Gleaning Secrets from the Transverse Profiles of AGN jets

Both the emission properties and evolution of Active Galactic Nuclei (AGN) radio jets are dependent on the magnetic fields that thread them. A better understanding of these magnetic fields is therefore important in helping our understanding of jets in AGN. Several observations of jets have suggested that, on parsec scales, the jet magnetic field may have a significant helical component. Using a model first proposed by Laing and developed by Papageorgiou and Cawthorne, all of the above observations can be reproduced by varying only the helical pitch angle of the magnetic field and the line of sight angle. In order to reduce the total polarization to agree with observed values, a tangled magnetic field component is also introduced to the model, via another parameter, the degree of entanglement. We are in the process of comparing data from observations of several AGN with this model, making it possible to derive estimates of the helical pitch angle, the viewing angle and the degree of entanglement for these jets in the framework of this model. The observed profiles are compared with a set of theoretical profiles. This proves to be an objective approach to profile fitting that should enable analysis of a large number of AGN jets, making it possible to look for trends. Results for Mrk501 are presented

Simulation of polarized emission from recollimation shocks in relativistic jets

This paper presents a method for obtaining synthetic images of linearly polarized synchrotron radiation from steady-state numerical simulations of relativistic jets, in which the magnetic field is assumed to be initially either partially or completely disordered. The method is based on the earlier work which characterized the deformation of the fluid using infinitessimal fluid elements that are initially cubic, and which evolve into parallelepipeds. The method is described for a range of models for the initial magnetic field, including ordered components that are axial, helical, and toroidal with a bi-directional axial component. The method is then applied to steady, axisymmetric simulations of initially overpressured jets and the initial results are discussed. Some characteristic patterns and trends in polarization angle are identified. Although the recollimation shocks that form in these jets are not clearly visible in the total intensity images presented here, arcs of strongly polarized emission associated with radial velocity gradients in the vicinity of the shock waves are seen, and might prove to be a useful diagnostic feature of this model