PhD. Thesis

This thesis presents the results of a series of polarization VLBI experiments on the well-known variable radio source BL Lacertae (BL Lac). VLBI provides the only means to study radio sources on scales ~ 1 mas, and the sensitivity to polarization allows the magnetic field structures in the source to be investigated, while also providing information about the environment of the source through the phenomenon of Faraday rotation. An attempt is also made to exploit the wealth of VLBI data on this source available in the literature to derive a kinematic model for superluminal components (knots) in its radio jet.

Four epochs of multi-frequency monitoring of BL Lac presented here demonstrate the extreme variability of this source in both total intensity and polarized flux density. A number of superluminal knots are identified in the jet with speeds ~ 5 c, comparable to those found in previous studies of this source. It has been noted in past observations that the electric vectors of knots in the jets of BL Lac objects tend, in general, to be parallel to the jet direction (implying that the magnetic field is dominated by a transverse component), in contrast to quasars wherein the electric vectors are usually transverse to the jet direction. While such parallel electric vectors are observed in some knots, some evidence is also presented here for knots with a large component of longitudinal magnetic field, possibly as the result of a shear interaction with the ambient medium. These include compact knots which move with superluminal velocities close to the central axis of the jet as well as a more diffuse, slowly moving component, located away from the central axis of the jet and reminiscent of the ``sheath" observed in 1055+018 (Attridge et al. 1999).

These observations also provide the first examples of non-uniform Faraday rotation to be detected in a BL Lac object. This finding has important consequences for the interpretation of magnetic field alignments in this source, as the rotation measure (RM) applicable to parsec scale knots is shown to differ from the integrated RM of the source derived from VLA observations. The detection of significant quantities of thermal gas in the immediate vicinity of this object is somewhat unexpected given that BL Lac objects are distinguished from quasars by the small equivalent widths of their optical emission lines. The results demonstrate that, in future, observations of Faraday rotation will provide important information on the relative quantities of thermal gas in BL Lac objects and quasars. If BL Lacs are shown not to be depleted in thermal gas on scales of ~ 1 pc, then it also seems likely that they are not depleted in gas on smaller scales and so some other explanation will be required to explain their low line luminosities.

A kinematic model derived for the jet in BL Lac suggests that the flux density evolution of knots in the jet is not determined solely by changes in the Doppler factor resulting from changes in the direction of a constant speed jet. Other factors may include expansion losses and/or deceleration of the fluid in the jet. Deceleration from relativistic to non-relativistic speeds on scales <= 2 kpc has been inferred for many FR I radio galaxies (Laing et al. 1999). That significant deceleration on scales of several hundred parsecs in the jet of BL Lac is allowed by the kinematic models is therefore consistent with the FR I/BL Lac unification scheme.

The high degree of polarization observed in component S2 in the region where the jet bends is difficult to reconcile with simple shock models, unless the fluid in the jet is moving more slowly than the observed pattern. It is interesting to note that in a similar study of the quasar 3C 345 by Wardle et al. (1994) the flux density evolution was successfully modelled without invoking the additional loss mechanisms required for BL Lac and the jet polarization indicated that for simple shock models the fluid in the jet should be moving at a greater speed than the observed pattern. Future studies will determine whether these represent real differences between the jets in quasars and BL Lac objects in general. This model constrains the jet angle to the line of sight in BL Lac to be less than 15 degrees, and is therefore consistent with the extreme beaming model for this source.

Finally, joint EVN/MERLIN observations presented here detect coherent structures on scales larger than those probed in conventional VLBI observations in 3 of 5 BL Lac objects observed. However, not all the flux density predicted to lie on these larger scales is detected. If the remaining ``missing" flux is all associated with a diffuse, unbeamed component, then the extended power in all the sources is above the FR I/FR II limit, providing a complication for currently favoured unification schemes. Having proven the feasibility of such a study with these observations, the experiment is to be repeated at lower frequency and with improved u,v coverage in order to constrain better the nature of these objects on intermediate scales.