Parsec-scale polarization of the jet in quasar 4C 71.07

Abstract

Magnetic fields are of considerable importance in the study of radio jets: longitudinal magnetic fields are thought to help stabilize the jet as it travels outwards from the nucleus and, since magnetic field and jet plasma are frozen together, changes in jet structure are reflected in changes in magnetic field structure. Polarization measurements provide the only way to study the magnetic field structures in extragalactic radio sources, and, therefore, are the only means of studying magnetic field alignments. From the alignment of magnetic field and jet structures, inferences may be made about dynamical changes occurring within the jet. Magnetic fields appear to be well aligned with the jets in quasars on kiloparsec-scales, but on parsec-scales, the picture is less clear with apparently large misalignments between field and jet seen in a number of sources. This raises the question: are jet magnetic fields becoming aligned on parsec-scales? This thesis presents a VLBI study of the causes of apparently misaligned fields on parsec-scales in the quasar 4C71.07.
The magnetic field alignments in quasar jets may be affected by a number of factors; external and internal Faraday rotation, unresolved changes in jet direction ("wiggles"), asymmetric magnetic field structures and gravitational lensing. Lensing rotates the image, leaving the polarization orientation unchanged, even when the lensing is sufficiently weak that only one image occurs. The jet of quasar 4C71.07 (0836+710) was chosen for this study because it shows a large misalignment (about 35 ° ) between inferred magnetic field and jet direction, and because it has a highly polarized jet with relatively simple structure. Two epochs of polarization-sensitive global VLBI observations at frequencies 5.00Hz and 8.40Hz are presented.
Although observations at two frequencies cannot identify Faraday rotation with certainty, they can be used to test the hypothesis that external Faraday rotation is responsible for the observed misalignment. The results show that external Faraday rotation does not greatly improve the alignments and the largest rotation measures observed are = lOOrad m 2 corresponding to only 20° of rotation at 6cm. Though non-linear internal Faraday rotation from an inhomogeneous medium may be responsible for the observed misalignment, it seems unlikely since the fractional polarization at the two frequencies shows no evidence of depolarization.
Misalignments are significantly reduced if both external Faraday rotation is assumed and the magnetic field compared to the high resolution local jet structure. A pattern of wiggles recurs throughout the jet and the observation of one such kink on the parsec-scale shows a magnetic field structure which follows the jet around the bends. Other instances of misalignment may be due to similar, but unresolved, jet wiggles.
Jet components show a range of apparent velocities from stationary to superluminal. No evidence was found for variations in the polarization properties of components as they move out. One prominent component shows evidence for non-ballistic motion since its linear trajectory does not align with the core position. The inferred magnetic field direction at this component appears to be well aligned with the local ridge-line of the jet rather than to the direction of motion.
Lensing models show that sufficient image rotation of the parsec-scale jet of 4C 71.07 is possible without multiple imaging, given a leusing potential characteristic of an intervening galaxy with mass M - 10 12M® , situated - 1.7 arcseconds from the source and at redshift z = 0.9. However, little observational evidence has as yet been found for such a galaxy. In addition, variations in alignment of magnetic field aloug the parsec-scale jet and the good alignments found on kiloparsecscales
mean lensing by a standard galaxy potential is unlikely to contribute significantly to the observed misalignment in quasar 4C 71.07.
In conclusion, the misalignment of the magnetic field structure in the parsec-scale jet structure of quasar 4C 71.07 is not consistent with an intrinsically longitudinal magnetic field apparently misaligned due to external Faraday rotation. However, alignments are improved when local structural variations are considered and a longitudinal magnetic field appears to follow variations in the structure of the jet. There is little evidence for components 'flowiug" along the wiggles in the jet. Rather, the jet appears to represent a locus of bright features which, upon reaching the parsecscale, move outwards in the same or similar directions. The alignment between the magnetic field and jet directions appears to already be in place when the jet reaches the parsec-scale.