Systematic Search for γ-Ray Periodicity in Active Galactic Nuclei Detected by the Fermi Large Area Telescope

We use nine years of γ-ray data provided by the Fermi Large Area Telescope (LAT) to systematically study the light curves (LCs) of more than 2000 active galactic nuclei (AGN) included in recent Fermi-LAT catalogs. Ten different techniques are used, which are organized in an automatic periodicity-search pipeline, in order to search for evidence of periodic emission in γ rays. Understanding the processes behind this puzzling phenomenon will provide a better view about the astrophysical nature of these extragalactic sources. However, the observation of temporal patterns in γ-ray LCs of AGN is still challenging. Despite the fact that there have been efforts to characterize the temporal emission of some individual sources, a systematic search for periodicities by means of a full likelihood analysis applied to large samples of sources was missing. Our analysis finds 11 AGN, of which 9 are identified for the first time, showing periodicity at more than 4σ in at least four algorithms. These findings will help in solving questions related to the astrophysical origin of this periodic behavior.