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 \gamma-rays. Understanding the spectral evolution of blazars in selected frequency ranges, as well as multifrequency correlations in various types of blazar sources, is of a primary importance for constraining blazar physics. Aims. 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 $\gamma$-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 to 2012. Results. We find significant overall color–magnitude correlations (meaning bluer-when-brighter spectral evolution) in 40% of the sample. The sources that do not display any clear chromatism in the full 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 of the analyzed sources. Finally, we find that the radio, optical, and $\gamma$-ray luminosities of the sources in the sample obey almost linear correlations, which seem induced, at least partly, by the redshift dependence and may also be affected by the non-simultaneity of the analyzed multifrequency dataset. Conclusions. We argue that the observed bluer-when-brighter behavior is intrinsic to the jet-emission regions, at least for some of these blazars, rather than resulting from the contamination of the measured flux by the starlight of host galaxies. We also conclude that the significance of color–magnitude scalings does not correlate with the optical color, but instead seems to depend on the source luminosity, in that these are the lowest luminosity BL Lac objects to display the strongest correlations