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 ...

Observations by the RHESSI satellite of large polarization of the prompt γ-ray emission from the Gamma Ray Burst GRB021206 (Coburn & Boggs 2003) imply that the magnetic field coherence scale is larger than the size of the visible emitting region, ∼ R/Γ, where R is the radius of the flow, Γ is the associated Lorentz factor. Such fields cannot be generated in a causally disconnected, hydrodynamically dominated outflow. Electromagnetic models of GRBs (Lyutikov & Blandford 2002), in which large scale, dynamically dominant, magnetic fields are present in the outflow from the very beginning, provide a natural explanation of this large reported linear polarization. We derive Stokes parameters of synchrotron emission of a relativistically moving plasma with a given magnetic field configuration and calculate the pulse averaged polarization fraction of the emission from a relativistically expanding shell carrying global toroidal magnetic field. For viewing angles larger than 1/Γ the observed patch of the emitting shell has almost homogeneous magnetic field, producing a large fractional polarization (56% for a power-law energy distribution of relativistic particles dn/dǫ ∝ ǫ −3 ). The maximum polarization is smaller than the theoretical upper limit for a stationary plasma in uniform magnetic field due to relativistic kinematic effects.Subject headings: gamma rays: bursts -MHD -polarization 1.

We investigate the linear stability of a hydrodynamic relativistic flow of magnetized plasma in the simplest case where the energy density of the electromagnetic fields is much greater than the energy density of the matter (including the rest mass energy). This is the force-free approximation. We considered the case of light cylindrical jet in cold and dense environment, so the jet boundary remains at rest. Continuous and discrete spectra of frequencies are investigated analytically. An infinite sequence of eigenfrequencies is found near the edge of Alfvén continuum. Numerical calculations showed that modes having reasonable values of azimuthal wavenumber m and radial number n are stable and have attenuation increment γ small. The dispersion curves ω = ω(k ) have a minimum for k 0 ≃ 1/R (R is the jet radius ). This results in accumulation of perturbations inside the jet with wavelength of the order of the jet radius. The wave crests of the perturbation pattern formed in such a way move along the jet with the velocity exceeding light speed. If one has relativistic electrons emitting synchrotron radiation inside the jet, than this pattern will be visible. This provide us with the new type of superluminal source. If the jet is oriented close to the line of sight, than the observer will see knots moving backward to the core.