Parsec-scale Circular Polarization Research Articles

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 m c 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 m c 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.

THE PARSEC-SCALE CIRCULAR POLARIZATION OF ACTIVE GALACTIC NUCLEI AT 22 AND 15 GHZ

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 22 and 15 GHz. The degrees of circular polarization mc for the VLBI core region at 15 GHz are similar to values reported earlier at this wavelength, with typical values of a few tenths of a percent. The origin of this polarization is almost certainly the conversion of linear to circular polarization during the propagation of the radiation through a magnetised plasma. We find that mc is as often higher as lower at the higher frequency, for reasons that are not clear. 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.

Detection and Measurement of Parsec-Scale Circular Polarization in Four [CLC]AGNs[/CLC

We present five epochs of 15 GHz VLBA observations of 13 AGNs. These observations were specially calibrated to detect parsec-scale circular polarization and our calibration techniques are discussed and analyzed in detail. We obtained reliable detections of parsec-scale circular polarization in the radio jets of four AGNs: 3C 84, PKS 0528+134, 3C 273, and 3C 279. For each of these objects our detections are at the level of approximately 0.3%–1% local fractional circular polarization. Each individual detection has a significance in the range of 3 to 10 σ. Our observations are consistent across multiple epochs (and different calibration techniques) in the sign and magnitude of the circular polarization observed. 3C 273 and 3C 279 both undergo core outbursts during our observations and changes in the circular polarization of both sources are correlated with these outbursts. In general, we observe the circular polarization to be nearly coincident with the strong VLBI cores of these objects; however, in 3C 84 the circular polarization is located a full milliarcsecond south of the source peak, and in the 1996.73 epoch of 3C 273 the circular polarization is predominately associated with the newly emerging jet component. Our observations support the theoretical conclusion that emission of circular polarization is a sensitive function of opacity, being strongest when the optical depth is near unity. Circular polarization may be produced as an intrinsic component of synchrotron radiation or by the Faraday conversion of linear to circular polarization. Our single-frequency observations do not easily distinguish between these possible mechanisms, but independent of mechanism, the remarkable consistency across epoch of the sign of the observed circular polarization suggests the existence of a long-term, stable, unidirectional magnetic field. Single-dish observations of 3C 273 and 3C 279 at 8 GHz by Hodge and Aller suggest that this stability may persist for decades in our frame of observation.

Detection of Parsec-Scale Circular Polarization with the VLBA

AbstractCircular polarization has been detected in 3C 279 and 3C 84 at multiple epochs, using the VLBA at 15 GHz. This is the first detection of circular polarization in a compact radio source at VLBI resolution. In the case of 3C 279 the polarization is most likely produced by the conversion of linear to circular polarization by low energy relativistic electrons in the emitting region. If this is so, the absence of Faraday rotation by the same electrons suggests that the jet consists of an electron-positron plasma.