Magnetic Field Geometry in "Red" and "Blue" BL Lacs

We compare the systematics of the magnetic field geometry in the "red" low-energy peaked BL Lacs (LBLs) and "blue" high-energy peaked BL Lacs (HBLs) using VLBI polarimetric images. The LBLs are primarily "radio--selected" BL Lacs and the HBLs are primarily "X-ray selected". In contrast to the LBLs, which show predominantly transverse jet magnetic fields, the HBLs show predominantly longitudinal fields. Thus, while the SED peaks of core-dominated quasars, LBLs and HBLs form a sequence of increasing frequency, the magnetic field geometry does not follow an analogous sequence. We briefly investigate possible connections between the observed parsec-scale magnetic field structures and circular polarization measurements in the literature on various spatial scales.Comment: 12 pages, 5 figures, Proceedings of the Amsterdam workshop on "Circular polarisation from relativistic jet sources", to be published in Astrophysics and Space Science, eds. Rob Fender & J-P Macquar

Surprising Evolution of the Parsec-scale Faraday Rotation Gradients in the Jet of the BL Lac Object B1803+784

Several multi-frequency polarization studies have shown the presence of systematic Faraday Rotation gradients across the parsec-scale jets of Active Galactic Nuclei (AGN), taken to be due to the systematic variation of the line-of-sight component of a helical magnetic (B) field across the jet. Other studies have confirmed the presence and sense of these gradients in several sources, thus providing evidence that these gradients persist over time and over large distances from the core. However, we find surprising new evidence for a reversal in the direction of the Faraday Rotation gradient across the jet of B1803+784, for which multi-frequency polarization observations are available at four epochs. At our three epochs and the epoch of Zavala & Taylor (2003), we observe transverse Rotation Measure (RM) gradients across the jet, consistent with the presence of a helical magnetic field wrapped around the jet. However, we also observe a "flip" in the direction of the gradient between June 2000 and August 2002. Although the origins of this phenomena are not entirely clear, possibly explanations include (i) the sense of rotation of the central supermassive black hole and accretion disc has remained the same, but the dominant magnetic pole facing the Earth has changed from North to South; (ii) a change in the direction of the azimuthal B field component as a result of torsional oscillations of the jet; and (iii) a change in the relative contributions to the observed rotation measures of the "inner" and "outer" helical fields in a magnetic-tower model. Although we cannot entirely rule out the possibility that the observed changes in the RM distribution are associated instead with changes in the thermal-electron distribution in the vicinity of the jet, we argue that this explanation is unlikely.Comment: 21 pages, 10 figures. Accepted for publication in MNRA

Magnetic field structure of the extended 3C 380 jet

An earlier study of the complex jet of 3C 380 by Papageorgiou et al. revealed total intensity and polarization structure associated with a bright knot K1 about 0.7 arcsec from the core that was reminiscent of that expected for a conical shock wave. In this new study, 1.42, 1.66 and 4.99 GHz total intensity, polarization and Faraday rotation images are presented and analysed. These images were derived from observations with the Very Long Baseline Array plus one antenna of the Very Large Array, obtained in 2006 March. These new images confirm the overall magnetic field structure of the knot K1 indicated in the earlier observations. In addition, a clear Faraday rotation gradient has been detected across the jet, extending roughly from 10 to 30 mas (70–200 pc) along the jet from the core (a radial distance of approximately two beamwidths). The gradient spans roughly 3.5 beamwidths in the transverse direction, and the difference in the rotation measures on either side of the jet is 4–5σ, demonstrating that the detection of the gradient is firm. We interpret this transverse Faraday rotation gradient as reflecting systematic variation of the line-of-sight component of a helical or toroidal magnetic field (B) associated with the jet of 3C 380. These results provide evidence that the helical field arising due to the joint action of the rotation of the central black hole and its accretion disc and the jet outflow can survive to distances of hundreds of parsecs from the central engine

Three dimensional magnetic field structure of six parsec-scale active galactic nuclei jets

The parsec-scale Faraday rotation measure (RM) distribution of six "blazars" is investigated using multi-frequency (4.6--43 GHz) polarization observations taken on 2006 July 2 with the VLBA. Analysis of the RM provides the direction of the line-of-sight (LoS) magnetic field component, as well as the intrinsic 2-D polarization distribution on the plane of the sky. Our results show that the magnitude of the core RM increases systematically with frequency, and is well described by a power-law, where |RM_{core}| \propto \nu^a. Our measured values of $a$ vary from 0.9 to 3.8, providing information on the assumed power-law fall-off in the electron density with distance from the central engine for each source. RM gradients were detected across the jets of three sources, supporting the presence of helical magnetic fields in a sheath or boundary layer surrounding their jets. We find a bi-modal distribution of the intrinsic jet polarization orientation; either aligned or orthogonal to the jet direction. A helical magnetic field geometry can neatly explain both the bi-model distribution of the jet polarization orientation and the ordered polarization structure detected on these scales. In half the sources, we find that the core RM changes sign with distance from the central engine. We provide an explanation for this by considering a boundary layer of Faraday rotating material threaded by a helical magnetic field, where bends in the relativistic jet or accelerating/decelerating flows give rise to changes in the dominant LoS components of the magnetic field, which in turn gives rise to different signs of the RM. (abridged)Comment: 29 pages, 26 figures, accepted for publication in MNRAS, v2 -> proof corrections: references update

A new method for estimating frequency-dependent core shifts in active galactic nucleus jets

We discuss the opacity in the core regions of active galactic nuclei observed with Very Long Baseline Interferometry (VLBI), and describe a new method for deriving the frequency-dependent shifts of the VLBI core from the frequency-dependent time lags of flares observed with single-dish observations. Application of the method to the core shifts of the quasar 3C 345 shows a very good agreement between the core shifts directly measured from VLBI observations and derived from flares in the total flux density using the proposed method. The frequency-dependent time lags of flares can be used to derive physical parameters of the jets, such as distance from the VLBI core to the base of the jet and the magnetic fields in the core region. Our estimates for 3C 345 indicate core magnetic fields ~0.1 G and magnetic field at 1 pc ~0.4 G.Comment: 8 pages, 6 figures, accepted for publication in MNRA

Investigating the Effects of Finite Resolution on Observed Transverse Jet Profiles

Both the emission properties and evolution of Active Galactic Nuclei (AGN) radio jets are dependent on the magnetic fields that thread them. Faraday Rotation gradients are a very important way of investigating these magnetic fields, and can provide information on the orientation and structure of the magnetic field in the immediate vicinity of the jet; for example, a toroidal or helical field component should give rise to a systematic gradient in the observed Faraday rotation across the jet, as well as characteristic intensity and polarization profiles. However, real observed radio images have finite resolution, usually expressed via convolution with a Gaussian beam whose size corresponds to the central lobe of the point source response function. This will tend to blur transverse structure in the jet profile, raising the question of how well resolved a jet must be in the transverse direction in order to reliably detect transverse structure associated with a helical jet magnetic field. We present results of simulated intensity, polarization and Faraday rotation images designed to directly and empirically investigate the effect of finite resolution on observed transverse jet structures

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