Gamma-ray and optical oscillations of 0716+714, MRK 421, and BL Lacertae

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 INFN–Istituto Nazionale di Fisica Nucleare, Sezione Trieste – Udine, via Valerio 2, 34127 Trieste, Italy
⁴ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20500 Turku, Finland
⁵ INAF–Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁶ FINCA–Centre for Astronomy with ESO, University of Turku, 20500 Turku, Finland
⁷ Harlingten New Mexico Observatory, USA
⁸ Institute of Astronomy, Bulgarian Academy of Sciences, 72 Tsarigradsko shosse Blvd., 1784 Sofia, Bulgaria

Received: 19 December 2016
Accepted: 16 January 2017

Abstract

We examine the 2008–2016 γ-ray and optical light curves of three bright BL Lac objects, 0716+714, MRK 421, BL Lac, which exhibit large structured variability. We searched for periodicities by using a fully Bayesian approach. For two out of three sources investigated, no significant periodic variability was found. In the case of BL Lac, we detected a periodicity of ~680 days. Although the signal related to this is modest, the coincidence of the periods in both gamma and optical bands is indicative of a physical relevance. Taking into consideration previous literature results, possibly related γ-ray and optical periodicities of about one year time scale are proposed in four bright γ-ray blazars out of the ten examined in detail. Compared with results from periodicity search of optical archives of quasars, the presence of quasi-periodicities in blazars may be more frequent by a large factor. This suggests the intriguing possibility that the basic conditions for their observability are related to the relativistic jet in the observer direction, but the overall picture remains uncertain.