| Description |
Supermassive black hole binaries (SMBHBs) should be common in galactic nuclei, but we have little evidence, to date, of their existence. I will review possibilities of electromagnetic (EM) emission from SMBHBs, which may aid in their identification. In particular, any detectable EM emission immediately preceding, during, and following the coalescence of the BHs is likely to be time-variable, and contain characteristic spectral signatures, which should aid in its identification. I will discuss recent hydrodynamical simulations, which suggest quasiperiodic signals due to the orbital motion prior to coalescence. Transient "after-glows" may also be produced by post-merger gas accretion and by merger-induced shocks in a circumbinary disk. These time-variable EM signatures may be used to identify unique counterparts of gravitational wave sources expected to be detected by (e)LISA and by Pulsar Timing Arrays, or to discover binary SMBHs in transient EM surveys.
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