Synchrotron radiation from relativistic jets in active galactic nuclei is linearly polarized. It is evidence for anisotropic distribution of magnetic fields in jets. Comparing the angle of polarisation with jet orientation we can learn about internal structure of magnetic fields. Toroidal magnetic fields produce always parallel polarisation and poloidal magnetic fields produce perpendicular polarisation. In case of more complicated distribution of magnetic fields resulting polarisation of synchrotron radiation depends on details of the model and its parameters.
The work “Polarization of synchrotron emission from relativistic reconfinement shocks with ordered magnetic fields” by Krzysztof Nalewajko (currently in JILA, Boulder, Co, previously Copernicus Center) and Marek Sikora (Copernicus Center, Warsaw) is the fourth in a series of papers devoted to focusing of shock waves. Such focusing of waves is proposed as a mechanism of energy dissipation in relativistic jets. Waves are formed as a result of jet interaction with external medium with high pressure. In this paper authors investigate various combinations of chaotic and ordered magnetic fields, in particular toroidal and helical. They show that even in case of preserved internal axial symmetry the resulting polarisation does not have to be precisely parallel or perpendicular. The transition between expansion and focusing of the jet is reflected on the polarisation maps as a turning point at which polarization vectors switch, e.g., from clockwise to counterclockwise.
