  We use full three-body scattering experiments to study the ejection of
hypervelocity stars (HVSs) by massive black hole binaries (MBHBs) at the center
of galaxies. Ambient stars drawn from a Maxwellian distribution unbound to the
binary are expelled by the gravitational slingshot. Accurate measurements of
thermally averaged hardening, mass ejection, and eccentricity growth rates (H,
J, and K) for MBHBs in a fixed stellar background are obtained by numerical
orbit integration from initial conditions determined by Monte Carlo techniques.
Three-body interactions create a subpopulation of HVSs on nearly radial,
corotating orbits, with a spatial distribution that is initially highly
flattened in the inspiral plane of the MBHB, but becomes more isotropic with
decreasing binary separation. The degree of anisotropy is smaller for unequal
mass binaries and larger for stars with higher kick velocities. Eccentric MBHBs
produce a more prominent tail of high-velocity stars and break planar symmetry,
ejecting HVSs along a broad jet perpendicular to the semimajor axis. The jet
two-sidedness decreases with increasing binary mass ratio, while the jet
opening-angle increases with decreasing kick velocity and orbital separation.
The detection of a numerous population of HVSs in the halo of the Milky Way by
the next generation of large astrometric surveys like GAIA may provide a unique
signature of the history, nature, and environment of the MBH at the Galactic
center.
