Search

We report measurements of time delays of up to 8 minutes in the centimeter wavelength variability patterns of the intra-hour scintillating quasar PKS 1257-326 as observed between the VLA and the ATCA on three separate epochs. These time delays confirm interstellar scintillation as the mechanism responsible for the rapid variability, at the same time effectively ruling out the coexistence of intrinsic intra-hour variability in this source. The time delays are combined with measurements of the annual variation in variability timescale exhibited by this source to determine the characteristic length scale and anisotropy of the quasar's intensity scintillation pattern, as well as attempting to fit for the bulk velocity of the scattering plasma responsible for the scintillation. We find evidence for anisotropic scattering and highly elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio > 10:1, extended in a northwest direction on the sky. The characteristic scale of the scintillation pattern along its minor axis is well determined, but the high anisotropy leads to degenerate solutions for the scintillation velocity. The decorrelation of the pattern over the baseline gives an estimate of the major axis length scale of the scintillation pattern. We derive an upper limit on the distance to the scattering plasma of no more than 10 pc., 27 pages, 6 figures, accepted for publication in ApJ

We are undertaking a large-scale, Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey of the northern sky, Dec > 0 deg, at 4.9 GHz with the VLA. Our objective is to construct a sample of 100 to 150 scintillating extragalactic sources with which to examine both the microarcsecond structure and the parent populations of these sources, and to probe the turbulent interstellar medium responsible for the scintillation. We report on our first epoch of observations which revealed variability on timescales ranging from hours to days in 85 of 710 compact flat-spectrum sources. The number of highly variable sources, those with RMS flux density variations greater than 4% of the mean, increases with decreasing source flux density but rapid, large amplitude variables such as J1819+3845 are very rare. When compared with a model for the scintillation due to irregularities in a 500 pc thick electron layer, our preliminary results indicate maximum brightness temperatures ~10E+12 K, similar to those obtained from VLBI surveys even though interstellar scintillation is not subject to the same angular resolution limit., 18 pages, 5 figures. To appear in the Astronomical Journal

The Micro-arcsecond Scintillation-Induced Variability (MASIV) Survey and its follow-up observations have provided large datasets of AGN intra-day variability (IDV) at radio wavelengths. These data have shown that IDV arises mainly from scintillation caused by scattering in the ionized interstellar medium (ISM) of our Galaxy, based on correlation with Galactic latitudes and line-of-sight Galactic electron column densities. The sensitivity of interstellar scintillation (ISS) towards source angular sizes has provided a new tool for studying the most compact components of radio-loud AGNs at microarcsecond (μas) scale resolution - much higher than any ground-based radio interferometer. We present here key results from the MASIV Survey and its follow-up observations, and point to relevant papers where these results have been published.

Interstellar scintillation (ISS) has been shown to be primarily responsible for the short term intraday variability (IDV) exhibited by extragalactic sources at centimeter wavelengths (e.g. Bignall et al. 2006 and references therein). For a source to scintillate its angular size must be comparable to that of the first Fresnel zone (Narayan 1992) which implies microarcsecond angular sizes for screen distances of tens to hundreds of parsecs. This has the potential to probe within a few light months of the central black hole (Bignall et al. 2006). The aim of the Microarcsecond Scintillation-Induced Variability (MASIV) survey was to provide a catalogue of at least a hundred AGNs that vary on timescales of hours to days to provide the basis of detailed studies of the IDV population drawn from a well-defined sample.

The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact (milliarcsecond) components located on opposite sides of the ring. We have obtained 22 GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components. Previous VLBI observations at lower frequencies indicate that the brightness temperatures of these components are significantly lower than 10E10 K, less than is typical for compact synchrotron radio sources and less than is implied by the short timescales of flux density variations. A possible explanation is that interstellar scattering is broadening the apparent angular size of the source and thereby reducing the observed brightness temperature. Our VLBA data support this hypothesis. At 22 GHz the measured brightness temperature is at least 10E11 K, and the deconvolved size of the core in the southwest compact component is proportional to the wavelength squared between 1.3 cm (22 GHz) and 18 cm (1.7 GHz). VLBI observations at still higher frequencies should be unaffected by interstellar scattering., Accepted by Ap.J. (Letters). Paper includes three postscript figures

We present the results of a radio variability study of the gravitational lens PMN J1838-3427. Our motivation was to determine the Hubble constant by measuring the time delay between variations of the two quasar images. We monitored the system for 4 months (approximately 5 times longer than the expected delay) using the Australia Telescope Compact Array at 9 GHz. Although both images were variable on a time scale of a few days, no correlated intrinsic variability could be identified, and therefore no time delay could be measured. Notably, the fractional variation of the fainter image (8%) was greater than that of the brighter image (4%), whereas lensed images of a point source would have the same fractional variation. This effect can be explained, at least in part, as the refractive scintillation of both images due to the turbulent interstellar medium of the Galaxy., To appear in AJ (8 pages, including 4 figures)

We have used the NASA Tidbinbilla 70-m antenna to search for emission from the 2(1)-3(0)E 19.9-GHz transition of methanol. The search was targeted towards 22 star formation regions which exhibit maser emission in the 107.0-GHz 3(1)-4(0)A+ methanol transition, as well as in the 6.6-GHz 5(1)-6(0)A+ transition characteristic of class II methanol maser sources. A total of 7 sources were detected in the 19.9-GHz transition, 6 of these being new detections. Many of the new detections are weak (< 0.5 Jy), however, they appear to be weak masers rather than thermal or quasi-thermal emission. We find a strong correlation between sources which exhibit 19.9-GHz methanol masers and those which both have the class II methanol masers projected against radio continuum emission and have associated 6035-MHz OH masers. This suggests that the 19.9-GHz methanol masers arise in very specific physical conditions, probably associated with a particular evolutionary phase. In the model of (Cragg, Sobolev & Godfrey 2002) these observations are consistent with gas temperatures of 50 K, dust temperatures of 150-200 K and gas densities of 10^6.5-10^7.5 cm^-3., 14 pages, 12 figures, landscape table in additional file, Accepted for publication in MNRAS

We present results from Chandra X-ray imaging and spectroscopy of a flux-limited sample of flat spectrum radio-emitting quasars with jet-like extended structure. Twelve of twenty quasar jets are detected in 5 ks ACIS-S exposures. The quasars without X-ray jets are not significantly different from those in the sample with detected jets except that the extended radio emission is generally fainter. New radio maps are combined with the X-ray images in order to elucidate the relation between radio and X-ray emission in spatially resolved structures. We find a variety of morphologies, including long straight jets and bends up to 90 degrees. All X-ray jets are one-sided although the radio images used for source selection often show lobes opposite the X-ray jets. The FR II X-ray jets can all be interpreted as inverse Compton scattering of cosmic microwave background photons by electrons in large-scale relativistic jets although deeper observations are required to test this interpretation in detail. Applying this interpretation to the jets as a population, we find that the jets would be aligned to within 30 degrees of the line of sight generally, assuming that the bulk Lorentz factor of the jets is 10., Comment: 25 pages with 5 pages of color figures; accepted for publication in the Astrophysical Journal Supplements; higher resolution jpeg images are available at http://space.mit.edu/home/jonathan/jets/

The recent discovery of radio components apparently moving away from a Galactic source of transient X-ray emission faster than the speed of light (superluminal motion) has identified a low-energy Galactic counterpart to quasars. Here we report high-resolution radio observations of a second Galactic superluminal radio source GRO J1655-40, which was detected as an X-ray transient on 27 July 1994. Our radio images reveal two components moving away from each other at an angular speed of 65 ± 5 mas d -1 , corresponding to superluminal motion at the estimated distance of 3–5 kpc. The 12-day delay between the X-ray and radio outbursts suggests that the ejection of material at relativistic speeds occurs during a stable phase of accretion onto a black hole, which follows an unstable phase with a high accretion rate.

Rapid radio intra-day variability (IDV) has been discovered in the southern quasar PKS 1257-326. Flux density changes of up to 40% in as little as 45 minutes have been observed in this source, making it, along with PKS 0405-385 and J1819+3845, one of the three most rapid IDV sources known. We have monitored the IDV in this source with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.6 GHz over the course of the last year, and find a clear annual cycle in the characteristic time-scale of variability. This annual cycle demonstrates unequivocally that interstellar scintillation is the cause of the rapid IDV at radio wavelengths observed in this source. We use the observed annual cycle to constrain the velocity of the scattering material, and the angular size of the scintillating component of PKS 1257-326. We observe a time delay, which also shows an annual cycle, between the similar variability patterns at the two frequencies. We suggest that this is caused by a small (~10 microarcsecond) offset between the centroids of the 4.8 and 8.6 GHz components, and may be due to opacity effects in the source. The statistical properties of the observed scintillation thus enable us to resolve source structure on a scale of ~10 microarcseconds, resolution orders of magnitude higher than current VLBI techniques allow. General implications of scintillation for the physical properties of sources and the turbulent ISM are discussed., Comment: 28 pages, 7 figures. Accepted for publication in the Astrophysical Journal

The discovery that interstellar scintillation (ISS) is suppressed for compact radio sources at z ≳ 2 has enabled ISS surveys to be used as cosmological probes. We discuss briefly the potential and challenges involved in such an undertaking, based on a dual-frequency survey of ISS carried out to determine the origin of this redshift dependence.

In 1998 September, the X-ray transient XTE J1550–564 underwent amajor outburst in soft and hard X-rays, followed by a radio flare. Australian Long Baseline Array imagesobtained shortly after the peak in the radio flare showed evolving structure.The components observed have an apparent separation velocity of >2c.

We report the discovery of a gravitationally lensed quasar resulting from our survey for lenses in the southern sky. Radio images of PMN J1632-0033 with the VLA and ATCA exhibit two compact, flat-spectrum components with separation 1.47" and flux density ratio 13.2. Images with the HST reveal the optical counterparts to the radio components and also the lens galaxy. An optical spectrum of the bright component, obtained with the first Magellan telescope, reveals quasar emission lines at redshift 3.42. Deeper radio images with MERLIN and the VLBA reveal a faint third radio component located near the center of the lens galaxy, which is either a third image of the background quasar or faint emission from the lens galaxy., Comment: 21 pp., including 4 figures; thoroughly revised in light of new MERLIN/HST data; accepted for publication in AJ

The radio-loud quasar PMN J0134-0931 was discovered to have an unusual morphology during our search for gravitational lenses. In VLA and MERLIN images, there are 5 compact components with maximum separation 681 millarcseconds. All of these components have the same spectral index from 5 GHz to 43 GHz. In a VLBA image at 1.7 GHz, a curved arc of extended emission joins two of the components in a manner suggestive of gravitational lensing. At least two of the radio components have near-infrared counterparts. We argue that this evidence implies that J0134-0931 is a gravitational lens, although we have not been able to devise a plausible model for the foreground gravitational potential. Like several other radio-loud lenses, the background source has an extraordinarily red optical counterpart., Comment: Accepted for publication in ApJ. 19 pp., 5 figs. See also the preprint astro-ph/0107441 by Gregg et al., who discovered this object independently

We present the first VLBI Space Observatory Programme (VSOP) observations of the gamma-ray blazar 3C 279 at 1.6 and 5 GHz. The combination of the VSOP and VLBA-only images at these two frequencies maps the jet structure on scales from 1 to 100 mas. On small angular scales the structure is dominated by the quasar core and the bright secondary component `C4' located 3 milliarcseconds from the core (at this epoch). On larger angular scales the structure is dominated by a jet extending to the southwest, which at the largest scale seen in these images connects with the smallest scale structure seen in VLA images. We have exploited two of the main strengths of VSOP: the ability to obtain matched-resolution images to ground-based images at higher frequencies and the ability to measure high brightness temperatures. A spectral index map was made by combining the VSOP 1.6 GHz image with a matched-resolution VLBA-only image at 5 GHz from our VSOP observation on the following day. The spectral index map shows the core to have a highly inverted spectrum, with some areas having a spectral index approaching the limiting value for synchrotron self-absorbed radiation of 2.5. Gaussian model fits to the VSOP visibilities revealed high brightness temperatures (>10^{12} K) that are difficult to measure with ground-only arrays. An extensive error analysis was performed on the brightness temperature measurements. Most components did not have measurable brightness temperature upper limits, but lower limits were measured as high as 5x10^{12} K. This lower limit is significantly above both the nominal inverse Compton and equipartition brightness temperature limits. The derived Doppler factor, Lorentz factor, and angle to the line-of-sight in the case of the equipartition limit are at the upper end of the range of expected values for EGRET blazars., 11 pages, 6 figures, emulateapj.sty, To be published in The Astrophysical Journal, v537, Jul 1, 2000

High angular resolution images of extragalactic radio sources are being made with the Highly Advanced Laboratory for Communications and Astronomy (HALCA) satellite and ground-based radio telescopes as part of the Very Long Baseline Interferometry (VLBI) Space Observatory Programme (VSOP). VSOP observations at 1.6 and 5 gigahertz of the milli–arc-second–scale structure of radio quasars enable the quasar core size and the corresponding brightness temperature to be determined, and they enable the motions of jet components that are close to the core to be studied. Here, VSOP images of the gamma-ray source 1156+295, the quasar 1548+056, the ultraluminous quasar 0014+813, and the superluminal quasar 0212+735 are presented and discussed.

We have measured a time delay of 26 (+4/-5) days and a magnification ratio of 1.52 +/- 0.05 in the strong radio gravitational lens PKS 1830-211. The observations were made over the 18 month period from 1997 January to 1998 July with the Australia Telescope Compact Array at 8.6 GHz, and have shown that the source started a large flux density outburst around 1997 June., Comment: 10 pages including 2 figures. To be published in ApJ Letters

We have observed, using very long baseline interferometry (VLBI), some EGRET-identified AGN in the Southern Hemisphere: PKS 0208−512; PKS 0521−365; PKS 0537−441; PKS 1127−145; PKS 1622−253; PKS 1622−297; PKS 1730−130; and PKS 1908−201 as well as some prominent, compact southern radio sources which are not gamma-ray loud: PKS 0438−438; PKS 0637−752; and PKS 1921−293. We are searching for radio wavelength properties on parsec-scales which can distinguish between gamma-ray loud and gamma-ray quiet AGN. Such properties might include relativistic beaming indicators such as parsec-scale jet proper motions and radio core brightness temperatures, as well as parsec-scale to kiloparsec-scale jet bend angles.

From several southern hemisphere radio-wavelength monitoring programs, we have compiled multi-wavelength observations of a number of both EGRET-detected sources and sources with similar radio-properties that have not been detected by EGRET. The radio observations were made at 0.8, 2, 5, 8, 90 and 230 GHz, though not all sources were monitored at all wavelengths. The data are compared with EGRET data to the end of Cycle 4 (October 1995) in an effort to confirm previously reported correlations between activity at radio wavelengths and EGRET detections, and to investigate whether the non-EGRET-detected sources were in radio- and gamma-ray–quiet phases during this period.

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., Accepted for publication by the Astrophysical Journal

Measurements of the properties of gravitational lenses have the power to tell us what sort of universe we live in. The brightest known radio Einstein ring/gravitational lens PKS 1830-211 (Jauncey et al., 1991), whilst obscured by our Galaxy at optical wavelengths, has recently been shown to contain absorption at the millimetre waveband at a redshift of 0.89 (Wiklind and Combes, 1996a). We report the detection of a new absorption feature, most likely due to neutral hydrogen in a second redshift system at z = 0.19. Follow-up VLBI observations have spatially resolved the absorption and reveal it to cover the NE compact component and part of the lower surface brightness ring. This new information, together with existing evidence of the unusual VLBI radio structure and difficulties in modeling the lensing system, points to the existence of a second lensing galaxy along our line of sight and implies that PKS 1830-211 may be a compound gravitational lens., 8 pages, 2 figures, LaTeX (aasms4.sty). Accepted for publication in ApJ Letters. Preprint also available at http://kerr.phys.utas.edu.au/preprints/

A sample of 38 southern peaked-spectrum radio sources from the Parkes Catalogue have been observed using single-baseline VLBI. Thirty three objects were successfully detected on baselines of > 30 Mλ at 2.3 GHz. For 21 of these sources, the flux density in the compact components contributes more than half the total flux density of the radio emission. Twenty sources showed structure more complex than a point-source.

We are undertaking an imaging survey with the Australia Telescope Compact Array (ATCA) to find gravitational lens candidates in flat-spectrum Parkes Catalogue radio sources. Flat-spectrum radio sources typically possess a single high brightness temperature nucleus of milliarcsecond size. Such sources, if lensed, will show multiply imaged nuclei with separations that are large compared to their milliarcsecond sizes. Our flat-spectrum sample was selected using the criteria α2.7/5.0 > −0.5 (S(ν) ∝ να), S 2.7 > 0.34Jy and δ ≤ −20°, and comprises a total of 461 sources.

Here we present very long baseline interferometry observations of the Galactic radio and X-ray source GRO J1655-40. These observations show that the radio source which appeared approximately two weeks after the initial X-ray outburst consisted of two prominent components which separated with an apparent speed of 1.5±0.4c. When the various possibilities for the geometry of the radio source are taken into account the apparent speed implies an intrinsic speed between 0.5c and 0.9c.

We present high-resolution very long baseline interferometry images of three southern radio sources that the Energetic Gamma-Ray Experiment Telescope (EGRET), on board the Compton Gamma Ray Observatory, has identified as greater than 100 MeV gamma-ray sources. These are the first results in a continuing program of VLBI observations of southern EGRET identifications. For two of these sources, PKS 0208-512 (at 4.851 GHz) and PKS 0537-441 (at 4.851 and 8.418 GHz), the images represent first-epoch observations. For the remaining lower redshift object, PKS 0521-365, we present images from three epochs at 4.851 GHz and an image from one further epoch at 8.418 GHz, spanning approximately 1 yr. We discuss the need for further extensive VLBI observations of EGRET-identified radio sources.

We present dual-frequency, co-eval very long baseline interferometry observations of the sub-parsec-scale radio source in Centaurus A which identify the core of the radio source, the origin of the parsec-scale and kiloparsec-scale radio emission. In addition we present multi-epoch observations at a frequency of 8.4 GHz which show the complex evolution of the sub-parsec-scale morphology over a 3.3 year period. Subluminal motion is seen and structural changes observed on time-scales shorter than four months.

We have been observing the strong radio source and Einstein ring gravitational lens PKS 1830–211 as part of a flux density monitoring program at 2.3 and 8.4 GHz using the 26 m antenna at the Mt Pleasant Observatory. As can be seen from Figure 1 this source is variable at both frequencies. The 2.3 GHz flux density decreased from 10 Jy in 1990 November to a minimum of 9 Jy during 1992, and has since steadily increased to its present ∼12 Jy. Over the same interval, the flux density at 8.4 GHz has exhibited greater variability, more than doubling from an initial value of 5 Jy in late 1990 to a peak of ∼11 Jy early in 1992, before decreasing to a level of ∼7.5 Jy. VLA observations by van Ommen et al. (1995) show that the 15 GHz total flux density increased before the 8.4 GHz brightening seen at Mount Pleasant. Although the 15 GHz time series is not well sampled, it is clear from these data that the 15 GHz outburst precedes the 8.4 GHz feature by ∼400 days.

We present a series of 8.4-GHz very-long-baseline radio interferometry images of the nucleus of Centaurus A (NGC5128) made with a Southern Hemisphere array, representing a 3.3-year monitoring effort. The nuclear radio jet is approximately 50 milliarcseconds in extent, or at the 3.5-megaparsec distance of NGC5128, approximately 1 parsec in length. Subluminal motion is seen and structural changes are observed on time scales shorter than 4 months. High-resolution observations at 4.8 and 8.4 GHz made in November 1992 reveal a complex morphology and allow us to unambiguously identify the self-absorbed core located at the southwestern end of the jet.

Rapid variability in the radio flux density of the BL Lac object PKS B1144–379 has been observed at four frequencies, ranging from 1.5 to 15 GHz, with the Very Large Array and the University of Tasmania's Ceduna antenna. Intrinsic and line-of-sight effects were examined as possible causes of this variability, with interstellar scintillation best explaining the frequency dependence of the variability timescales and modulation indices. This scintillation is consistent with a compact source 20-40 μas or 0.15-0.3 pc in size. The inferred brightness temperature for PKS B1144–379 (assuming that the observed variations are due to scintillation) is 6.2 × 1012 K at 4.9 GHz, with approximately 10% of the total flux in the scintillating component. We show that scintillation surveys aimed at identifying variability timescales of days to weeks are an effective way to identify the active galactic nuclei with the highest brightness temperatures.

The Southern Hemisphere VLBI Experiment (SHEVE) program is aimed at producing high-resolution images of southern radio sources. The radio telescopes of the present SHEVE array are described below and some recent results presented.

The extreme, intra-hour and > 10% rms flux density scintillation observed in AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has been attributed to scattering in highly turbulent, nearby regions in the interstellar medium. Such behavior has been found to be rare. We searched for rapid scintillators among 128 flat spectrum AGNs and analyzed their properties to determine the origin of such rapid and large amplitude radio scintillation. The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two hour intervals over 11 days. We detected six rapid scintillators with characteristic time-scales of 10%. We found strong lines of evidence linking rapid scintillation to the presence of nearby scattering regions, estimated to be 11 day variations, suggesting that the highly turbulent cloud responsible for its extreme scintillation has moved away, with its scintillation now caused by a more distant screen ~ 50 to 150 pc away.

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., 13 pages, 11 figures, ApJ in press

The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z > 2, indicating an apparent increase in angular diameter or a decrease in flux density of the most compact components of these sources, relative to their extended emission. This can result from intrinsic source size effects or scatter broadening in the Intergalactic Medium (IGM), in excess of the expected (1+z)^0.5 angular diameter scaling of brightness temperature limited sources due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations and data analysis for a sample of 140 compact, flat-spectrum sources which may allow us to determine the origin of this angular diameter-redshift relation by exploiting their different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide additional insight into source morphologies and the characteristics of ISS. As in the MASIV Survey, the variability of the sources is found to be significantly correlated with line-of-sight H-alpha intensities, confirming its link with ISS. For 25 sources, time delays of about 0.15 to 3 days are observed between the scintillation patterns at both frequencies, interpreted as being caused by a shift in core positions when probed at different optical depths. Significant correlation is found between ISS amplitudes and source spectral index; in particular, a large drop in ISS amplitudes is observed at spectral indices of < -0.4 confirming that steep spectrum sources scintillate less. We detect a weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2 sources relative to the z < 2 sources, as opposed to the factor of 3 decrease observed at 4.9 GHz. This suggests scatter broadening in the IGM., 30 pages, 14 figures, accepted for publication in the Astronomical Journal

Centaurus A is the closest active galactic nucleus. High resolution imaging using Very Long Baseline Interferometry (VLBI) enables us to study the spectral and kinematic behavior of the radio jet-counterjet system on sub-parsec scales, providing essential information for jet emission and formation models. Our aim is to study the structure and spectral shape of the emission from the central-parsec region of Cen A. As a target of the Southern Hemisphere VLBI monitoring program TANAMI (Tracking Active Galactic Nuclei with Milliarcsecond Interferometry), VLBI observations of Cen A are made regularly at 8.4 and 22.3 GHz with the Australian Long Baseline Array (LBA) and associated telescopes in Antarctica, Chile, and South Africa. The first dual-frequency images of this source are presented along with the resulting spectral index map. An angular resolution of 0.4 mas x 0.7 mas is achieved at 8.4 GHz, corresponding to a linear scale of less than 0.013 pc. Hence, we obtain the highest resolution VLBI image of Cen A, comparable to previous space-VLBI observations. By combining with the 22.3 GHz image, which has been taken without contributing transoceanic baselines at somewhat lower resolution, we present the corresponding dual-frequency spectral index distribution along the sub-parsec scale jet revealing the putative emission regions for recently detected gamma-rays from the core region by Fermi/LAT. We resolve the innermost structure of the milliarcsecond scale jet and counterjet system of Cen A into discrete components. The simultaneous observations at two frequencies provide the highest resolved spectral index map of an AGN jet allowing us to identify multiple possible sites as the origin of the high energy emission.

The VSOP Survey Program contains 402 bright, small diameter extragalactic radio sources. Pre-launch observations indicated that 113 sources were probably undetectable on space-ground baselines, and so VSOP observations are being undertaken at 5 GHz of the remaining 289 sources. Progress to date is described.

We present results of the first space VLBI observations of PKS 1921-293. An inner jet component about 1.5 mas north of the core is revealed for the first time. The compact core is partially resolved, but still has a brightness temperature (at the source rest frame) of 3.0×1012 K. A spectral index map made by combining the 1.6 GHz VSOP image with the 5.0 GHz VLBA+Y image at the first epoch is also presented.

We have identified a new class of object that we term PRONGS (powerful radio objects nested in galaxies with star formation). These are powerful radio sources whose optical properties are that of spiral/star-forming galaxies, unlike classic powerful radio sources which are typically hosted by elliptical galaxies in the local Universe. Here we present a first look at these enigmatic sources.

The first VSOP (VLBI Space Observatory Programme) observations of the quasar 3C279 at 1.6 and 5 GHz were conducted in 1998 January. The quasar core and a secondary component 3 milli-arcseconds away dominate the compact radio emission, with synchrotron self-absorption in the core resulting in the jet component appearing brighter at 1.6 GHz.

Radio-wave scattering in the Vela supernova remnant acts as an imperfect lens to resolve the pulsar’s radio emission region. We use this lens to measure the pulsar’s emission region. We suggest that refraction of radiation within the pulsar’s magnetosphere is responsible for the observed size.

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.

PKS 1830–211 is the strongest known radio gravitational lens by almost an order of magnitude and has the potential to provide a measurement of H0, provided the lensing system can be parameterized. Attempts to identify optical counterparts, to measure redshifts, have so far proved unsuccessful and this has lead to radio and millimetre spectral line observations. We present our discovery of an absorption system at z = 0.19. A brief description is also made of our ATCA observations to measure the lensing time delay for this source.

From the combination of VLBI phase-referenced observations and Hipparcos satellite data, we have found evidence of a low-mass object orbiting the late-type star AB Doradus. The mass of the new object is near the hydrogen burning limit and will constitute a precise point for calibrating the low end of the main sequence. This represents the first detection of a low-mass stellar companion using the VLBI technique, which could become an important tool in future searches for planets and brown dwarfs orbiting other stars.

We have undertaken VLBI observations of 8 Southern Hemisphere EGRET radio sources. Using our data as well as data obtained from the literature we have examined the difference in radio properties between gamma-ray loud and gamma-ray quiet radio sources. In particular, we find no evidence that gamma-ray loud radio sources lie preferentially in sources with straight radio jets as has been suggested.

PKS 1934–638 is an archetypal GPS source, peaking at 1.4 GHz and exhibits almost no flux density variability. VLBI images at frequencies of .843, 2.3, 4.8, & 8.4 were made with the southern hemisphere VLBI array and they reveal that the source is a 42 mas compact double. There is no detectable change in separation over the last 20 years, yielding an upper limit of ~ 0.03c ± 0.2c on any expansion velocity. The spectral shapes of the two components are remarkably similar, despite indications of finer structure on longer baselines. Magnetic field calculations indicate fields of a few mGauss and the results are consistent with equipartition.

PKS 1718–649 has a classic GPS spectrum and is identified with the galaxy NGC 6328. At a distance of 56 Mpc, PKS 1718–649 becomes the nearest known GPS radio source.

Centaurus A is the closest active extragalactic radio source, at a distance of approximately 3.5 Mpc, and is identified with the peculiar elliptical galaxy NGC 5128. As such it is a very important target for observations of the small-scale (sub-parsec) and large-scale (kpc) structures in extragalactic jets. Here we present Mk-II VLBI observations made at 8.4 GHz over a 4.3 year period from early 1991 until mid-1995, as well as a 4.8 GHz observation that was co-eval with one of the 8.4 GHz observations. All of the observations were made with the SHEVE array except for the last observation which was made with the VLBA. The dual-frequency observations identify the core of the radio source, while the multi-epoch observations show the complex structural evolution at a resolution of 0.1 pc. Subluminal motion of ≈ 0.15c is evident. Structural changes are observed on time scales shorter than four months.

In this paper we identify PKS 1718-649, at a distance of 56 Mpc (z = 0.014; H(sub o) = 75 km/s/Mpc, q(sub o) = 0), as the nearest GHz peaked-spectrum (GPS) radio galaxy, more than four times closer than any previously known. Extensive observations at radio wavelengths with the Australia Telescope Compact Array, the Southern Hemisphere VLBI Experiment array, and the Swedish-ESO Submillimetre Telescope have allowed us to determine the properties of the radio source: PKS 1718-649 consists of two compact sub-pc-scale components separated by approximately 2 pc, the overall radio polarization is low, and the radio spectrum is peaked near 3 GHz. Order-of-magnitude agreement between the quantitative model for GPS sources of Bicknell et al. and the radio data we present, as well as data at optical wavelengths from the literature, raises the interesting possibility that PKS 1718-649 may be frustrated in its development by the nuclear environment of its host galaxy, NGC 6328. The model of Bicknell et al. suggests free-free absorption as an explanation of the PKS 1718-649 radio spectrum. However, both free-free absorption and synchrotron self-absorption mechanisms are plausible for this source and both may contribute to the overall radio spectrum. PKS 1718-649 provides evidence to strengthen the speculative suggestion that GPS sources arise as a consequence of galaxy merger activity.

We present high-resolution very long baseline interferometry images of three southern radio sources that the Energetic Gamma-Ray Experiment Telescope (EGRET), on board the Compton Gamma Ray Observatory, has identified as greater than 100 MeV gamma-ray sources. These are the first results in a continuing program of VLBI observations of southern EGRET identifications. For two of these sources, PKS 0208-512 (at 4.851 GHz) and PKS 0537-441 (at 4.851 and 8.418 GHz), the images represent first-epoch observations. For the remaining lower redshift object, PKS 0521-365, we present images from three epochs at 4.851 GHz and an image from one further epoch at 8.418 GHz, spanning approximately 1 yr. We discuss the need for further extensive VLBI observations of EGRET-identified radio sources.