POLAMI: Polarimetric Monitoring of Active Galactic Nuclei at Millimetre Wavelengths – II. Widespread circular polarization

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. © 2016 The Authors.
We are grateful to Daniel Homan (Denison University, USA) for valuable comments and for sharing unpublished data, and to Ioannis Myserlis (MPIfR, Germany) for a careful reading of the manuscript. We thank an anonymous referee for insightful and helpful comments. This paper is based on observations carried out with the IRAM 30-m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). CT thanks the GILDAS software group, notably Sebastien Bardeau, for competent and rapid support. IA acknowledges support by a Ramon y Cajal grant of the Ministerio de Economia y Competitividad (MINECO) of Spain. The research at the IAA-CSIC was supported in part by the MINECO through grants AYA2016-80889-P, AYA2013-40825-P and AYA2010-14844, and by the regional government of Andalucia through grant P09-FQM-4784. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We have also made use of NASA's Astrophysics Data System Bibliographic Services.