The cm-λ properties of BL Lac objects are discussed.

The compact steep-spectrum source 3C 119 has been mapped using the VLBA at four frequencies spread over the 5 GHz observing band. The high rotation-measure reported at short wavelengths is found to continue unbroken beyond the depolarization wavelength indicating that 3C 119 is depolarized by an external medium.

We discuss the preliminary results from radio polarization observations of 22 compact steep spectrum sources made with the VLA at 1.3 cm wavelength. We confirm that 1) the detection of a flat-spectrum synchrotron halo and a radio bridge in two CSSs, suggesting that they may be young sources, 2) CSS galaxies with two lobes of emission asymmetric to the core position show strong polarization asymmetry with the highest polarized emission arising from the component farther away from the core and 3) Source components which are nearest to the cores are generally unpolarized even at such short wavelength, suggesting that the spread in the λ2 polarization spectra is much narrower than in extended sources.

We present multi-frequency VLBA polarimetric observations of a small sample of quasars revealing their Faraday rotation measure (RM) structure at ~ 1 mas resolution. Two of the sources (OQ 172 and 3C 216) were chosen on the basis of high RMs seen by the VLA and single dish telescopes. A few more typical quasars (3C 273, 3C 279, 3C 345 and 3C 380) were also included in the study. In OQ 172, 3C 273, 3C 279 and 3C 380 we find high RMs (>3000 rad m−2) associated with the nuclear region. Only 1-10 mas from the nucleus the jet |RM|s in all 4 of these sources fall to less than 100 rad m−2. This RM structure may be produced by strong magnetic fields in the the narrow line region of the quasar.

We have performed simultaneous multi-frequency polarization VLBA observations of the compact radio sources 3C 395 and 4C 39.25 which show both stationary and superluminal components in their parsec-scale structures. Those of 3C 395 have been reported elsewhere. Here we report on high resolution maps of the total intensity and polarized radio emission of 4C 39.25, trace the magnetic field configuration along the jet, and explore different possibilities for the nature of the components within the framework of the bent shocked relativistic jet model.

VLBA polarimetric observations at 8.15 and 8.54 GHz of the quasar 3C 287 are presented and discussed. The source structure is complex, and the magnetic field geometry does not help in the interpretation of the source.

We present 1.3 cm and 7 mm VLBA observations of the radio galaxy 3C 120 at epochs November 11 and December 22, 1996. The 7 mm maps, with linear resolution of ~0.1 pc, show a very rich structure consisting of up to eight superluminal (~ 7 c) components.

The orthodox model of active galactic nuclei (AGN), as is generally accepted, is that of Rees. We have found an AGN (3C 147) which has a counter-jet much longer than the main jet. It also has an unusual sideways ejection from the nuclear region. Both of these and the 3-jet structure per se do not seem to conform with the general accepted model, and are also not explained by relativistic beaming effects. We speculate that this is due to coronal mass ejection (CME) in the accretion disk corona. Recently, we have found convincing new evidence that this is true. We have also found direct evidence of helical magnetic fields in the core region of 3C 147. These results may be of importance to the understanding of AGN.

Circular polarization has been detected in 3C 279 and 3C 84 at multiple epochs, using the VLBA at 15 GHz. This is the first detection of circular polarization in a compact radio source at VLBI resolution. In the case of 3C 279 the polarization is most likely produced by the conversion of linear to circular polarization by low energy relativistic electrons in the emitting region. If this is so, the absence of Faraday rotation by the same electrons suggests that the jet consists of an electron-positron plasma.

Parsec-scale polarization observations of the quasar 4C 71.07 show a misalignment between the inferred magnetic field direction and the structural direction of the jet. Dual frequency VLBI polarimetry was used to to determine whether Faraday rotation was the origin of the misalignment. We find insufficient rotation to realign the field and jet. Other possible mechanisms which may result in a misalignment are discussed.

We have obtained 15 and 22 GHz linear polarization VLBA images of a sample of strong and rapidly variable polarized sources at six epochs over one year. We observe rapid changes, in both cores and jet components, in total intensity, both with and without associated changes in polarization. Changes in the cores are probably associated with the births of new components. Images of one quasar (3C 273) and one BL Lacertae object (OJ 287) from two epochs are shown here. No significant Faraday rotation is observed at any epoch in any position in either source.

VLBA+Effelsberg observations of the radio galaxy 3C 111 at 7 mm show remarkable rapid changes in structure which cannot be described in terms of simple component motions or changes in the structure of fixed components.

We observed the four OH 18 cm transitions against the Cen A VLBI source. With high spatial resolution we detect the same molecular absorption as previously seen with ATCA. Both the excitation and VLBI absorption suggest a cloud size of ≲ 1 pc.

Multi-frequency VLBA observations were used to study the magnitude and radial dependence of free-free absorption of the radio emission from the counterjet in 3C 84.

The maximum brightness temperature limit for synchrotron sources derived from inverse Compton catastrophe is usually taken to be 1012 K. Readhead has argued in 1994 that the equipartition brightness temperature may be a better estimate for Tb,lim. It provides a value of 5 × 1010 K. We suggest that a reasonable estimate of the value of Tb,lim can also be achieved by comparing Doppler boosting factors from total flux density variations at 22 and 37 GHz with traditional Doppler factors from the SSC X-ray flux. We also compare our variability brightness temperatures with values calculated for individual sources observed with VLBI, obtaining other independent estimates of Tb,lim.

We examine the effects of Doppler beaming on flux-limited samples of compact extragalactic radio sources using Monte Carlo simulations. We incorporate a luminosity function and z-distribution for the parent population, and investigate models in which the unbeamed synchrotron luminosity L of a relativistic jet is related to its bulk Lorentz factor Γ. The predicted flux density, redshift, monochromatic luminosity, and apparent velocity distributions of our simulated flux-limited samples are compared to the Caltech-Jodrell Bank (CJF) sample of flat-spectrum, radio core-dominated active galactic nuclei (AGNs).

We find that a relation between L and Γ is not needed to reproduce the characteristics of the CJF sample. Introducing a positive correlation between these quantities results in an underabundance of objects with high viewing angles, while a negative correlation gives generally poor fits to the data.

We present simulated maps showing the appearance in total intensity of flows computed using a relativistic hydrodynamic code (Duncan k. Hughes 1994: ApJ, 436, L119). The radiation transfer calculations allow for full treatment of relativistic effects, such as Doppler boosting and time delay. Depending on viewing angle, and the speed of emitting features, the appearance of a jet can be strongly influenced by either effect; in both cases the map differs dramatically from the morphology exhibited by the hydrodynamic quantities.

In this paper we report the first results of the 327 MHz VLBI survey which has been carried out in the last two years to select potential candidate sources for the future SVLBI missions and to search for directions of small scattering-“holes” in the interstellar scattering medium. During the three VLBI sessions conducted so far, we observed about 80 sources. Preliminary data analyzed from the first part of the survey shows that at least 17 out of 50 sources have compact components with a size smaller than 30 mas.

The VSOP Survey Program will observe a complete survey of flat-spectrum sources at 5 GHz. In order to determine which of the sample sources are strong and small enough to be detected with VSOP, a pre-launch survey of 377 continuum sources north of δ > −43° was conducted at 5 GHz with the VLBA over 24 hours in June 1996. About 250 of the sources had correlated flux densities ≥ 0.4 Jy between MK–SC and have been included in the VSOP survey sample.

We have selected a field from the VLA FIRST Survey which is typical in all aspects except one: it contains a bright, but extremely compact VLBI calibrator - J1159+291. 127 unresolved FIRST sources with SFIRST ≥ 10 mJy lie within 2.5° of this calibrator. λ6 cm MERLIN observations with a resolution ~ 60 mas detect around half the sources. These detections form the basis of a sample of faint but compact radio sources which are ideally suited to follow-up VLBI observations.