2018A&A...615A.118S

We review the reports of possible year-long quasi-periodicities of BL Lac objects in the γ-ray and optical bands, and present a homogeneous time analysis of the light curves of PKS2155-304, PG1553+113, and BL Lac. Based on results from a survey covering the entire Fermiγ-ray sky we have estimated the fraction of possible quasi-periodic BL Lac objects. We compared the cyclical behaviour in BL Lac objects with that derived from the search of possible optical periodicities in quasars, and find that at z≤1 the cosmic density of quasi-periodic BL Lac objects is larger than that of quasi-periodic quasars. If the BL Lac quasi-periodicities were due to a supermassive binary black hole (SBBH) scenario, there could be a tension with the upper limits on the gravitational wave background measured by the pulsar timing array. The argument clearly indicates the difficulties of generally associating quasi-periodicities of BL Lac objects with SBBHs.