On Supermassive Binary Black Holes in AGNs

Supermassive binary black holes (SBBHs) are a natural outcome of hierarchical galaxy mergers, yet phenomenological evidence for the existence of close (separation d <= 0.1 pc) SBBHs, that are able to coalesce within a Hubble time, still remains ambiguous. We show that low-frequency quasi-periodic variability observed from (jet-aligned) radio-loud Active Galactic Nuclei (AGNs) can provide substantial inductive support for the presence of close SBBHs at their centers: It is argued on physical grounds that such close binary systems are likely to give rise to different (although not independent) periodicities in the radio, optical and X-ray/TeV regime, and, hence that detection of appropriate period ratios significantly corroborates the SBBH interpretation. We point out that a binary framework may further allow to reconcile some of the apparently divergent central mass estimates inferred from host galaxy and fast high energy variability observations, and in particular allow to relax minimum variability source constraints. Applications are discussed for the candidate sources Mkn 501, OJ 287 and PKS 2155-304.