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. Analogous 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 FeK alpha emission from accreting gas. Near-future X-ray telescopes such as Astro-H and Athena will have the energy resolution (delta E/E less than or similar to 10(-3)) to search nearby active galactic nuclei (AGN) for the presence of binaries with mass ratios q greater than or similar to 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 overestimate 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.