Chalk Talk: Refining the Search for Supermassive Black-Hole Binaries
Electromagnetic transient surveys have been systematically searching for periodic quasars that may host supermassive black-hole (SMBH) binaries at sub-pc to milli-pc separations. At these separations, gravitational waves (GWs) start to dominate the orbital decay and are at frequencies that pulsar timing arrays may be sensitive to. There are currently over 100 binary candidates and as new transient surveys come online, the catalog of candidates will continue to grow. Pulsar timing array upper limits on the nanohertz stochastic GW background, however, suggest that not all candidates can be real binaries. I will first discuss circumbinary accretion variability and relativistic Doppler boosting as the possible electromagnetic emission mechanisms for SMBH binaries. I will then discuss the GW-driven orbital decay of SMBH binaries and how this might be used as an additional test of the binary hypothesis for current and future SMBH binary candidates.
