10 Summary

No completely convincing case of a bound, binary SBH has yet been discovered. “Dual” SBHs, i.e. two, widely-separated SBHs in a single system, have been seen in some interacting galaxies and binary quasars. However none has a projected separation less than $~ 1kpc$ . Strong limits can be placed on the binarity of the Milky Way SBH.
The evolution of binary SBHs in gas-poor galaxies is dominated by the gravitational slingshot ejection of stars that pass near the binary, carrying away energy and angular momentum. Once the binary has ejected all stars on intersecting orbits, continued hardening depends on a refilling of the binary’s “loss cone”. Possible mechanisms for loss-cone refilling include star-star gravitational scattering, chaotic orbits in non-axisymmetric potentials, and perturbations from additional massive objects.
Most $N$ -body simulations of binary evolution have been based on such small particle numbers that the binary’s loss cone was refilled at a spuriously high rate by gravitational encounters, Brownian motion of the binary and other finite- $N$ effects. As a result, many results from these studies can not usefully be extrapolated to the large- $N$ regime; in particular, they do not make useful predictions about the binary hardening rates expected in real galaxies.
$N$ -body studies do appear to make robust predictions about the “mass deficit”, the mass in stars ejected from a galactic nucleus by an evolving binary. Observed mass deficits are of order 1-2 times the SBH mass, consistent with $N$ -body predictions.
Studies of the interaction of binary SBHs with gas are in their infancy. Major uncertainties are the amount, distribution and thermodynamic state of gas very near the centers of galaxies containing massive binaries.
Binary coalescence can have a large influence on SBH spins; even mass ratios as extreme as 10:1 can substantially spin up or re-orient a SBH. Evidence for spin-flips may have been observed in the so-called X-shaped radio sources.