A 6.4yr optical quasi-periodic oscillations in SDSS J075217.84+193542.2:
  a new candidate for central binary black hole system
  In this manuscript, a 6.4yr optical quasi-periodic oscillations (QPOs) is
detected in the quasar SDSS J075217.84+193542.2 (=\obj) at a redshift 0.117, of
which 13.6yr-long light curve from CSS and ASAS-SN directly described by a
sinusoidal function with a periodicity 6.4yr. The 6.4yr QPOs can be further
confirmed through the Generalized Lomb-Scargle periodogram with confidence
level higher than 99.99\%, and through the auto-correlation analysis results,
and through the WWZ technique. The optical QPOs strongly indicate a central
binary black hole (BBH) system in \obj. The determined two broad Gaussian
components in the broad H$\alpha$ can lead to the BBH system with expected
space separation about 0.02pc between the expected two central BHs with
determined virial BH masses about $8.8\times10^7{\rm M_\odot}$ and
$1.04\times10^9{\rm M_\odot}$. Meanwhile, we check the disk precessions applied
to explain the optical QPOs. However, under the disk precession assumption, the
determined optical emission regions from central BH have sizes about $40{\rm
R_G}$ two times smaller than sizes of the expected NUV emission regions through
the correlation between disk size and BH mass, indicating the disk precessions
are not preferred. And due to the lower radio loudness around 0.28, jet
precessions can be also totally ruled out. Furthermore, only 0.08\% probability
can determined as the QPOs mis-detected through light curves randomly created
by the CAR process, re-confirming the reported optical QPOs.
