Recurring flares from supermassive black hole binaries: implications for
  tidal disruption candidates and OJ 287
  I discuss the possibility that accreting supermassive black hole (SMBH)
binaries with sub-parsec 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 single SMBHs, while
the second can trigger quasiperiodic fluctuations in luminosity. I argue that
the quasiperiodic 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, fueling 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 quasiperiodic recurrence is observed, or
if they are produced by very massive 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
specta. I also apply the model to the active galaxy OJ 287, whose production of
quasiperiodic pairs of optical flares has long fueled speculation that it hosts
a SMBH binary.
