A Possible ~20 yr Periodicity in Long-term Optical Photometric and
  Spectral Variations of the Nearby Radio-Quiet Active Galactic Nucleus Ark 120
  We study the long-term variability in the optical monitoring database of
Ark~120, a nearby radio-quiet active galactic nucleus (AGN) at a distance of
143 Mpc (z=0.03271). We compiled the historical archival photometric and
spectroscopic data since 1974 and conducted a new two-year monitoring campaign
in 2015-2017, resulting in a total temporal baseline over four decades. The
long-term variations in the optical continuum exhibit a wave-like pattern and
the Hbeta integrated flux series varies with a similar behavior. The broad
Hbeta profiles have asymmetric double peaks, which change strongly with time
and tend to merge into a single peak during some epochs. The period in the
optical continuum determined from various period-search methods is about 20 yr
and the estimated false alarm probability with null hypothesis simulations is
about 1*10^-3. The overall variations of the broad Hbeta profiles also follow
the same period. However, the present database only covers two cycles of the
suggested period, which strongly encourages continued monitoring to track more
cycles and confirm the periodicity. Nevertheless, in light of the possible
periodicity and the complicated Hbeta profile, Ark~120 is one candidate of the
nearest radio-quiet AGNs with possible periodic variability, and it is thereby
a potential candidate host for a sub-parsec supermassive black hole binary.
