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Earth-space interferometry with RadioAstron provides the highest direct angular resolution ever achieved in astronomy at any wavelength. RadioAstron detections of the classic quasar 3C273 on interferometric baselines up to 171,000 km suggest brightness temperatures exceeding expected limits from the "inverse-Compton catastrophe" by two orders of magnitude. We show that at 18 cm, these estimates most probably arise from refractive substructure introduced by scattering in the interstellar medium. We use the scattering properties to estimate an intrinsic brightness temperature of 7*10^12 K, which is consistent with expected theoretical limits, but which is ~15 times lower than estimates that neglect substructure. At 6.2 cm, the substructure influences the measured values appreciably but gives an estimated brightness temperature that is comparable to models that do not account for the substructure. At 1.35 cm, the substructure does not affect the extremely high inferred brightness temperatures, in excess of 10^13 K. We also demonstrate that for a source having a Gaussian surface brightness profile, a single long-baseline estimate of refractive substructure determines an absolute minimum brightness temperature, if the scattering properties along a given line of sight are known, and that this minimum accurately approximates the apparent brightness temperature over a wide range of total flux densities., Comment: 6 Pages, 3 Figures, Accepted for publication in ApJ Letters

Continuing our program of spectroscopic observations of ICRF sources, we present redshifts for 120 quasars and radio galaxies. Data were obtained with five telescopes: the 3.58m ESO New Technology Telescope (NTT), the two 8.2m Gemini telescopes, the 2.5m Nordic Optical Telescope (NOT), and the 6.0m Big Azimuthal Telescope (BTA) of the Special Astrophysical Observatory in Russia. The targets were selected from the International VLBI Service for Geodesy & Astrometry (IVS) candidate International Celestial Reference Catalogue which forms part of an observational VLBI program to strengthen the celestial reference frame. We obtained spectra of the potential optical counterparts of more than 150 compact flat-spectrum radio sources, and measured redshifts of 120 emission-line objects, together with 19 BL Lac objects. These identifications add significantly to the precise radio--optical frame tie to be undertaken by Gaia, due to be launched in 2013, and to the existing data available for analysing source proper motions over the celestial sphere. We show that the distribution of redshifts for ICRF sources is consistent with the much larger sample drawn from FIRST and SDSS, implying that the ultra-compact VLBI sources are not distinguished from the overall radio-loud quasar population. In addition, we obtained NOT spectra for five radio sources from the FIRST and NVSS catalogs, selected on the basis of their red colors, which yielded three quasars with z>4., Comment: Accepted to AJ 2013

Intraday variability (IDV) of the radio emission from active galactic nuclei is now known to be predominantly due to interstellar scintillation (ISS). The MASIV (The Micro-Arcsecond Scintillation-Induced Variability) survey of 443 flat spectrum sources revealed that the IDV is related to the radio flux density and redshift. A study of the physical properties of these sources has been severely handicapped by the absence of reliable redshift measurements for many of these objects. This paper presents 79 new redshifts and a critical evaluation of 233 redshifts obtained from the literature. We classify spectroscopic identifications based on emission line properties, finding that 78% of the sources have broad emission lines and are mainly FSRQs. About 16% are weak lined objects, chiefly BL Lacs, and the remaining 6% are narrow line objects. The gross properties (redshift, spectroscopic class) of the MASIV sample are similar to those of other blazar surveys. However, the extreme compactness implied by ISS favors FSRQs and BL Lacs in the MASIV sample as these are the most compact object classes. We confirm that the level of IDV depends on the 5\,GHz flux density for all optical spectral types. We find that BL Lac objects tend to be more variable than broad line quasars. The level of ISS decreases substantially above a redshift of about two. The decrease is found to be generally consistent with ISS expected for beamed emission from a jet that is limited to a fixed maximum brightness temperature in the source rest frame., Comment: Accepted for publication by the Astrophysical Journal

We present deep {\it HST, Chandra, VLA} and {\it ATCA} images of the jets of PKS 0208--512 and PKS 1202--262, which were found in a {\it Chandra} survey of a flux-limited sample of flat-spectrum radio quasars with jets (see Marshall et al., 2005). We discuss in detail their X-ray morphologies and spectra. We find optical emission from one knot in the jet of PKS 1202--262 and two regions in the jet of PKS 0208--512. The X-ray emission of both jets is most consistent with external Comptonization of cosmic microwave background photons by particles within the jet, while the optical emission is most consistent with the synchrotron process. We model the emission from the jet in this context and discuss implications for jet emission models, including magnetic field and beaming parameters., Comment: 13 pages, 11 figures, ApJ in press

The remarkable rapid variations in radio flux density and polarization of the quasar PKS 0405-385 observed in 1996 are subject to a correlation analysis, from which characteristic time scales and amplitudes are derived. The variations are interpreted as interstellar scintillations. The cm wavelength observations are in the weak scintillation regime for which models for the various auto- and cross-correlations of the Stokes parameters are derived and fitted to the observations. These are well modelled by interstellar scintillation (ISS) of a 30 by 22 micro-as source, with about 180 degree rotation of the polarization angle along its long dimension. This success in explaining the remarkable intra-day variations (IDV)in polarization confirms that ISS gives rise to the IDV in this quasar. However, the fit requires the scintillations to be occurring much closer to the Earth than expected according to the standard model for the ionized interstellar medium (IISM). Scattering at distances in the range 3-30 parsec are required to explain the observations. The associated source model has a peak brightness temperature near 2.0 10^{13}K, which is about twenty-five times smaller than previously derived for this source. This reduces the implied Doppler factor in the relativistic jet, presumed responsible to 10-20, high but just compatible with cm wavelength VLBI estimates for the Doppler factors in Active Galactic Nuclei (AGNs)., Comment: 43 pages 15 figures, accepted for ApJ Dec 2002

VLBI observations of intraday variable (IDV) quasars found in the MASIV (Micro-Arcsecond Scintillation-Induced Variability) 5 GHz VLA Survey of 500 flat-spectrum sources in the northern sky have shown that these sources are extremely compact, often unresolved, on milliarcsecond scales, and more core-dominated than their non-IDV counterparts. VAST: an ASKAP Survey for Variables and Slow Transients, proposes to observe 10,000 square degrees of southern sky daily for 2 years in the VAST-Wide survey component. This is expected to reveal of order 30,000 compact sources brighter than 10 mJy showing refractive interstellar scintillation (the cause of centimeter-wavelength IDV) at the survey frequency of about 1.4 GHz. Many of these sources may be suitable astrometric calibrators for VLBI at higher frequencies.

A sub-parsec scale radio counterjet has been detected in the nucleus of the closest radio galaxy, Centarus A (NGC 5128), with VLBI imaging at 2.3 and 8.4 GHz.

Accurate radio positions with a precision of about 0.01 arcsec are reported for eight compact extragalactic radio sources south of -45-deg declination. The radio positions were determined using VLBI at 8.4 GHz on the 9589 km Tidbinbilla (Australia) to Hartebeesthoek (South Africa) baseline. The sources were selected from the Parkes Catalogue to be strong, flat-spectrum radio sources with bright optical QSO counterparts. Optical positions of the QSOs were also measured from the ESO B Sky Survey plates with respect to stars from the Perth 70 Catalogue, to an accuracy of about 0.19 arcsec rms. These radio and optical positions are as precise as any presently available in the far southern sky. A comparison of the radio and optical positions confirms the estimated optical position errors and shows that there is overall agreement at the 0.1-arcsec level between the radio and Perth 70 optical reference frames in the far south.

A third list of stars is presented which have been tested by speckle interferometry for use in the Hubble Space Telescope link between the Hipparcos astrometric reference frame and the extragalactic VLBI reference frame. Structural information on angular scales of 0.15-1.2 arcsec for 34 Southern Hemisphere stars is reported from observations made with the Imperial College Speckle Interferometer mounted on the Mount Stromlo 1.9-m telescope. Twenty-four percent of the stars (8 out of the 34) show evidence of multiplicity, in agreement with previous observations in this program.

A list of bright (less than 17 mag) southern QSOs and bright (less than 11 mag) stars that may be suitable for the Hubble Space Telescope link between the Hipparcos astrometric reference frame and the VLBI extragalactic frame is presented. 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 Catalog, and supplemented with stars measured from the SERC IIIa-J Schmidt survey. These QSOs and stars have been included in the Hipparcos and HST schedule of observations.

A complete sample, taken from the Parkes 2.7 GHz catalog, of flat-spectrum radio sources with flux densities greater than 0.5 Jy is described. The sample covers all right ascensions and declinations for +10 deg to -45 deg, excluding the Galactic plane, and contains 403 sources. Attention is drawn to the advantages of radio surveys over optical surveys. The survey is used to highlight some selection effects found in optical surveys. How this sample can be used to give information on the early universe is discussed.

Optical identifications are presented for 158 radio sources, mostly from the Southern Hemisphere, based on the coincidence between the position of the optical object and the compact milliarcsecond radio nucleus. Radio positions with an accuracy of typically 0.3 arcsec rms were measured from the observed delay and fringe rate of VLBI observations at 2.29 GHz on an Australia-to-South Africa baseline. Optical identifications and positions were measured from the UK Schmidt Telescope deep IIIa-J Southern Sky Survey plates, where available.

VLBI observations of the nucleus of Centaurus A have been made at two frequencies with an array of five Australian radio telescopes as part of the Southern Hemisphere VLBI Experiment. Observations were made at 2.3 GHz with all five antennas, while only two were employed at 8.4 GHz. At 2.3 GHz seven tracks in the (u,v) plane with coverage of 6-8 hr each were obtained, yielding significant information on the structure of the nuclear jet. At 8.4 GHz a compact unresolved core was detected as well. It is found that the source consists of the compact self-absorbed core, a jet containing a set of three knots extending from 100 to 160 mas from the core, and a very long, narrow component elongated along the same position angle as the knots. The allowable range for the position angle of the jet is 51 + or - 3 deg, in agreement with that of the radio and X-ray structure on arcsecond and arcminute scales. The jet has brightened at 2.3 GHz by about 4 Jy, a factor of nearly 3, since the early 1970s, 1.8 Jy of which has occurred in the last 2 yr with no discernable changes in structure.

Six radio telescopes were operated as the first Southern Hemisphere VLBI array in April and May 1982. Observations were made at 2.3 and 8.4 GHz. This array provided VLBI modeling and hybrid imaging of celestial radio sources in the Southern Hemisphere, high-accuracy VLBI geodesy between Southern Hemisphere sites, and subarcsecond radio astrometry of celestial sources south of declination -45 deg. The goals and implementation of the array are discussed, the methods of modeling and hybrid image production are explained, and the VLBI structure of the sources that were observed is summarized.