Joanne Merrill Attridge

Radio Jet-Ambient Medium Interactions on Parsec Scales in the Blazar 1055+018

As part of our study of the magnetic fields of active galactic nuclei, we have recently observed a large sample of blazars with the Very Long Baseline Array. Here we report the discovery of a striking two-component jet in the source 1055+018 that consists of an inner spine with a transverse magnetic field and a fragmentary but distinct boundary layer with a longitudinal magnetic field. The polarization distribution in the spine strongly supports shocked-jet models, while that in the boundary layer suggests interaction with the surrounding medium. This behavior suggests a new way to understand the differing polarization properties of strong- and weak-lined blazars.

Parsec-Scale Circular Polarization Observations of 40 Blazars

We present circular polarization results from a 5 GHz survey of the parsec-scale polarization properties of 40 active galactic nuclei (AGNs) made with the VLBA. We find 11 circular polarization detections at the 3 σ level or higher. This nearly quadruples the number of sources detected in circular polarization at VLBI resolution. We find no correlation between fractional linear and circular polarization across our sample. A likely explanation is external Faraday depolarization in the cores of AGNs which reduces linear polarization but leaves circular polarization unchanged. In comparing ours and other recent results to observations made ~20 years ago, we find that, in five of six cases, sources have the same sign of circular polarization today as they did 20 years ago. This suggests the presence of a long-term property of the jets, such as the polarity of a net magnetic flux, which is stable on timescales much longer than those of individual outbursts.

Concurrent 43 and 86 GHz Very Long Baseline Polarimetry of 3C 273

We present submilliarcsecond resolution total intensity and linear polarization VLBI images of 3C 273, using concurrent 43 and 86 GHz data taken with the Very Long Baseline Array in 2002 May. The structure seen in the innermost jet suggest that we have fortuitously caught the jet in the act of changing direction. The polarization images confirm that the core is unpolarized (fractional polarization ≤1%) at 86 GHz, but also show well ordered magnetic fields (m ~ 15%) in the inner jet, at a projected distance of 2.3 pc from the core. In this strongly polarized region, the rotation measure changes across the jet by ~4.2 × 104 rad m-2 over an angular width of about 0.3 mas. If the lack of polarization in the core is also attributed to a Faraday screen, then a rotation measure dispersion 5.2 × 104 rad m-2 must be present in or in front of that region. These are among the highest rotation measures reported so far in the nucleus of any active galaxy or quasar, and must occur outside (but probably close to) the radio emitting region. The transverse rotation measure gradient is in the same sense as that observed by Asada and coworkers and by Zavala and Taylor at greater core distances. The magnitude of the transverse gradient decreases rapidly with distance down the jet, and appears to be variable.

86 GHz Very Long Baseline Polarimetry of 3C 273 and 3C 279 with the Coordinated Millimeter VLBI Array

Very long baseline polarimetry at 86 GHz probes magnetic field structures within the cores of active galactic nuclei at higher angular resolutions and a spectral octave higher than previously achievable. Observations of 3C 273 and 3C 279 taken in 2000 April with the Coordinated Millimeter VLBI Array have resulted in the first total intensity (Stokes I) and linear polarization VLBI images reported of any source at 86 GHz. These results reveal the 86 GHz electric vector position angles within the jets of 3C 273 and 3C 279 to be orthogonal to each other and the core of 3C 273 to be unpolarized. If this lack of polarization is due to Faraday depolarization alone, the dispersion in rotation measure is 90,000 rad m-2 for the core of 3C 273.

VLBA Imaging of a Large Sample of Blazars

As part of our continuing study of parsec-scale magnetic fields in active galactic nuclei we have obtained deep polarization-sensitive images of the blazar 1055+018 with the VLBA at 5 GHz. These dramatic images reveal a magnetized layer of material on the outer surfaces of the jet, distinct from the bulk of the jet. This morphology suggests interaction of the jet with the surrounding medium, the resulting shear stretching the magnetic field lines in the direction of the flow. Further multi-frequency polarization-sensitive observations of 1055+018 offer the opportunity to study radio jet-ambient medium interactions in detail.