Fe K_alpha line: A tool to probe massive binary black holes in Active Galactic Nuclei?

Hierarchical mergers of galaxies can form binary black holes (BBHs) since many or most galaxies have central massive black holes (BHs). It is possible that some BBHs exist in active galactic nuclei (AGNs). We argue that each BH may be surrounded by an accretion disc with a different inclination angle to the line of sight (due to different BH spin directions and the Bardeen-Petterson effect). The observed Fe K_alpha line profile from a BBH system is a combination of the lines from the inner regions of the two discs, which is significantly affected by the inclination angles of the two discs. The Fe K_alpha line profile associated with BBHs may have an unusual shape with double or more peaks as well as short-term variability, which can be distinguished from the Fe K_alpha line properties of some other possible models. We suggest that with the improvement of resolution in X-ray astronomy, Fe K_alpha line profiles be a potential tool to probe the existence of massive BBHs in AGNs. The Fe K_alpha line profile associated with BBHs may also provide a tool to investigate the dynamics in strong gravitation field (e.g. providing evidence of the Bardeen-Petterson effect).