A1 Journal article – refereed
Quasi-periodicities of BL Lacertae objects

List of Authors: Sandrinelli A, Covino S, Treves A, Holgado AM, Sesana A, Lindfors E, Ramazani VF
Publication year: 2018
Journal: Astronomy and Astrophysics
Journal name in source: ASTRONOMY & ASTROPHYSICS
Journal acronym: ASTRON ASTROPHYS
Volume number: 615
Number of pages: 5
ISSN: 1432-0746

We review the reports of possible year-long quasi-periodicities of BL Lac objects in the gamma-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 Fermi gamma-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 less than or similar to 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.

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