Observational Signatures of Binary Supermassive Black Holes
  Observations indicate that most massive galaxies contain a supermassive black
hole, and theoretical studies suggest that when such galaxies have a major
merger, the central black holes will form a binary and eventually coalesce.
Here we discuss two spectral signatures of such binaries that may help
distinguish them from ordinary AGN. These signatures are expected when the mass
ratio between the holes is not extreme and the system is fed by a circumbinary
disk. One such signature is a notch in the thermal continuum that has been
predicted by other authors; we point out that it should be accompanied by a
spectral revival at shorter wavelengths and also discuss its dependence on
binary properties such as mass, mass ratio, and separation. In particular, we
note that the wavelength $\lambda_n$ at which the notch occurs depends on these
three parameters in such a way as to make the number of systems displaying
these notches $\propto \lambda_n^{16/3}$; longer wavelength searches are
therefore strongly favored. A second signature, first discussed here, is hard
X-ray emission with a Wien-like spectrum at a characteristic temperature $\sim
100$ keV produced by Compton cooling of the shock generated when streams from
the circumbinary disk hit the accretion disks around the individual black
holes. We investigate the observability of both signatures. The hard X-ray
signal may be particularly valuable as it can provide an indicator of black
hole merger a few decades in advance of the event.
