We describe bright microwave events that were first detected with the Parkes 64-m telescope at 8.4 or 22 GHz from six active-chromosphere stars. In some flares spectral data were obtained over a large frequency range from simultaneous measurements with the Parkes reflector (8.4 or 22 GHz), the Tidbinbilla interferometer (8.4 and 2.29 GHz), the Fleurs synthesis telescope (1.42 GHz) and the Molonglo Observatory synthesis telescope (0.843 GHz). Data on circular polarization were obtained from the Parkes observations at 8.4 GHz.

The stars were in a wide variety of evolutionary states, ranging from a single pre-main-sequence star (HD 36705), two RS CVn binaries (HD 127535, HD 128171), an Algol (HD 132742) and two apparently single K giants (HD 32918 and HD 196818). Their high brightness temperatures, positive spectral indices and low polarization are consistent with optically thick gyrosynchrotron emission from mildly relativistic electrons with average energies 0.5 to 3 MeV gyrating in inhomogeneous magnetic fields of 5 to 100 G.

We present a list of bright (< 17 mag) southern QSOs and bright (< 11 mag) stars that may be suitable for the Hubble Space Telescope link between the Hipparcos astrometric reference frame and the VLBI extragalactic frame. The QSOs have been selected from various lists of radio objects and identifications. The stars have been selected from the Strasbourg (CDS) data base and from the Preliminary Second Cape Photographic Catalogue, and supplemented with stars measured from the SERC IIIa-J Schmidt survey. The list of QSOs and stars have been included in the Hipparcos and HST schedule of observations.

Radio measurements of the outer planets at wavelengths longer than ~ 10 cm are difficult: the emission is weak (generally following an optically thick thermal spectrum — i.e. S ∝ λ-2) and the confusion due to background sources may be large.

We present here the low-dispersion optical spectra of 295 QSO candidates. The great majority of the objects were originally selected as QSOs from the Parkes 2700 MHz radio survey, although we have also included spectra of several optically selected QSOs. A few of the QSO candidates are now better described as radio galaxies and BL Lac objects. This collection of spectra is not suitable for statistical studies unless due consideration is given to selection effects.

Radio position measurements with an error of <2” arc rms allow reliable optical identifications of compact radio sources to be made solely on the basis of radio-optical position coincidence. In this way neutral or red stellar objects, faint compact galaxies and faint QSOs can be reliably identified. Such identifications are of particular interest because they are rich in BL Lac objects, high-redshift QSOs, QSOs with unusual optical emission or absorption spectra and galaxies with active nuclei (see Jauncey et al. 1978).

We present an overview of the survey for radio emission from active stars that has been in progress for the last six years using the observatories at Fleurs, Molonglo, Parkes and Tidbinbilla. The role of complementary optical observations at the Anglo-Australian Observatory, Mount Burnett, Mount Stromlo and Siding Spring Observatories and Mount Tamborine are also outlined. We describe the different types of star that have been included in our survey and discuss some of the problems in making the radio observations.

The recent upgrading of the Tidbinbilla two-element interferometer to simultaneous S-band (2.3 GHz) and X-band (8.4 GHz) operation has provided a powerful new astronomical facility for weak radio source measurement in the Southern Hemisphere. The new X-band system has a minimum fringe spacing of 38 arcsec, and about the same positional measurement capability (approximately 2 arcsec) and sensitivity (1 s rms noise of 10 mJy) as the previous S-band system. However, the far lower confusion limit will allow detection and accurate positional measurements for sources as weak as a few millijanskys. This capability will be invaluable for observations of radio stars, X-ray sources and other weak, compact radio sources.

The ‘holographic’ technique for accurately measuring the surface figure of large reflector antennas, described by Bennet et al, (1976) and Scott and Ryle (1977), has many advantages over older conventional survey methods. These include high speed, low cost, and the absence of any need for additional complex mechanical or optical survey devices. In essence, the technique consists of measuring the complex far-field response of the antenna at a single frequency using a terrestrial, satellite-borne or celestial radiation source of small angular diameter. This two-dimensional pattern is then Fourier-transformed to yield the complex illumination function across the antenna aperture. Antenna surface deviations are manifested as phase fluctuations in this function. In practice, a second antenna is needed to provide a phase reference.

The primary function of the 64-m telescope, situated at Tidbinbilla near Canberra, is to communicate with and to control NASA’s unmanned interplanetary space craft (Reid et al. 1973). Similar telescopes are situated at Goldstone in California and in Madrid and are part of the NASA-JPL Deep Space Network (DSN). Under the Host Country Radio Science Agreement between the U.S. and Australia the telescope is available to Australian astronomers at times outside those committed to normal tracking. A radiometer operating at λ = 13.5 mm has been built by CSIRO for use on the 64-m telescope. It was initially used for VLBI observations of water vapour masers on baselines between the U.S., Australia and USSR (Batchelor et al. 1976).

Savage et al. (1977) found that the radio source PKS 1448-232 coincided with a stellar object of about magnitude 16.4 having an ultraviolet excess. A low resolution spectrum obtained with the Anglo-Australian Telescope (AAT) confirmed this object as a QSO with zem = 2.22 and revealed many absorption lines short-ward of the La emission. Consequently this object was included in a programme of spectroscopy at intermediate resolution with the AAT to investigate QSO absorption lines. Savage et al. have given a finding chart with an optical position of 14h48m09s.3, −23°17′10″ (1950.0). The radio fluxes are 0.40 Jy at 2.7 GHz and 0.31 Jy at 5.0 GHz.

Following the ‘prompt’ radio outburst seen soon after the neutrino emission in SN 1987A (Turtle et al. 1987), we initiated a program to monitor the supernova at the Tidbinbilla Deep Space Communications Complex at 8.4 GHz in a search for radio emission from the expanding remnant. No radio emission has been detected to date (DOY 151, May 30 1988).

During its burst of intraday variation (IDV) in June 1996 quasar PKS 0405–385 exhibited rapid changes in the Stokes parameters Q and U that were even faster than the 1 hr timescale changes in I. Auto- and cross-correlation analyses of I,Q, and U at 8.6 GHz show systematic relations between the variations in I,Q, and U. These are well modelled by interstellar scintillation (ISS) of a 14 × 20 μ as source, with about 180 degree rotation of the polarisation angle along its long dimension. This success in explaining the remarkable IDV in polarisation confirms that ISS gives rise to the IDV in this quasar.

This triennium began with an action to re-create the Terms of Reference for the Working Group Global VLBI (WG-GV). These had been lost over the years since the Group was established in 1990. Fortunately, the personal archive of one long-term member yielded a copy of the original memorandum by R. D. Ekers, which was found to coincide quite well with current practice and areas of interest. New Terms of Reference, based on modern conditions, were drafted and accepted by both IAU and URSI.

PKS 1257—326 is one of three quasars known to show unusually large and rapid, intra-hour intensity variations, as a result of scintillation in the turbulent Galactic interstellar medium. We have measured time delays in the variability pattern arrival times at the VLA and the ATCA, as well as an annual cycle in the time-scale of variability for this source. Results of the two-station time delay observations are presented here. Implications for the scintillation of this source are discussed in the light of these results, together with results from two years of monitoring with the ATCA.

We discuss conditions which have to be satisfied for observable signatures of diffractive scintillation in intraday variable (IDV) sources. We apply this discussion to the quasar PKS 0405-385 which had a scintillating component with angular size of 5 μas (Kedziora-Chudczer et al. 1997).

We discuss the detection of variable circular polarization (CP) in several scintillating radio sources: PKS 1519-273, PKS 0405-385 and Sgr A*. In all cases the CP is variable, with the fractional variability in the CP greatly exceeding that in total intensity.

The accumulation of evidence now strongly favours interstellar scintillation (ISS) as the principal mechanism causing intra-day variability (IDV) at cm wavelengths. While ISS reduces the implied brightness temperatures, they remain uncomfortably high. The distance to the scattering screen is an important parameter in determining the actual brightness temperature encountered. The high brightness temperatures, the presence of strong and variable circular polarization and the observed lifetimes of a decade or more for several IDV sources, pose significant problems for synchrotron theory.

Intra-day variability (IDV) of active galactic nuclei (AGN) has been detected from gamma-ray energies to radio wavelengths. At high energies, such variability appears to be intrinsic to the sources themselves. However, at radio wavelengths, brightness temperatures as high as 1018 to 1021 K are encountered if the IDV is intrinsic to the source. We discuss here the accumulating evidence showing that, at radio wavelengths where the highest brightness temperatures are encountered, interstellar scintillation (ISS) is the principal mechanism causing IDV. While ISS reduces the implied brightness temperatures, they still remain uncomfortably high.

We present results of the ATCA IDV Survey of southern extragalactic radio sources. We discuss briefly the properties of the 22 new intraday variable sources discovered in the Survey. The follow-up observations of a few extreme examples of strong intraday variability are presented. We find that the characteristics of the total flux density fluctuations at different wavelengths are consistent with intersteller scintillations (ISS) of the microarcsecondsize soorten components. However, the scintillating components of a few extreme IDVs are characterized by the brightness temperatures far exceeding the TB = 1012 K limit. The relativistic beaming invoked in such sources would require Doppler factors up to as high as δ ~ 103.

A full version of this paper was presented at Workshop on Optical Surveys for Quasars (Tucson, January 1988) and will be published in the Proceedings.