Observable Consequences of Merger-Driven Gaps and Holes in Black Hole Accretion Disks. (arXiv:1207.0296v1 [astro-ph.HE])

Observable Consequences of Merger-Driven Gaps and Holes in Black Hole Accretion Disks. (arXiv:1207.0296v1 [astro-ph.HE]):
We calculate the observable signature of a black hole accretion disk with a
gap or hole created by a secondary black hole embedded in the disk. We find
that for an interesting range of parameters of black hole masses (~10^6 to 10^9
Msun), orbital separation (~1 AU to ~1 pc), and gap width (10--180 disk scale
heights), the missing thermal emission from a gap manifests itself in an
observable decrement in the spectral energy distribution. We present
observational diagnostics in terms of powerlaw forms that can be fit to
line-free regions in AGN spectra or in fluxes from sequences of broad filters.
Most interestingly, the change in slope in the broken powerlaw is almost
entirely dependent on the width of gap in the accretion disk, which in turn is
uniquely determined by mass ratio of the black holes, such that it scales
roughly as $q^{5/4}$. Thus one can use spectral observations of the continuum
of bright active galactic nuclei to infer not only the presence of a closely
separated black hole binary but also the mass ratio. When the black hole merger
opens a hole in the inner disk, the broad band SED of the AGN or quasar may
serve as a diagnostic. Such sources should be especially luminous in optical
bands but intrinsically faint in X-rays (i.e., not merely obscured). We briefly
note that viable candidates may have already been identified.