Electromagnetic signatures of Binary Supermassive Black Holes

LIGO has now detected almost a dozen gravitational wave (GW) signatures from merger stellar-mass black holes. Supermassive black holes (SMBHs) also emit GW, at nanohertz frequencies expected to be detected by pulsar timing arrays in the next decade. In the mean time, the number of candidate SMBH-binary systems identified in electromagnetic surveys is rapidly growing, although none have yet been confirmed. In this talk, I will discuss our results modeling the expected properties and detection rates of SMBH binaries as photometrically-variable active galactic nuclei (AGN). While many of the existing variability candidates are likely false positives, we find that numerous true binaries should exist in the data. While our results hint at what candidate parameters are most likely real sources, I will also briefly discuss what the contamination tells us about accretion processes in single AGN. Finally, I will present recent work examining the signatures and detection prospects of spectroscopic binary AGN, and those from VLBI and micro-lensing surveys. The multi-messenger landscape of SMBH binary observations in both EM and GW spectra offer an exciting future of discoveries in the physics of galactic nuclei, SMBH feeding, and the dynamics of galaxy mergers and many-body systems.