Electromagnetic Emission from Supermassive Black Hole Mergers

I will review ideas that may be useful in identifying electromagnetic (EM) emission from supermassive black hole (SMBH) binaries. In particular, any detectable EM emission is likely to be time-variable, which should aid in its identification. I will discuss four possibilities for such variable emission: (i) roughly periodic signals due to the orbital motion prior to coalescence, (ii) a transient pre-cursor caused by the gas trapped inside the binary's orbit, and transients "after-glows" produced by (iii) post-merger gas accretion and (iv) by merger-induced shocks in a circumbinary disk. I will argue that 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. I will also highlight the extra science that will be enabled if an EM counterpart is found, such as constraints on SMBH accretion physics, cosmology, and gravitational physics.