Quasi-periodicities of BL Lacertae objects

¹ Università dell’Insubria, Dipartimento di Scienza ed Alta Tecnologia, Via Valleggio 11, 22100 Como, Italy
e-mail: asandrinelli@yahoo.it
² INAF-Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate (LC), Italy
³ Department of Astronomy and National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
⁴ School of Physics and Astronomy and Institute of Gravitational Wave Astronomy, University of Birmingham, Edgbaston B15 2TT, UK
⁵ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Turku, Finland

Received: 26 December 2017
Accepted: 18 January 2018

Abstract

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 Fermi γ-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.