科研之友
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摘要
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 have studied the mass function of binary massive black holes in nearby AGNs based on the observed AGN black-hole mass function and the theory of evolution of binary massive black holes interacting with a massive circumbinary disk within the framework of the 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 nonfragmenting with a fixed fraction of the Eddington accretion rate, which is typically one tenth of the Eddington value. The timescale of orbital decay is then estimated to be similar to 10(8) yr for equal mass black holes, being independent of the black-hole mass, semimajor 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 the Hubble time under the binary disk interaction. We find that (1.5% +/- 0.6%) of the total number of nearby AGNs for the equal-mass ratio and (1.3% +/- 0.5%) for the one-to-ten mass ratio have close binary massive black holes with an orbital period of less than 10 yr in their centers, detectable with ongoing 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 10% of AGNs with black-hole masses of 10(6.5-7) M-circle dot have close binaries, and thus provide the best chance to detect them.
- 出版日期2010
