A Possible ~20 yr Periodicity in Long-term Optical Photometric and Spectral Variations of the Nearby Radio-Quiet Active Galactic Nucleus Ark 120

Understand

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.