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I discuss the possibility that accreting supermassive black hole (SMBH)binaries with sub-parsec separations produce periodically recurring luminousoutbursts that interrupt periods of relative quiescence. This hypothesis ismotivated by two characteristics found generically in simulations of binariesembedded in prograde accretion discs: (i) the formation of a central,low-density cavity around the binary, and (ii) the leakage of gas into thiscavity, occurring once per orbit via discrete streams on nearly radialtrajectories. The first feature would reduce the emergent optical/UV flux ofthe system relative to active galactic nuclei powered by single SMBHs, whilethe second can trigger quasiperiodic fluctuations in luminosity. I argue thatthe quasiperiodic accretion signature may be much more dramatic than previouslythought, because the infalling gas streams can strongly shock-heat viaself-collision and tidal compression, thereby enhancing viscous accretion. Anyoptically thick gas that is circularized about either SMBH can accrete beforethe next pair of streams is deposited, fueling transient, luminous flares thatrecur every orbit. Due to the diminished flux in between accretion episodes,such cavity-accretion flares could plausibly be mistaken for the tidaldisruptions of stars in quiescent nuclei. The flares could be distinguishedfrom tidal disruption events if their quasiperiodic recurrence is observed, orif they are produced by very massive SMBHs that cannot disrupt solar-typestars. They may be discovered serendipitously in surveys such as LSST oreROSITA. I present a heuristic toy model as a proof of concept for theproduction of cavity-accretion flares, and generate mock light curves andspecta. I also apply the model to the active galaxy OJ 287, whose production ofquasiperiodic pairs of optical flares has long fueled speculation that it hostsa SMBH binary.

Recurring flares from supermassive black hole binaries: implications for tidal disruption candidates and OJ 287