2009A&A...501..925R

Active galactic nuclei (AGN) 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 such as the component mass, the orbital semi-major-axis and eccentricity. 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 of bound binary black holes in the core of AGN. 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. 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 that can be derived as a higher harmonic from the 72-day period.