Quasars can Signpost Supermassive Black Hole Binaries

TL;DR

This paper explores whether quasars are good indicators of supermassive black hole binaries by combining gravitational wave data, AGN observations, and quasar statistics, suggesting quasars are promising targets for SMBHB detection.

Contribution

It provides the first multimessenger estimate of the likelihood that quasars host SMBHBs, guiding future gravitational wave searches.

Findings

Quasars are at most five times more likely to host SMBHBs than random galaxies.

Pulsar timing arrays could improve SMBHB detection by focusing on quasars.

The study constrains the fraction of quasars with SMBHBs using combined observational data.

Abstract

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 five times as likely to host a SMBHB as a random galaxy. Pulsar timing arrays may therefore be more likely to find SMBHBs by prioritizing quasars over a random selection of galaxies in their searches.