Longterm Optical Monitoring of Bright BL Lacertae Objects with ATOM: Spectral Variability and Multiwavelength Correlations

Context. Blazars are the established sources of an intense and variable non-thermal radiation extending from radio wavelengths up to high and very high energy γ-rays. Understanding the spectral evolution of blazars in selected frequency ranges, as well as multi-frequency correlations in various types of blazar sources, is of a primary importance for constraining the blazar physics. Aims. Here we present the results of a long-term optical monitoring of a sample of 30 blazars of the BL Lac type, most of which are the confirmed TeV emitters. We study the optical color–magnitude correlation patterns emerging in the analyzed sample, and compare the optical properties of the targets with the high-energy γ-ray and high-frequency radio data. Methods. The optical observations were carried out in R and B filters using the Automatic Telescope for Optical Monitoring (ATOM) located at the site of the H.E.S.S. Array. Each object in the sample was observed during at least 20 nights in the period 2007–2012. Results. We find significant global color–magnitude correlations (meaning bluer-when-brighter spectral evolution) in 40 % of the sample. The sources which do not display any clear chromatism in the gathered entire datasets often do exhibit bluer-when-brighter behavior but only in isolated shorter time intervals. We also discovered spectral state transitions at optical wavelengths in several analyzed sources. Finally, we find that the radio, optical, and γ-ray luminosities of the sources in the sample obey almost linear corre-lations, which seem however induced, at least partly, by the redshift dependance, and may be also affected by non-simultaneousness of the analyzed multifrequency dataset