Binary Supermassive Black Holes and their Electromagnetic Counterparts
Nanohertz gravitational waves (GW) from supermassive black holes (SMBHs) are expected to be detected by pulsar timing arrays (PTA) in the next decade. In the meantime, there is a rapidly growing population of candidate SMBH-binary systems identified in electromagnetic (EM) surveys, although none have yet been confirmed. In this talk, I will discuss the interplay between GW upper-limits from PTA, and these emerging candidate systems. Excitingly, we are now in a regime in which astrophysically-meaningful constraints can be made at the interface of these measurements. Using theoretical models of SMBH-binary formation and evolution, I will present results on the expected populations of both photometrically-variable and spectroscopically-offset AGN induced by a binary companion. Finally, I will suggest some future discovery spaces which will be opened by multi-messenger observations with PTA and eventually with LISA, concerning the physics of SMBH feeding, emission signatures of binary accretion disks, and SMBH--galaxy coevolution.