Probing a Supermassive Binary Black Hole with MAXI

摘要

Supermassive black holes are considered to have been coevolved with their host galaxies. This strongly suggests that black hole growth is mainly caused by black hole mergers and subsequent accretion of gas in the course of the galaxy merger. If so, a supermassive binary black hole with a sub-parsec scale separation are inevitably formed before the black holes merge by emitting gravitational radiation. However, there is yet no definitive observational evidence for the supermassive binary black hole in spite of some claims (e.g. quasi-periodic light variations from OJ287). In this paper, we study the smoothed particle hydrodynamics simulations of accretion flows around the sub-parsec scale binary black hole with a moderate orbital eccentricity. In the simulations we consider a triple-disk system composing of two accretion disks around black holes and one circumbinary disk surrounding the two. Here, the circumbinary disk works as a mass reservoir. We confirm that X-ray luminosity from accretion disks significantly depends on the orbital phase because of the eccentric orbit of the binary black hole. Note also that the X-ray luminosity exhibits the double peaks every binary orbit in the case of binary black hole with low mass ratios. Such properties could not only explain a basic feature of light variations from OJ287 but also provide a potentially important observational signature of the sub-parsec binary black hole in active galactic nuclei with MAXI.