The optical and infrared emission of blazars

We have carried out a polarization study of 44 blazars and candidate blazars observed at optical and near-infrared frequencies. The spectrum of the radiation is discussed in terms of shock acceleration models. Frequency dependence is a common feature of the polarization behaviour, with the polarization and spectral index generally increasing towards higher frequencies in those BL Lac objects which were observed. No evidence was found to support the previous claim that frequency dependence is related to high levels of polarization. No characteristic form of blazar variability was evident from the data. The intrinsic frequency-dependent polarization of the BL Lac objects is the result of spectral curvature, due to source inhomogeneity. (author)