The 15-43 GHz parsec-scale circular polarization of 41 active galactic nuclei

We present the 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. The degrees of circular polarization m(c) for the Very Long Baseline Interferometry (VLBI) core region at 15 GHz are similar to the values reported earlier at this wavelength, with typical values of a few tenths of a per cent. We find that mc as often rises as falls with increasing frequency between 15 and 22 GHz, while the degree of circular polarization at 43 GHz is in all cases higher than at 22 and 15 GHz. This behaviour seems contrary to expectations, since the degree of circular polarization from both synchrotron radiation and the Faraday conversion of linear to circular polarization - the two main mechanisms considered thus far in the literature - should decrease towards higher frequencies if the source is homogeneous. The increase in mc at 43 GHz may be due to the presence of regions of both positive and negative circular polarization 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-Konigl like jet. In several objects, the detected circular polarization appears to be near, but not coincident with, the core, although further observations are needed to confirm this. We find several cases of changes in sign with frequency, most often between 22 and 43 GHz. We find 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 the Faraday conversion in a helical magnetic field. Our results confirm the earlier finding that the sign of the circular polarization at a given observing frequency is generally consistent across epochs separated by several years or more, suggesting stability of the magnetic-field orientation in the innermost jets

Radio Circular Polarization Produced in Helical Magnetic Fields in Eight Active Galactic Nuclei

Homan & Lister (2006) have recently published circular-polarization (CP) detections for 34 objects in the MOJAVE sample - a set of bright, compact AGN being monitored by the Very Long Baseline Array at 15 GHz. We report the detection of 15-GHz parsec-scale CP in two more AGN (3C345 and 2231+114), and confirm the MOJAVE detection of CP in 1633+382. It is generally believed that the most likely mechanism for the generation of this CP is Faraday conversion of linear polarization to CP. A helical jet magnetic-field (B-field) geometry can facilitate this process - linearly polarized emission from the far side of the jet is converted to CP as it passes through the magnetised plasma at the front side of the jet on its way toward the observer. In this case, the sign of the generated CP is essentially determined by the pitch angle and helicity of the helical B field. We have determined the pitch-angle regimes and helicities of the helical jet B fields in 8 AGN for which parsec-scale CP has been detected, and used them to predict the expected CP signs for these AGN if the CP is generated via conversion in these helical fields. We have obtained the intriguing result that our predictions agree with the observed signs in all eight cases, provided that the longitudinal B-field components in the jets correspond to South magnetic poles. This clearly non-random pattern demonstrates that the observed CP in AGN is directly associated with the presence of helical jet B fields. These results suggest that helical B fields are ubiquitous in AGN jets.Comment: 24 pages, 6 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS

Polarization observations of the OH maser W75N on the Nançay radio telescope

International audienceResults of observations of the OH maser in W75N at 18 cm are reported. The observations were obtained on the radio telescope of the Nancay Radio Astronomy Observatory (France) from October 2007 to April 2009. The profiles of the Stokes parameters I, Q, U, and V in the 1665 and 1667 MHz OH lines have been measured. A technique taking into account instrumental polarization has been developed and applied. The emission in the OH lines is strongly polarized both linearly and circularly. The degree of polarization of some emission features reaches almost 100%. There were two flares of the maser emission in 2008-2009. During a flare at a radial velocity of +5.5 km/s, a Zeeman pattern was detected in the 1667 MHz line. The measured intensity of the line-of-sight component of the magnetic field was -1.1 mG, which corresponds to the field being directed away from the observer. The maser flares and the associated enhancement of the magnetic field could be associated with the compression of gas at a shock front