Binary SMBHs and the Case of OJ 287: Downsizing the Giant Primary Mass, And Other Recent Results

Galaxy mergers and the binary SMBHs at their centers are a key component in understanding thegalaxy-SMBH (co-)evolution. Gas-rich mergers represent phases of extreme accretion and SMBHgrowth, a high fraction of mergers is now discovered with JWST at high redshift, and the recentdetection of a GW background signal is consistent with a population of massive binary SMBHmergers across cosmic times. Therefore, the search for individual, nearby, evolved binaries is in fullswing. The best-known candidate for such a compact binary system is the blazar OJ 287. Here, wepresent new results from the project MOMO. It is the longest, densest, multi-frequency monitoringprogram of any binary SMBH candidate carried out to date (e.g. Komossa et al. 2023). MOMOalso provides multiwavelength coverage quasi-simultaneous to Event Horizon Telescope (EHT) observations of OJ 287, and sheds new light on disk-jet physics independent of a binary’s presence.Among other results, MOMO has clearly established that the timing of OJ 287’s famous, giant,double-peaked outbursts is inconsistent with a heavily precessing orbit. The first direct primarySMBHmass measurement yields 108Msun, a factor of 100 lower than used in the previously leadingbinary model. These results also rule out the previously leading binary model and the physical processes it involved, with significant implications for binary populations and binary search strategiesin upcoming transient surveys at multiple wavelengths.