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 ( \sim 10 ^ { 6 } – 10 ^ { 9 } { { M } } _ { \scriptscriptstyle \odot } ) , orbital separation ( \sim 1 \mathrm { { AU } } to \sim 0.1 \mathrm { { pc } } ) , and gap width ( 10–190 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 power-law 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 power-law 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 / 12 } .
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 an entire hole ( or cavity ) 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 , though extant detailed modeling of those with high quality data have not yet revealed an inner cavity .