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Supermassive binary black holes (BBHs) are unavoidable products of galaxymergers and are expected to exist in the cores of many quasars. Great efforthas been made during the past several decades to search for BBHs among quasars;however, observational evidence for BBHs remains elusive and ambiguous, whichis difficult to reconcile with theoretical expectations. In this paper, we showthat the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted asemission from accretion flows onto a BBH, with a semimajor axis of ~590AU andan orbital period of ~1.2 year. The flat optical and UV continua are mainlyemitted from a circumbinary disk and a mini-disk around the secondary blackhole (BH), respectively; and the observed sharp drop off and flux deficit atwavelength lambda ~ 4000-2500 Angstrom is due to a gap (or hole) opened by thesecondary BH migrating within the circumbinary disk. If confirmed by futureobservations, this BBH will provide a unique laboratory to study the interplaybetween BBHs and accretion flows onto them. Our result also demonstrates a newmethod to find sub-parsec scale BBHs by searching for deficits in theoptical-to-UV continuum among the spectra of quasars.

A probable Milli-Parsec Supermassive Binary Black Hole in the Nearest Quasar Mrk 231