A probable Milli-Parsec Supermassive Binary Black Hole in the Nearest
  Quasar Mrk 231
  Supermassive binary black holes (BBHs) are unavoidable products of galaxy
mergers and are expected to exist in the cores of many quasars. Great effort
has been made during the past several decades to search for BBHs among quasars;
however, observational evidence for BBHs remains elusive and ambiguous, which
is difficult to reconcile with theoretical expectations. In this paper, we show
that the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted as
emission from accretion flows onto a BBH, with a semimajor axis of ~590AU and
an orbital period of ~1.2 year. The flat optical and UV continua are mainly
emitted from a circumbinary disk and a mini-disk around the secondary black
hole (BH), respectively; and the observed sharp drop off and flux deficit at
wavelength lambda ~ 4000-2500 Angstrom is due to a gap (or hole) opened by the
secondary BH migrating within the circumbinary disk. If confirmed by future
observations, this BBH will provide a unique laboratory to study the interplay
between BBHs and accretion flows onto them. Our result also demonstrates a new
method to find sub-parsec scale BBHs by searching for deficits in the
optical-to-UV continuum among the spectra of quasars.
