G. D. Nicolson

PKS 0405–385: The Smallest Radio Quasar?

We have observed profound variability in the radio flux density of the quasar PKS 0405-385 on timescales of less than an hour; this is unprecedented among extragalactic sources. If intrinsic to the source, these variations would imply a brightness temperature TB~1021 K, some 9 orders of magnitude larger than the inverse Compton limit for a static synchrotron source, and still a million times greater than can be accommodated with bulk relativistic motion at a Lorentz factor γ~10. The variability is intermittent with episodes lasting a few weeks to months. Our data can be explained most sensibly as interstellar scintillation of a source component that is less than 5 μas in size—a source size which implies a brightness temperature TB>5×1014 K, still far above the inverse Compton limit. Simply interpreted as a steady, relativistically beamed synchrotron source, this would imply a bulk Lorentz factor γ~103.

The Subparsec-Scale Structure and Evolution of Centaurus A: The Nearest Active Radio Galaxy

?????The subparsec-scale structure of Cen A is complex, consisting of a bright jet and a fainter counterjet. The bright jet contains components that have subluminal speeds of approximately 0.1c and undergo irregular episodes of rapid internal evolution. The rapid evolution sometimes observed could be interpreted as evidence for an underlying jet flow much faster (>0.45c) than observed from the proper motion of components (~0.1c). Considering the large-scale morphology of the source, the motions and temporal variations in the jet, and the detection of a counterjet, we conclude that the axis of the Cen A jet lies between ~50? and ~80? to our line of sight. We find that the estimated times of component ejection from the compact core are reasonably coincident with enhancements in hard X-ray intensity and 22 GHz flux density. In the context of the radio galaxy population, Cen A is a low-luminosity FR I?type source and in general has the properties observed in other FR I radio galaxies. Overall, the observations of Cen A presented here, and from other investigations, are consistent with the idea that sources with an FR I appearance are not aligned with our line of sight and have relativistic flow on the subparsec scale. The apparently subluminal subparsec-scale jet components are interpreted as being slow patterns on the relativistic flow.

A Radio Reference Frame

A catalogue is presented based on the radio positions of 436 extragalactic sources distributed over the entire sky. The positional accuracy of the sources is better than 3 milliarcsec (mas) in both coordinates, with the majority of the sources having errors better than 1 mas. This catalogue is based upon a general solution of all applicable dual frequency 2.3 and 8.4 GHz Mark-III VLBI data available through the end of 1993 consisting of 1,015,292 pairs of group delay and phase delay rate observations. Details and positions are also given for an additional 124 objects that either need further observation or are currently unsuitable for the definition of a reference frame. The final orientation of the catalogue has been obtained by a rotation of the positions into the system of the International Earth Rotation Service and is consistent with the FK5 J2000.0 optical system, within the limits of the link accuracy.

Discovery of a Subparsec Radio Counterjet in the Nucleus of Centaurus A

A subparsec scale radio counterjet has been detected in the nucleus of the closest radio galaxy, Centaurus A (NGC 5128), with VLBI imaging at 2.3 and 8.4 GHz. This is one of the first detections of a VLBI counterjet and provides new constraints on the kinematics of the radio jets emerging from the nucleus of Cen A. A bright, compact core is seen at 8.4 GHz, along with a jet extending along P.A. 51°. The core is completely absorbed at 2.3 GHz. Our images show a much wider gap between the base of the main jet and the counterjet at 2.3 GHz than at 8.4 GHz and also that the core has an extraordinarily inverted spectrum. These observations provide evidence that the innermost 0.4-0.8 pc of the source is seen through a disk or torus of ionized gas which is opaque at low frequencies due to free-free absorption.

VLBI Observations of Southern Hemisphere ICRF Sources. I

We present 8.4 GHz very long baseline interferometry (VLBI) observations of 69 southern hemisphere extragalactic sources in the International Celestial Reference Frame. These are the first in a series of observations intended to image all such sources at milliarcsecond resolution in order to determine their continued suitability for reference-frame use based on intrinsic structure. We use the resultant images to calculate a core fraction, that is, the ratio of core flux density to total flux density, for all observed sources. The resulting distribution, with a mean value of 0.83, suggests that most sources are relatively compact. However, just over half the observed sources show significant extended emission in the form of multiple compact components. These sources are probably poorly suited for high-accuracy reference-frame use unless intrinsic structure and potential variability can be taken into account. Our observations represent the first large, comprehensive VLBI imaging survey in the southern hemisphere, significantly extending the existing limited VLBI surveys and, along with some well-known objects, containing many sources that have never been imaged at milliarcsecond resolution. The overlap with Very Long Baseline Array images of sources between 0° and -35° declination helps determine the limits to imaging with the southern hemisphere–accessible telescopes.

A 5 GHz Southern Hemisphere VLBI Survey of Compact Radio Sources. II.

We report the results of a 5 GHz Southern Hemisphere snapshot VLBI observation of a sample of blazars. The observations were performed with the Southern Hemisphere VLBI Network plus the Shanghai station in 1993 May. Twenty-three flat-spectrum, radio-loud sources were imaged. These are the first VLBI images for 15 of the sources. Eight of the sources are EGRET (>100 MeV) γ-ray sources. The milliarcsecond morphology shows a core-jet structure for 12 sources and a single compact core for the remaining 11. No compact doubles were seen. Compared with other radio images at different epochs and/or different frequencies, three core-jet blazars show evidence of bent jets, and there is some evidence for superluminal motion in the cases of two blazars. Detailed descriptions for individual blazars are given.

VLBI Observations of Southern EGRET Identifications. 1; PKS 0208-512, PKS 0521-365 and PKS 0537-441

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.

A speckle hologram of the interstellar plasma

Observations of a speckle hologram of scattering material along the line of sight to the Vela pulsar indicate that this material is concentrated in the Vela supernova remnant, deep within the Gum Nebula. The speckle hologram is observed through the amplitude and phase variations of the interferometric cross-power spectrum with time and frequency. These variations describe the density fluctuations of the interstellar plasma, in a holographic fashion. The decorrelation due to the phase variations of the speckles yields the angular size of the scattering disk; comparison with the bandwidth of their amplitude variations yields a characteristic distance from earth to the scattering material of 0.81 +/- 0.03 of the distance from earth to the pulsar. This result is consistent with theories of irregularities associated with particle acceleration in shocks in supernova remnants.

VSOP Space VLBI and Geodetic VLBI Investigations of Southern Hemisphere Radio Sources

We present images from VLBI Space Observatory Programme (VSOP) observations of 14 compact extragalactic southern hemisphere radio sources, including a description of the observations, the data reduction techniques, and the parameters of the resulting images and model fits. These images provide the highest resolution information to date for many of these objects. Comparisons are made between VSOP and previous ground-based VLBI results, including images from data extracted from the geodetic VLBI archive at the United States Naval Observatory. From the VSOP data, we find that the two radio galaxies observed have lower peak brightness temperatures than the 12 quasars. Also, these data show (1) no evidence for obvious differences between the brightness temperature distributions of gamma-ray-loud and gamma-ray-quiet radio-loud active galactic nuclei and (2) no evidence for obvious correlations between brightness temperature and spectral index, radio polarization, flux density, or month timescale modulation index. These results are consistent with previous work by Lister, Tingay, & Preston, who found that the only observable significantly correlated with VSOP-derived brightness temperature is intraday variability, which is strongly correlated with many relativistic beaming indicators. For one source, PKS 1127-145, we undertake a detailed investigation of the milliarcsecond-scale component positions as a function of time, taking data from the literature and the current work, to estimate proper motions. As a result, we suggest that two components previously reported as stationary, C1 and C2, have apparent transverse speeds of (9.1 ± 3.8) and (5.3 ± 2.3) h-1c, respectively. We also make the first investigation of the apparent motion in the nearest GHz-peaked spectrum radio galaxy, PKS 1718-649, finding an upper limit on the apparent separation speed of 0.08c. Comparison of geodetic VLBI and VSOP data show no significant detection of component motion in PKS 0208-512, (2.4 ± 3.1) h-1c, and only a tentative detection in PKS 0537-441, (2.8 ± 2.2) h-1c. A significant detection of component motion is found in PKS 1610-771, solely from the geodetic VLBI data, (9.4 ± 3.5) h-1c.

The Nearest GHz Peaked-Spectrum Radio Galaxy, PKS 1718-649

In this paper we identify PKS 1718-649, at a distance of 56 Mpc (z=0.014; H0=75 km s-1 Mpc-1, q0=O), 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. [ApJ (1997) (in press)] 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. (1997) 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.