The 15--43-GHz Parsec-scale Circular Polarization of AGN

Abstract

Vitrishchak et al. [20] have recently published results of parsec-scale circular polarization measurements based on Very Long Baseline Array data for a number of radio-bright, core-dominated Active Galactic Nuclei obtained simultaneously at 15, 22 and 43 GHz. There are very few multifrequency VLBI circular-polarization results to date, making these of considerable interest; we summarize these results here. The degree of circular polarization m c as often rises as falls with increasing frequency between 15 and 22 GHz, while the degree of circular polarisation at 43 GHz is in all cases higher than at 22 and 15 GHz. This is somewhat surprising, since the degree of circular polarization from both synchrotron radiation and Faraday conversion of linear to circular polarization should decrease towards higher frequencies if the source is homogeneous. The increase in m c at 43 GHz may be due to the presence of regions of both positive and negative circular polarisation with different frequency dependences (but decreasing with increasing frequency) on small scales within the core region; alternatively, it may be associated with the intrinsic inhomogeneity of a Blandford-Königl-like jet. There are several cases of changes in sign with frequency, usually between 22 and 43 GHz, and tentative evidence for transverse structure in the circular polarization of 1055+018 and 1334−127 that is consistent with their being generated by either the synchrotron mechanism or Faraday conversion in a helical magnetic field.