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

Abstract

I discuss the possibility that accreting supermassive black hole (SMBH) binaries with subparsec separations produce periodically recurring luminous outbursts that interrupt periods of relative quiescence. This hypothesis is motivated by two characteristics found generically in simulations of binaries embedded in prograde accretion discs: (i) the formation of a central, low-density cavity around the binary and (ii) the leakage of gas into this cavity, occurring once per orbit via discrete streams on nearly radial trajectories. The first feature would reduce the emergent optical/UV flux of the system relative to active galactic nuclei powered by a single SMBH, while the second can trigger quasi-periodic fluctuations in luminosity. I argue that the quasi-periodic accretion signature may be much more dramatic than previously thought, because the infalling gas streams can strongly shock-heat via self-collision and tidal compression, thereby enhancing viscous accretion. Any optically thick gas that is circularized about either SMBH can accrete before the next pair of streams is deposited, fuelling transient, luminous flares that recur every orbit. Due to the diminished flux in between accretion episodes, such cavity-accretion flares could plausibly be mistaken for the tidal disruptions of stars in quiescent nuclei. The flares could be distinguished from tidal disruption events if their quasi-periodic recurrence is observed, or if they are produced by very massive ({similar or greater-than}109M⊙) SMBHs that cannot disrupt solar-type stars. They may be discovered serendipitously in surveys such as LSST or eROSITA. I present a heuristic toy model as a proof of concept for the production of cavity-accretion flares, and generate mock light curves and spectra. I also apply the model to the active galaxy OJ 287, whose production of quasi-periodic pairs of optical flares has long fuelled speculation that it hosts an SMBH binary. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.