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.
