Multiwavelength periodicity study of Markarian 501

Context: Active Galactic Nuclei are highly variable emitters of electromagnetic waves from the radio to the gamma-ray regime. This variability may be periodic, which in turn could be the signature of a binary black hole. Systems of black holes are strong emitters of gravitational waves whose amplitude depends on the binary orbital parameters as the component mass, the orbital semi-major-axis and eccentricity. Aims: It is our aim to prove the existence of periodicity of the AGN Markarian 501 from several observations in different wavelengths. A simultaneous periodicity in different wavelengths provides evidence for bound binary black holes in the core of AGN. Methods: Existing data sets from observations by Whipple, SWIFT, RXTE and MAGIC have been analysed with the Lomb-Scargle method, the epoch folding technique and the SigSpec software. Results: Our analysis shows a 72-day period, which could not be seen in previous works due to the limited length of observations. This does not contradict a 23-day period which can be derived as a higher harmonic from the 72-day period.