Simultaneous optical and infrared polarization measurements of blazars

The authors present measurements of the polarization and flux of a sample of 28 blazars (21 BL Lacs and 7 OVV quasars) at optical and near-infrared wavelengths, with repeated observations for some objects. As in earlier work, the authors find that the level of polarization in the infrared are generally equal to, or lower than, those in the optical, although for some objects with low levels of polarization (P < 10 per cent), the polarization in the infrared can exceed that in the optical. Large wavelength dependences of polarization, in which the polarization increases with decreasing wavelength, were observed for both high and modest (P ≡ 10 per cent) values of the optical polarization of an object. Marked variations in the wavelength dependence of polarization were observed on consecutive nights for some of the objects. Models consisting of two synchrotron sources, with different spectral indices, can satisfactorily account for the wavelength dependence of both the polarization and flux.