FACT - Probing the Periodicity of Active Galactic Nuclei

Periodic signals from Active Galactic Nuclei could arise from Keplerian motion in binary black hole systems, accretion flow instabilities, jet motion or other reasons. With the HEGRA telescope system, a hint for a periodic modulation of 23-days was seen for Mrk 501 at TeV energies in coincidence with a hint for modulation in X-rays. Recently, a quasi-periodic modulation of PG1553+113 of about 2 years was observed with Fermi at GeV energies supported by consistent data points from MAGIC at TeV energies. Since most models for periodic modulations of TeV emission predict time scales in the order of weeks, only unbiased and continuous long-term monitoring can lead to conclusive results. With its dedicated observation strategy for monitoring the brightest blazars, the First G-APD Cherenkov Telescope (FACT) is ideally suited for such a study. This talk presents the result of a periodicity analysis on five years of data taken on Mrk 501 and Mrk 421 analysed with a Lomb-Scargle algorithm.