Abstract
Radio polarimetry is an invaluable tool to investigate the physical conditions and variability processes in active galactic nuclei (AGN) jets. However, detecting their linear and circular polarization properties is a challenging endeavor due to their low levels and possible depolarization effects. We have developed an end-to-end data analysis methodology to recover the polarization properties of unresolved sources with high accuracy. It has been applied to recover the linear and circular polarization of 87 AGNs measured by the F-GAMMA program from July 2010 to January 2015 with a mean cadence of 1.3 months. Their linear polarization was recovered at four frequencies between 2.64 and 10.45 GHz and the circular polarization at 4.85 and 8.35 GHz. The physical conditions required to reproduce the observed polarization properties and the processes which induce their variability were investigated with a full-Stokes radiative transfer code which emulates the synchrotron emission of modeled jets. The model was used to investigate the conditions needed to reproduce the observed polarization behavior for the blazar 3C 454.3, assuming that the observed variability is attributed to evolving internal shocks propagating downstream.
Highlights
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany; Academic Editors: Jose L
The variability is usually attributed to propagating shocked regions in the jet, where the magnetic field is compressed, resulting in an increase of the polarization degree, e.g., [2,3,4]
We developed an end-to-end polarimetric data analysis methodology which can be used to recover the linear and circular polarization properties of unresolved sources in the radio window
Summary
We developed an end-to-end polarimetric data analysis methodology which can be used to recover the linear and circular polarization properties of unresolved sources in the radio window. The methodology was developed using data obtained with the 4.85-GHz and 8.35-GHz receivers of the 100-m Effelsberg telescope, equipped with circularly polarized feeds It eliminates a number of systematics bringing the uncertainty to levels as low as 0.1% for linear polarization degree, 0.5◦. We use the Stokes Q and U datasets obtained from linearly unpolarized sources to create a model of the instrumental linear polarization signals for every observing session. This model is used to recreate the expected shape and amplitude of the instrumental polarization signals which are subtracted from each measurement
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