  Supermassive black holes (SMBHs) are found in the centers of massive
galaxies, and galaxy mergers should eventually lead to SMBH mergers. Quasar
activity has long been associated with galaxy mergers, so here we investigate
if supermassive black hole binaries (SMBHBs) are preferentially found in
quasars. Our multimessenger investigation folds together a gravitational wave
background signal from NANOGrav, a sample of periodic AGN candidates from the
Catalina Real-Time Transient Survey, and a quasar mass function, to estimate an
upper limit on the fraction of quasars which could host a SMBHB. We find at
95\% confidence that quasars are at most seven times as likely to host a SMBHB
as a random galaxy. Quasars should therefore be prioritized as targets for
SMBHB searches in pulsar timing arrays.
