1303.6279 (Takamitsu L. Tanaka)

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

Takamitsu L. Tanaka

I discuss the possibility that accreting, supermassive black hole (SMBH) binaries with sub-parsec separations produce luminous, periodically recurring outbursts that interrupt periods of relative quiescence. This hypothesis is motivated by two characteristics found in simulations of binaries embedded in prograde accretion discs: (i) the formation of a central, low-density cavity, and (ii) the leakage of circumbinary gas into this cavity, occurring once per orbit, via discrete streams on nearly radial trajectories. The first feature will diminish the emergent optical/UV flux of the system relative to active galactic nuclei (AGN) powered by single SMBHs, while the second is likely to trigger periodic fluctuations in the emergent flux. I propose a simple toy model in which a leaked stream crosses its own orbit and shocks, converting its bulk kinetic energy to heat. The result is a hot, optically thick flow that is quickly accreted and produces a flare with an AGN-like spectrum that peaks in the UV and ranges from the optical to the soft X-ray. Due to the preceding quiescence, such a flare could plausibly be mistaken for the tidal disruption of a star. For typical binary periods of years to decades, the event rate in an individual system can be much higher than that predicted for stellar tidal disruptions but infrequent enough to hinder tests of periodicity. The flares proposed here can be produced by very massive (>10^8 Msol) SMBHs that would not tidally disrupt solar-type stars. They could be discovered serendipitously in the future by observatories such as LSST or eROSITA. I apply the model to the active galaxy OJ 287, whose production of periodic optical flares has long fueled speculation that it hosts a SMBH binary.

View original: http://arxiv.org/abs/1303.6279