Detection of radial velocity shifts due to black hole binaries near
  merger
  The barycenter of a massive black hole binary will lie outside the event
horizon of the primary black hole for modest values of mass ratio and binary
separation. Analagous to radial velocity shifts in stellar emission lines
caused by the tug of planets, the radial velocity of the primary black hole
around the barycenter can leave a tell-tale oscillation in the broad component
of Fe K$\alpha$ emission from accreting gas. Near-future X-ray telescopes such
as Astro-H and Athena will have the energy resolution ($\delta E/E \lesssim
10^{-3}$) to search nearby active galactic nuclei (AGN) for the presence of
binaries with mass ratios $q \gtrsim 0.01$, separated by several hundred
gravitational radii. The general-relativistic and Lense-Thirring precession of
the periapse of the secondary orbit imprints a detectable modulation on the
oscillations. The lowest mass binaries in AGN will oscillate many times within
typical X-ray exposures, leading to a broadening of the line wings and an
over-estimate of black hole spin in these sources. Detection of periodic
oscillations in the AGN line centroid energy will reveal a massive black hole
binary close to merger and will provide an early warning of gravitational
radiation emission.
