FeK alpha line: A tool to probe massive binary black holes in Active Galactic Nuclei?

Since many or most galaxies have central massive black holes (BHs), mergers of galaxies can form massive binary black holes (BBHs). It is possible tha t some BBHs exist in active galactic nuclei (AGNs). We argue that each BH m ay 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-Pet terson 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 do uble or more peaks as well as short-term variability (e.g.similar to 10(4) s), which can be distinguished from the Fe K alpha line properties of some other possible models. The precession of the two discs could introduce two un-related quasi-periodic signals in the X-ray continuum emission, which wo uld be useful to constrain both of the BH masses. 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 al pha line profile associated with BBHs may also provide a tool to investigat e the dynamics in strong gravitation field (e.g. providing evidence of the Bardeen-Petterson effect).