Supermassive black hole (SMBH) binaries are inevitably produced during
galaxy formation, but observational evidence for them remains elusive.
I will discuss the coupled dynamics of a SMBH binary with a
circumbinary gas disk, and the expected characteristics of
electromagnetic (EM) emission from such a system. In particular, the
emission is likely time-variable, and contain unique spectral
signatures, which should aid in the identification of SMBH binaries.
We have performed hydrodynamical simulations and found that binaries
can be fueled efficiently, and that the accretion rates onto the BHs
have quasi-periodic modulations. The periodicity pattern depends on
the mass ratio, and the strong periodic emission persists all the way
to the merger. This may be used to identify unique counterparts of
gravitational wave sources expected to be detected by Pulsar Timing
Arrays and by LISA, and to discover wider binary SMBHs in time-domain
EM surveys. We have identified a handful of quasars with periodic
optical variability on the timescale of O(year). I will comment on the
interpretation of these quasars as SMBH binary candidates, and on the
possibility of seeing an analogous "X-ray chirp" during the late-stage
inspiral a LISA binary.
