The bright radio nucleus 3C84 of the peculiar galaxy NGC1275 was among the first sources studied by VLBI techniques sixteen years ago (Clark et al., 1968). Early observations at short centimeter wavelengths using arrays of up to four telescopes (Schilizzi et al., 1975, and references therein) demonstrated the presence of structure significantly more complex than that seen in other strong compact objects, but were insufficient to determine the brightness distribution with the relatively primitive methods then in use. By the time of the most recent international VLBI symposium (Aug. 1978, in Heidelberg, FRG), Pauliny-Toth et al. (1976) had succeeded in modelling the structure seen by a 4-station transatlantic array, revealing three distinct bright emission regions aligned in position angle −9°, and connected by non-colinear, more diffuse features. Further observations and a reanalysis of earlier data by Preuss et al. (1979) confirmed previous inferences that the three major condensations remained stationary but varied in brightness.

Models are here presented interpreting the arcsecond radio structure found in the quasar 3C418 as a precessing jet. No simple solution exists and it has been found necessary to complicate the model geometry by allowing the precession cone-angle to increase with time. The timescales suggest the presence of a close binary black-hole system within the quasar core. The modelled behaviour of the precession cone close to the core is explained either by interaction of the jet with the surrounding galactic material or by the observation of the source during the capture of one black-hole by the other.

We have observed the nucleus of the elliptical galaxy M87 with VLBI arrays at 18 cm wavelength at two epochs separated by about two years. The results of the first epoch observations taken at 1980.12 are described in detail by Reid et al (1982, Ap. J. 263, pp 615-623). They indicated that the nucleus of M87 exhibited a highly asymmetric emission pattern with a bright “core” and long, thin “jet” extending for more than 0.05 arcsec towards the 20 arcsec jet seen in radio, optical, and X-ray radiation. This morphology is remarkably similar to that seen in the quasar 3C273. Asymmetric core-jet structures, possibly coupled with super-luminal motions as in 3C273, have been interpreted in the framework of relativistic beaming models (cf. Blandford, McKee, and Rees: 1977, Nature 267, pp 211–216). Therefore, we conducted second epoch observations of M87 at 1982.27 to look for such motions.

Continuing a long project of investigations of the optical morphology of the M87 jet (see review in Nieto, 1983), we have obtained at the Cassegrainan focus of the CFH telescope in March 1983 three UV exposures with the wide-field electronographic camera (Lallemand et al., 1970). The aim was to achieve the same order of resolution as that of the photographic plates obtained by Nieto and Lelièvre (1982), but with a much higher signal to noise ratio.

3C454.3 has a core-jet structure on the milliarcsec scale. Following a flux density outburst, the core showed a “superluminal brightening”. Comparison with older observations suggests superluminal motion in the jet. 2134+004, in contrast, is a double source, with no significant (v/c < 1) relative motion of the components.

Observations with arcsecond-resolution interferometers can be used to detect motion or changes of the large scale structure associated with core dominated objects. If these arcsecond structures are expanding at relativistic velocities, detectable changes should be measureable within a few years. Indirect estimates of jet velocities can be made by a number of arguments, but none are model independent or free of dependence upon poorly established parameters. Application of some of these methods are given on sources with established VLB structure.

The importance of studying the statistics of the occurrence of superluminal motion in an unbiased sample of radio sources is stressed, and methods for selecting such samples are reviewed. The weak radio cores of quasars exhibiting extended, double-lobed emission can be used for this purpose. Recent studies of the superluminal motion in the core of 3C 179, a quasar selected from such a sample, are described.

We present Mark III VLBI results for a sample of weak cores associated with quasars with extended double lobes. The m.a.s. scale structure is found to be well aligned with the outer structure.

The maps that we will show you today probably have the lowest resolution of any that you will see this week. They were made from data taken by the Westerbork Synthesis Radio Telescope (WSRT), which has a maximum baseline of 2.8 km, yielding a resolution of 3″.5 (λ 6 cm), 12″.5 (λ 21 cm) or 29″ (λ 49 cm). Because the WSRT is an EW-array the resolution in declination is worse by a factor 1/sin δ. This is unfortunate since three interesting superluminal sources are close to the equator. For two of these, namely 3C120 and 3C273 we nevertheless have obtained some interesting results.

The flux variability of extragalactic radio sources at decimetric wavelengths (L.F.V.) is now considered a classical astrophysical subject connected with compact radio sources.

A wide range of models have been proposed to account for low-frequency (≲ 1 GHz) variability in cosmologically-distant radio sources. To address these models we (Payne et al. 1982) are monitoring the 0.3–1.4 GHz spectra of such sources. Results from three years of monitoring indicate that two distinct types of low-frequency variables may exist.

In Table I we present the list of 38 celestial objects that have been observed since January 1978 at 2.7 and 8.1 GHz with the Green Bank interferometer. The sources fall naturally into three categories: radio stars, possibly Galactic sources, and extragalactic sources. SS433, Cyg X-3, and each extrgalactic source is measured several times per day while the other sources are measured once every three days. Reports on the entire program will be found in Geldzahler et al. (1983a), and on specific sources: SS433—Johnston et al. (1983a), BL Lac—Johnston et al. (1983b), Cyg X-3—Geldzahler et al. (1983b) and elsewhere in this volume), and CTA 26—Spencer et al. (1983).

Optical and radio behaviours of OJ 287 in the recent outburst are presented. The light curve is similar to that in the previous outburst in 1970–1971, although the maximum brightness is about 1 mag fainter. The intraday variations are found in both optical and radio regions. We find no periodicity of time scales less than 0.2 days.

Total flux observations of variable sources (Andrew et al. 1978) have been re-examined to test their consistency, or lack of it, with the Doppler beaming hypothesis of Scheuer and Readhead (1979).

Multifrequency observations of a variable extragalactic object, when all acquired within the inferred variability time scale of the source, can provide clues to the source's energy mechanisms. A guest observer program with the International Ultraviolet Explorer satellite has provided the focus for such measurements of a few BL Lac-type and related objects at frequencies in the radio, mm, IR, visual and UV. Earlier-epoch X-ray measurements are included in subsequent model fitting. This paper summarizes some of the observations in this program. Synchrotron self-Compton (SSC) models are then applied to the data, leading to the conclusions that the objects are relativistically beamed and that radio emission, at least below a frequency of ~20 GHz, is from a separate source region.

I shall compare the luminosities in the X-ray, optical, and radio wavebands of two types of radio source - compact flat-spectrum sources and extended double sources - and attempt to place limits on models which unify these types.

Is the theme of core-brightening by relativistic beaming, so useful for compact sources, helpful in explaining the nuclear properties of classical doubles? Certainly one can construct an attractive model - involving the usual beaming together with an anisotropic dust distribution at r ≲ 1 pc - to explain the differences in nuclear radio and optical properties of radiogalaxies as due differences in orientation with respect to us. There are clear predictions about the strength of broad Pα. Preliminary results indicate that this kind of model is not correct.

I present some results of a study of the statistics of bent jets. The general conclusion is that non relativistic jets appear less bent than they really are, while relativistic jets appear more bent.

The observed superluminal components have (deprojected) lengths of ~ 1020 cm, and imply relativistic bulk motions on these scales. There are, however, persuasive reasons for attributing the primary energy production to scales 1014–1015 cm. Moreover, the initial bifurcation and collimation must also be imposed on these small scales if the long-term stability of the jet axis in extended sources is due to the gyroscopic effect of a spinning black hole (Rees 1978). The issues I shall address in this talk are: how the jet gets from ~ 1015cm to ~ 1019 cm; and what VLBI data can tell us about the properties of galactic nuclei on scales below ~ 1019 cm — scales where optical and X-ray studies provide some evidence, but where there is no short-term hope of achieving spatial resolution.