Event

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.