Polarization and structure of relativistic parsec-scale AGN jets

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 Γ. 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 behavior. (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 Γ ≫ 1 whose axis makes a small angle to the line of sight, θ ∼ 1/Γ, 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 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 Bφ/Bz ≥ Γ. (iv) The emission-weighted average pitch angle [email protected] Work supported in part by the Department of Energy Contract DE-AC03-76SF00515 SLAC-PUB-10491 astro-ph/0406144 June 2004 Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309