv:1810.00901
 Bibcode:
 2019ApJ...871...91Z
 Keywords:

 binaries: close;
 black hole physics;
 globular clusters: general;
 gravitational waves;
 methods: numerical;
 stars: kinematics and dynamics;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology
 EPrint:
 18 pages, 6 figures. Published in The Astrophysical Journal