E. A. King

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.

Astrometric Detection of a Low Mass Companion Orbiting the Star AB Doradus

We report submilliarcsecond-precise astrometric measurements for the late-type star AB Doradus via a combination of VLBI (very long baseline interferometry) and HIPPARCOS data. Our astrometric analysis results in the precise determination of the kinematics of this star, which reveals an orbital motion readily explained as caused by gravitational interaction with a low-mass companion. From the portion of the reflex orbit covered by our data and using a revised mass of the primary star (0.76 M☉) derived from our new value of the parallax (66.3 mas < π < 67.2 mas), we find the dynamical mass of the newly discovered companion to be between 0.08 and 0.11 M☉. If accurate photometric information can be obtained for the low-mass companion, our precise mass estimate could serve as an accurate calibration point for different theoretical evolutionary models of low-mass objects. This represents the first detection of a low-mass stellar companion using VLBI, a technique that will become an important tool in future searches for planets and brown dwarfs orbiting other stars.

A sample of southern Compact Steep Spectrum radio sources: The VLBI observations

A small sample of 7 southern Compact Steep Spectrum (CSS) radio sources has been selected as part of the study of a larger flux-limited complete sample of radio sources. High resolution images, using the VLBI network in the southern hemisphere and the high resolution MERLIN array, are presented for all sources in the CSS sample. The overall morphology of each source consists of well-defined double lobes but with substantial diffuse and extended components present. In the majority of cases only a fraction of the total flux density is detected on the VLBI baselines, indicating the presence of larger extended radio structures. However, all sources are unresolved at arcsecond scales and are of sub-galactic size, with linear size in the range 0.1-2 kpc. The radio properties of the sources agree well with CSS sources in other samples.

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.

Size of the Vela Pulsar's Radio Emission Region: 500 Kilometers

We use interstellar scattering of the Vela pulsar to determine the size of its emission region. We find that radio-wave scattering in the Vela supernova remnant broadens the source by 3.3 ± 0.2 mas × 2.0 ± 0.1 mas, with the major axis at a position angle of 92° ± 10°. From the modulation of the pulsars scintillation, we infer a size of 500 km for the pulsars emission region, with an estimated uncertainty of about a factor of 2, including systematic errors. We suggest that radio-wave refraction within the pulsars magnetosphere may plausibly explain this size.

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.

Evidence of a Low-Mass Companion to AB Doradus

A b s t r a c t . 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.

Southern Hemisphere VLBI Observations of the Expansion of GRO J1655–40

Abstract. We present high-resolution radio observations of the second Galactic superluminal radio source GRO1655-40, which was detected as an X-ray transient on 1994 July 27. Our radio radio images reveal two components moving away from each other at an angular speed of 65±5 mas/day, corresponding to superluminal motion (v/c= 1.4 ± 0.4) 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. A complete description and discussion of these observations can be found in Tingay et al 1995 (Nature, 374, pp 141-143).