We consider the polarization properties of optically thin synchrotron radiation emitted by relativistically moving electron-positron jets carrying large-scale helical magnetic fields. In our model, the jet is cylindrical and the emitting plasma moves parallel to the jet axis with a characteristic Lorentz factor Gamma. We draw attention to the strong influence that the bulk relativistic motion of the emitting relativistic particles has on the observed polarization. Our computations predict and explain the following behaviour. (i) For jets unresolved in the direction perpendicular to their direction of propagation, the position angle of the electric vector of the linear polarization has a bimodal distribution, being oriented either parallel or perpendicular to the jet. (ii) If an ultra-relativistic jet with Gamma >> 1 whose axis makes a small angle to the line of sight, theta similar to 1/Gamma, experiences a relatively small change in the direction of propagation, velocity or pitch angle of the magnetic fields, the polarization is likely to remain parallel or perpendicular; on the other hand, in some cases, the degree of polarization can exhibit large variations and the polarization position angle can experience abrupt 90 degrees. changes. This change is more likely to occur in jets with flatter spectra. (iii) In order for the jet polarization to be oriented along the jet axis, the intrinsic toroidal magnetic field ( in the frame of the jet) should be of the order of or stronger than the intrinsic poloidal field; in this case, the highly relativistic motion of the jet implies that, in the observer's frame, the jet is strongly dominated by the toroidal magnetic field B(phi)/B(z) >= Gamma. (iv) The emission-weighted average pitch angle of the intrinsic helical field in the jet must not be too small to produce polarization along the jet axis. In force-free jets with a smooth distribution of emissivities, the emission should be generated in a limited range of radii not too close to the jet core. ( v) For mildly relativistic jets, when a counter-jet can be seen, the polarization of the counter-jet is preferentially orthogonal to the axis, unless the jet is strongly dominated by the toroidal magnetic field in its rest frame. ( vi) For resolved jets, the polarization pattern is not symmetric with respect to jet axis. Under certain conditions, this can be used to deduce the direction of the spin of the central object ( black hole or disc), whether it is aligned or anti-aligned with the jet axis. (vii) In resolved 'cylindrical shell' type jets, the central parts of the jet are polarized along the axis, while the outer parts are polarized orthogonal to it, in accordance with observations. We conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. Since the typical degrees of polarization are <= 15 per cent, the emitting parts of the jets must have comparable rest-frame toroidal and poloidal fields. In this case, most relativistic jets are strongly dominated by the ...

Evidence has been mounting that many of the transverse jet B fields observed in BL Lac objects on parsec scales represent the dominant toroidal component of the intrinsic jet B fields. Such fields could come about, for example, as a result of the 'winding up' of an initial 'seed' field with a significant longitudinal component by the rotation of the central accreting object. If this is the case, this should give rise to gradients in the rotation measure across the jets, due to the systematic change in the line-of-sight component of the jet B field. We present evidence for transverse rotation measure gradients in four BL Lac objects, strengthening arguments that the jets of these objects do indeed have toroidal or helical B fields. This underlines the view of the jets as fundamentally electromagnetic structures, and suggests that they may well carry non-zero currents. It also provides a natural means to collimate the jets

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We use observations of six 'blazars' with the Very Long Baseline Array (VLBA), at eight frequencies (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22.2 and 43.1 GHz), to investigate the frequency-dependent position of their VLBI cores ('core-shift') and their overall jet spectral distribution. By cross-correlating the optically thin jet emission, we are able to accurately align the multifrequency images of three of the jets (1418+546, 2007+777, 2200+420), whose core-shifts and spectra we find consistent with the equipartition regime of the Blandford & Konigl conical jet model, where the position of the radio core from the base of the jet follows r(core)(nu) proportional to nu-1. For the jet of 0954+658, we align the higher frequency images using our lower frequency measurements assuming equipartition in the radio core from 4.6-43 GHz. The jet emission of the other two sources in our sample (1156+295, 1749+096) is too sparse for our alignment technique to work. Using our measured core-shifts, we calculate equipartition magnetic field strengths of the order of 10s to 100s of mG in the radio cores of these four AGN from 4.6-43 GHz. Extrapolating our results back to the accretion disc and black hole jet-launching distances, we find magnetic field strengths consistent with those expected from theoretical models of magnetically powered jets