Evidence for a milliparsec-separation Supermassive Binary Black Hole with quasar microlensing

We report periodic oscillations in the 15-year long optical light curve of
the gravitationally lensed quasar QJ0158-4325. The signal is enhanced during a
high magnification microlensing event undergone by the fainter lensed image of
the quasar, between 2003 and 2010. We measure a period of $P_{o}=172.6\pm0.9$
days. We explore four scenarios to explain the origin of the periodicity: 1-
the high magnification microlensing event is due to a binary star in the
lensing galaxy, 2- QJ0158-4325 contains a massive binary black hole system in
its final dynamical stage before merging, 3- the quasar accretion disk contains
a bright inhomogeneity in Keplerian motion around the black hole, and 4- the
accretion disk is in precession. Among these four scenarios, only a
supermassive binary black hole can account for both the short observed period
and the amplitude of the signal, through the oscillation of the accretion disk
towards and away from high-magnification regions of a microlensing caustic. The
short measured period implies that the semi-long axis of the orbit is
$\sim10^{-3}$pc, and the coalescence timescale is $t_{coal}\sim1000$ years,
assuming that the decay of the orbit is solely powered by the emission of
gravitational waves. The probability of observing a system so close to
coalescence suggests either a much larger population of supermassive black hole
binaries than predicted, or, more likely, that some other mechanism
significantly increases the coalescence timescale. Three tests of the binary
black hole hypothesis include: i) the recurrence of oscillations in photometric
monitoring during any future microlensing events in either image, ii)
spectroscopic detection of Doppler shifts (up to 0.01$c$), and iii) the
detection of gravitational waves through Pulsar Timing Array experiments, such
as the SKA, which will have the sensitivity to detect the $\sim$100 nano-hertz
emission.