Optical and radio variability of the northern VHE gamma-ray emitting BL Lacertae objects

¹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
² Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁴ Aalto University Department of Radio Science and Engineering, PO Box 13000, 00076 Aalto, Finland
⁵ Foundation for Research and Technology – Hellas, IESL, Voutes, 7110 Heraklion, Greece
⁶ Department of Physics and Institute for Plasma Physics, University of Crete, 71003 Heraklion, Greece
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Cahill Center for Astronomy & Astrophysics, Caltech, 1200 E. California Blvd, Pasadena, CA 91125, USA

Received: 2 March 2016
Accepted: 4 June 2016

Abstract

We compare the variability properties of very high energy gamma-ray emitting BL Lac objects in the optical and radio bands. We use variability information to distinguish multiple emission components in the jet, to be used as a guidance for spectral energy distribution modelling. Our sample includes 32 objects in the northern sky for which we have data for at least two years in both bands. We use optical R-band data from the Tuorla blazar monitoring program and 15 GHz radio data from the Owens Valley Radio Observatory blazar monitoring program. We estimate the variability amplitudes using the intrinsic modulation index, and study the time-domain connection by cross-correlating the optical and radio light curves assuming power law power spectral density. Our sample objects are in general more variable in the optical than radio. We find correlated flares in about half of the objects, and correlated long-term trends in more than 40% of the objects. In these objects we estimate that at least 10−50% of the optical emission originates in the same emission region as the radio, while the other half is due to faster variations not seen in the radio. This implies that simple single-zone spectral energy distribution models are not adequate for many of these objects.