FACT - Searching for periodicity in five-year light-curves of Active Galactic Nuclei

Abstract

The First G-APD Cherenkov Telescope (FACT) has been monitoring Active Galactic Nuclei (AGN) for the past five years. The use of robust silicon photomultipliers (SiPMs) allows for a continuous, unbiased sampling even during bright-light conditions. This dataset promises insights into the core regions of AGN by investigating the periodicity of the sources. Periodic changes in the flux could indicate a binary nature of the supermassive black holes. A study using the Lomb-Scargle periodogram to find periodicity in monitored AGN is presented. Repeating patterns in the observation times, like moon periods and seasonal effects, affect the analysis by introducing spurious peaks into the periodogram. The zenith-dependence of the observed γ-ray rates further complicate the interpretation. Showing no variability at TeV energies, the γ-ray flux of the Crab Nebula is used to characterize this latter effect, before applying the Lomb-Scargle algorithm.