The optical and infrared-emission of blazars

A polarization study is presented of 44 blazars and candidate blazars observed at optical and near-IR frequencies. The spectrum of the radiation is discussed in terms of shock acceleration models. Frequency dependence is a common feature of the polarization behavior, with the polarization and spectral index generally increasing towards higher frequencies in those BL Lac objects which were observed. The intrinsic frequency-dependent polarization of the BL Lac objects is the result of spectral curvature, due to source inhomogeneity. It is suggested that the observed spectral, temporal and polarization behavior is the result of a polarized cut-off component, tentatively identified with emission from a shock and an unpolarized steep spectrum component. This can explain the observations of 1253-055 (3C 279) and many of the other observed cases of frequency-dependent polarization.