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A search for recoiling supermassive black hole candidates recently yieldedthe best candidate thus far, SDSS J092712.65+294344.0 reported by Komossa etal. Here we propose the alternative hypothesis that this object is asupermassive black hole binary. From the velocity shift imprinted in theemission-line spectrum we infer an orbital period of ~190 years for a binarymass ratio of 0.1, a secondary black hole mass of 100 million solar masses, andassuming inclination and orbital phase angles of 45 degrees. In this model theorigin of the blueshifted narrow emission lines is naturally explained in thecontext of an accretion flow within the inner rim of the circumbinary disk. Weattribute the blueshifted broad emission lines to gas associated with a diskaround the accreting secondary black hole. We show that, within theuncertainties, this binary system can be long lived and thus, is not observedin a special moment in time. The orbital motion of the binary can potentiallybe observed with the VLBA if at least the secondary black hole is a radioemitter. In addition, for the parameters quoted above, the orbital motion willresult in a ~100 km/s velocity shift of the emission lines on a time scale ofabout a year, providing a direct observational test for the binary hypothesis.

SDSS J092712.65+294344.0: Recoiling Black Hole or A Sub-parsec Binary Candidate?