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Abstract: If the activity of active galactic nuclei AGNs is predominantly induced bymajor galaxy mergers, then a significant fraction of AGNs should harbor binarymassive black holes in their centers. We study the mass function of binarymassive black holes in nearby AGNs based on the observed AGN black-hole massfunction and theory of evolution of binary massive black holes interacting witha massive circumbinary disk in the framework of coevolution of massive blackholes and their host galaxies. The circumbinary disk is assumed to be steady,axisymmetric, geometrically thin, self-regulated, self-gravitating butnon-fragmenting with a fraction of Eddington accretion rate, which is typicallyone tenth of Eddington value. The timescale of orbital decay is {then}estimated as ~10^8yr for equal mass black-hole, being independent of the blackhole mass, semi-major axis, and viscosity parameter but dependent on theblack-hole mass ratio, Eddington ratio, and mass-to-energy conversionefficiency. This makes it possible for any binary massive black holes to mergewithin 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 thetotal number of nearby AGNs have close binary massive black holes with orbitalperiod less than ten years in their centers, detectable with on-going highlysensitive X-ray monitors such as Monitor of All-sky X-ray Image and-orSwift-Burst Alert Telescope. Assuming that all binary massive black holes havethe equal mass ratio, about 20% of AGNs with black hole masses of10^{6.5-7}M sun has the close binaries and thus provides the best chance todetect them.



Author: Kimitake Hayasaki, Yoshihiro Ueda, Naoki Isobe

Source: https://arxiv.org/



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