A Possible ~20 yr Periodicity in Long-term Optical Photometric and Spectral Variations of the Nearby Radio-quiet Active Galactic Nucleus Ark 120
Yan-Rong Li et al 2019 ApJS 241 33
Optical monitoring of Ark 120, a nearby radio-quiet active galactic nucleus, reveals a 20-year periodicity that potentially indicates this galaxy hosts a sub-parsec supermassive black hole binary.