Multi-messenger Constraints on Supermassive Black Hole Binaries.

Pulsar timing array experiments around the world have reported evidence of a stochastic gravitational wave background (GWB) at nanohertz frequencies. This background is thought to be sourced by a cosmic population of supermassive black hole binaries (SMBHBs) — systems of two gravitationally bound supermassive black holes emitting gravitational waves as they co-orbit each other at sub-parsec separations. SMBHBs may also be traced by quasars, which can be triggered by the same galaxy mergers that form SMBHBs. In this talk, I will discuss efforts to constrain the mass, volume, and local number density of the SMBHB population using a multi-messenger, quasar-based SMBHB population model. I will then talk about how candidate binary quasars — identified via periodicities in their light curves — can be used to constrain the fraction of quasars hosting a SMBHB. I will also compare this to the fraction of galaxies hosting a SMBHB. Finally, I will discuss some preliminary work on how SMBHB population models can be used to interpret the observed spectrum of the GWB and whether or not we should be surprised by what we see.