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We analyse the circular polarization data accumulated in the first 7 years of the Polarimetric Monitoring of Active Galactic Nuclei at MillimetreWavelengths (POLAMI) project introduced in an accompanying paper. In the 3-mm wavelength band, we acquired more than 2600 observations, and all but one of our 37 sample sources were detected, most of them several times. For most sources, the observed distribution of the degree of circular polarization is broader than that of unpolarized calibrators, indicating that weak (≲0.5 per cent) circular polarization is present most of the time. Our detection rate and the maximum degree of polarization found, 2.0 per cent, are comparable to previous surveys, all made at much longer wavelengths. We argue that the process generating circular polarization must not be strongly wavelength dependent, and we propose that the widespread presence of circular polarization in our short wavelength sample dominated by blazars is mostly due to Faraday conversion of the linearly polarized synchrotron radiation in the helical magnetic field of the jet. Circular polarization is variable, most notably on time-scales comparable to or shorter than our median sampling interval of ≲1 month. Longer time-scales of about 1 yr are occasionally detected, but severely limited by the weakness of the signal. At variance with some longer wavelength investigations we find that the sign of circular polarization changes in most sources, while only seven sources, including three already known, have a strong preference for one sign. The degrees of circular and linear polarization do not show any systematic correlation. We do find however one particular event where the two polarization degrees vary in synchronism during a time span of 0.9 yr. This paper also describes a novel method for calibrating the sign of circular polarization observations. [ABSTRACT FROM AUTHOR]

We present γ-ray, X-ray, ultraviolet, optical, and near-infrared light curves of 33 γ-ray bright blazars over 4 years that we have been monitoring since 2008 August with multiple optical, ground-based telescopes and the Swift satellite, and augmented by data from the Fermi Gamma-ray Space Telescope and other publicly available data from Swift. The sample consists of 21 flat-spectrum radio quasars (FSRQs) and 12 BL Lac objects (BL Lacs). We identify quiescent and active states of the sources based on their γ-ray behavior. We derive γ-ray, X-ray, and optical spectral indices, αγ, αX, and αo, respectively (Fν∝να), and construct spectral energy distributions during quiescent and active states. We analyze the relationships between different spectral indices, blazar classes, and activity states. We find (1) significantly steeper γ-ray spectra of FSRQs than for BL Lacs during quiescent states, but a flattening of the spectra for FSRQs during active states while the BL Lacs show no significant change; (2) a small difference of αX within each class between states, with BL Lac X-ray spectra significantly steeper than in FSRQs; (3) a highly peaked distribution of X-ray spectral slopes of FSRQs at ∼ –0.60, but a very broad distribution of αX of BL Lacs during active states; (4) flattening of the optical spectra of FSRQs during quiescent states, but no statistically significant change of αo of BL Lacs between states; and (5) a positive correlation between optical and γ-ray spectral slopes of BL Lacs, with similar values of the slopes. We discuss the findings with respect to the relative prominence of different components of high-energy and optical emission as the flux state changes. [ABSTRACT FROM AUTHOR]