Radio circular polarization produced in helical magnetic fields in eight active galactic nuclei

Abstract

Homan & Lister have recently published circular polarization (CP) detections for 34 objects in the MOJAVE sample - a set of bright, compact active galactic nuclei (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 (3C 345 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 (LP) 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 magnetized plasma at the front side of the jet on its way towards 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 eight 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

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

“…Furthermore, the field is predominantly longitudinal in the VLBI core: quasi-simultaneous optical and 15+22+43-GHz VLBA polarization observations showed that the optical and Faraday-rotation-corrected VLBI core polarization angles were aligned to within 5 • , with both being orthgonal to the jet direction [4]; since the optical emission is clearly optically thin, this implies a longitudinal core B field. Thus, the observed transverse CP structure in 1334−127 is also consistent with its linear polarisation structure: the simple helical-field CP model of [5] predicts that CP of a single sign that is displaced toward one edge of the jet should be observed for jets whose helical fields have pitch angles somewhat less than ≃ 45 • and are viewed at angles not too different from ≃ 90 • in the jet rest frame. Thus, the 43-GHz CP images for 1334−127 and 1055+018 provide tantalizing evidence that a self-consistent picture of the observed intensity, linear polarisation and circular polarisation structures may be beginning to emerge.…”

“…on different sides of the jet; see Fig. 6 of [5], which could be blended in the observed "core." It is also quite possible that the frequency dependence for the observed CP is associated to a considerable extent with the intrinsic inhomogeneity of the jets.…”

“…This leads to a delay of E relative to E ⊥ , manifest as the introduction of a small amount of circular polarization. There is some evidence that the CP in AGN may be generated by Faraday conversion in helical jet magnetic fields [5].…”

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

“…Similar considerations regarding the survival of helical fields could apply for the large scale fields of stars and accretion disks. Another implication of slow diffusion of helical fields is that the observation of a helical large scale field in jets from Faraday rotation (Asada et al 2008;Gabuzda et al 2008; does not guarantee magnetic energy domination (Lyutikov et al 2005) on the observed scales (BS13).…”

“…From the jets of AGN, Faraday rotation from ordered helical magnetic fields is directly observed (Asada et al 2008;Gabuzda et al 2008;Gabuzda et al 2012). Because jets are anchored in the accretion engines, they play a role in extracting the angular momentum that allows remaining disc material to accrete.…”