Detection of radial velocity shifts due to black hole binaries near merger

TL;DR

This paper proposes a method to detect black hole binaries near merger by observing radial velocity shifts in X-ray emission lines, which can reveal binaries and predict gravitational wave events.

Contribution

It introduces a novel technique to identify close black hole binaries through X-ray spectral oscillations caused by their orbital motion.

Findings

Radial velocity oscillations can indicate black hole binaries.

Upcoming X-ray telescopes can detect these oscillations in nearby AGN.

Detection can serve as an early warning for gravitational wave emission.

Abstract

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.