Mass Function of Binary Massive Black Holes in Active Galactic Nuclei
  If the activity of active galactic nuclei (AGNs) is predominantly induced by
major galaxy mergers, then a significant fraction of AGNs should harbor binary
massive black holes in their centers. We study the mass function of binary
massive black holes in nearby AGNs based on the observed AGN black-hole mass
function and theory of evolution of binary massive black holes interacting with
a massive circumbinary disk in the framework of coevolution of massive black
holes and their host galaxies. The circumbinary disk is assumed to be steady,
axisymmetric, geometrically thin, self-regulated, self-gravitating but
non-fragmenting with a fraction of Eddington accretion rate, which is typically
one tenth of Eddington value. The timescale of orbital decay is {then}
estimated as ~10^8yr for equal mass black-hole, being independent of the black
hole mass, semi-major axis, and viscosity parameter but dependent on the
black-hole mass ratio, Eddington ratio, and mass-to-energy conversion
efficiency. This makes it possible for any binary massive black holes to merge
within a Hubble time by the binary-disk interaction. We find that (1.8+-0.6%)
for the equal mass ratio and (1.6+-0.4%) for the one-tenth mass ratio of the
total number of nearby AGNs have close binary massive black holes with orbital
period less than ten years in their centers, detectable with on-going highly
sensitive X-ray monitors such as Monitor of All-sky X-ray Image and/or
Swift/Burst Alert Telescope. Assuming that all binary massive black holes have
the equal mass ratio, about 20% of AGNs with black hole masses of
10^{6.5-7}M_sun has the close binaries and thus provides the best chance to
detect them.
