We highlight a few aspects of the radio jet of quasar 0153+744. Some challenge the relativistic beaming model to explain a stationary jet of short length and ending in a bright secondary component, extreme bending of the jet, and the lack of any emission on the counter jet side. We discuss a model of a precessing mildly relativistic jet for 0153+744.

VLBA observations of NGC 4261 (3C 270) reveal highly symmetric radio structures at both 1.6 and 8.4 GHz. There is little evidence for free-free absorption in the inner few pc, despite the fact that HST imaging shows this galaxy to contain a nearly edge-on disk of gas and dust in its nucleus. However, at our highest resolution we find a narrow gap in emission just east of the radio core which we interpret as evidence for a small (sub-parsec) nearly edge-on accretion disk which is obscuring the base of the counterjet. The position angle of the pc-scale radio axis agrees with the position angle of the VLA-scale jets, which differs from the apparent rotation axis of the nuclear disk seen by HST.

The results of VLBI observations of the quasar 3C 273, obtained during a multi-frequency campaign in late 1992 in the radio, millimeter, and X-ray bands are presented. The aim of the campaign was to test the application of the SSC (Synchrotron Self-Compton) model to 3C 273. Independent estimates are obtained through the assumption of the energy equipartition between particles and magnetic field.

We have investigated the sub-milliarcsecond scale structure of 41 γ-ray bright blazars with the VLBA at frequencies of 22, 43, and occasionally 8.4 GHz. All of the blazars have dominant, unresolved (less than 0.1 to 0.2 milliarcsec) cores, with the exception of 0954+556, which has no structure apparent on scales less than about 1 milliarcsec. Some have prominent jets, others show only faint secondary components or no non-core structure at all. Apparent velocities have been measured for 13 sources. These range from no detected motion to > 25c.

The blazar 3C 279 is one of the strongest extragalactic sources of γ-rays, and is also one of the best studied superluminal radio sources. Definitive testing of models of the broad-band spectral energy distribution, especially in the X-ray and γ-ray regions requires knowledge of the evolution of the spectrum with time. Within the context of the relativistic jet model, multi-wavelength monitoring of the parsec-scale radio jet is also required. We present here the first steps toward such a test, using extensive VLBI monitoring over a 13 year interval at 22 GHz, and γ-ray observations between 1991 and 1996.

The fine core-jet structure of 3C 345 and its low-frequency variability are discussed.

The highly redshifted quasar S5 0836+710 (z = 2.172) displays an outstanding one sided jet with complex morphology. The continuous jet shows several remarkable lateral displacements (kinks) of its ridgeline on various length scales. Despite the strength of these distortions the jet is not destroyed by them and remains well collimated. We investigated the possibility to explain the displacements by Kelvin-Helmholtz instabilities.

Imaging of the OVV quasar 3C 454.3 (z=0.859) shows that a new component was ejected from the core at epoch 1981 ± 1, close to the time of a major radio outburst. The component moved along a curved path with a mean apparent velocity of ~ 17c (Ho = 100 · h km s−1 Mpc−1). The motion, however, showed variations: from ~ 11c (1984–1985), to ~ 22c (1985–1988.5), then to ~ 14c (1988.5–1991.9).

Multi-epoch VLBI observations of the quasar 3C 380 reveal a bent parsec-scale radio jet with complex substructure and superluminal motion out to ~100 pc from the core.

We report on proper motions in the core, jet, and counter-jet of 3C 84, monitored by the VLBA at 2cm and 7mm.

Many FR 2 narrow-line radio galaxies also display polarized broad lines. The broad-line region is hidden from direct view and is seen by reflection (ie scattering). In these objects the spectral classification is controlled by the aspect at which they are viewed.

We report the first VLBA images of the nuclei in eight objects from the 3CR complete sample of 25 lobe-dominated quasars (LDQs): 3C 207, 3C 208, 3C 212, 3C 245, 3C 249.1, 3C 263, 3C 270.1, and 3C 275.1. These images reveal core-jet structures of considerable extent and complexity. In combination with earlier VLBI work on the 3CR LDQ sample, the partial distribution of apparent parsec-scale jet speeds for ten objects is relatively flat up to ~ 5h−1c, or perhaps even ~ 10h−1c. While this is not inconsistent with simple beaming models and quasar-FR II radio galaxy unification, it is crucial to obtain the complete speed distribution for truly definitive tests. Work is in progress on multi-frequency, dual-polarization, and phasereferencing VLBA observations on selected objects in the sample.

From VLBI observations of 11 FR I radio galaxies we find that: 1. parsec scale jets are relativistic; 2. 3C 264 shows a relativistic jet decelerating moving from the core to the extended lobes; 3. 3C 338 is a source with asymmetric parsec scale structure and morphological changes, implying proper motion on both sides of the source; 4. 1144+35 is an extended low power radio galaxy with an apparent superluminal motion in its parsec scale radio structure.

We have been observing the sub-parsec-scale radio jet in the nucleus of Centaurus A, the closest active radio galaxy, at 8.4 GHz over the last 6 years with the SHEVE array and most recently with the VLBA at 8.4 and 22.2 GHz. In this paper we will review the results of these observations and give a brief summary of our interpretation, concentrating on the evolution of the sub-parsec-scale jet structure with time; subluminal component motions and rapid short timescale evolution. A full description of this work is soon to be published in a major journal article.

We used VLA and MERLIN observations of 4 FRI radio galaxies to study the radio jet properties in the inner kiloparsecs from the core. We present evidence for relativistic motion in the inner regions.

We have observed the nucleus of Centaurus A with the VLBA and the SHEVE arrays, individually and in combination. In sensitive experiments at 8.4 GHz which use both arrays together we have detected components in a sub-parsec-scale counterjet. The counterjet is also seen in 2.3 and 22 GHz images. The nuclear region appears to be partially obscured by a free-free absorbing structure of about 1 pc in extent. The jet axis appears to lie at a 50°–70° angle to our line of sight.

The high energy gamma-ray flares observed in PKS 0528+134 are interpreted in terms of the external inverse Compton scattering (EICS) mechanism. The evolutional relationship between the gamma-ray flares and the associated mm-radio outbursts is investigated. The TeV/X-ray flare detected in May of 1994 from Mrk 421 is interpreted in terms of the SSC mechanism and it is shown that it may be due to the acceleration of relativistic electrons with an initially flat energy spectrum (N(E)∝E−s with s~1.5), rather than just a flattening of the high energy tail in the electron energy distribution of the source in the quiescent state.

We made a November 1995 observation of the close (33 arc sec) pair 1038+528 A,B using the VLBA+Effelsberg at 2.3 and 8.4 GHz, and the VLBA alone at 15 GHz. Our analysis of the precise astrometric separation between the core peak in A and a component ~ 1.8 mas from the core in B sheds new light on the observed changes with time and frequency.

We review VLBI polarization results. In particular, we discuss the a) “shock in jet paradigm”, b) the orientation of the magnetic field in jets as a function of optical identification, c) rotation measure and Faraday dispersion measurements as a probe of the narrow line region, and d) future directions of polarization observations. Results we emphasize are i) there is still a strong correlation between optical L/C ratio or EW and magnetic field orientation in the jets of blazars, even for high redshift weak-lined objects, ii) observed rotation measures are much smaller than expected from the properties of the NLR, except for some CSS sources. Also iii) a faint boundary layer or sheath (with a parallel magnetic field) has been observed around the jet of the weak-lined blazar 1055+018, and iv) circular polarization has been detected for the first time in the jets of 3C 84 and 3C 279.

We discuss the rotation measure properties of the CSS quasar 3C 309.1, determined from high dynamic range, large field of view VLBA images at several frequencies between 1.4 and 15 GHz. We consider the general properties of the ambient medium by considering the structure of the Faraday screen across various parts of the radio source.