| Compact radio sources | Supersoft sources | Pulsar interferometry | PKS 1718-649 |

Radio interferometry research

ATCA Monitoring Observations of 202 Compact Radio Sources in Support of the VSOP AGN Survey

The Australia Telescope Compact Array (ATCA) has been used in support of the VLBI Space Observatory Programme (VSOP) space VLBI mission, to monitor the total and polarised flux densities of 202 objects that make up 83% of the VSOP all-sky Survey of compact extragalactic radio sources south of +10°. The primary goal of the ATCA observations is to provide information on the total and polarised emission from the compact components in these sources, for correlation with parameters obtained from VSOP imaging observations. These data represent the first high resolution, long timescale flux density monitoring observations of a large number of southern compact radio sources. In the future, comparison of the ATCA and VSOP data will be used to investigate relativistic beaming models and identify similarities or differences between the major classes of extragalactic radio sources. As an illustration of the scientific value of the ATCA data we undertake a comparison of the properties of the gamma-ray loud and gamma-ray quiet AGN in the southern component of the VSOP Survey sample, finding that in a flat-spectrum sub-sample the gamma-ray loud AGN are more variable than the gamma-ray quiet AGN.

This work has been published by Tingay, S.J. et al., in the April 25, 2003, issue of the Publications of the Astronomical Society of Japan (volume 55). The links below contain a summary of the results of this work, in the form of radio light-curves for 185 radio sources at frequencies of 1.4, 2.5, 4.8, and 8.6 GHz. These light-curves are fully described in the aforementioned publication. Please refer to this publication if citing this work.

Supersoft X-ray sources

Supersoft X-ray sources are believed to consist of a white dwarf and sub-giant companion with a high accretion rate. The large accretion rate onto the white dwarf in these objects creates steady hydrogen burning on the white-dwarf surface thus creating the X-ray emission. We have undertaken a deep survey of all Northern Hemisphere supersoft sources within the VLA field of view.

While these sources are rare (only four are known to exist in this area of sky) we have detected one source: AG Dra. Using both the VLA and a follow-up high-resolution with MERLIN we confirmed a double radio-source structure (Ogley et al. 2001). Secondary observations have been taken combining the high-resolution of Merlin with the VLA to bring out any extended detail, we aim to investigate any nebulosity in this source.

Wide Bandwidth Pulsar Interferometry

A new PhD is being offered to measure Southern Hemisphere radio pulsar positions, velocities and parallaxes. These will help to establish the pulsar velocity distribution, distance scale and tie the radio and planetary reference frames. Simulations will be carried out to examine the Galactic population of pulsars and address the potential of the SKA for pulsar astronomy.

For this and other pulsar topics, the Swinburne Pulsar Group has much information.

An investigation of synchrotron self-absorption and free-free absorption models in explanation of the GHz-peaked spectrum of PKS 1718-649

The Australia Telescope Compact Array (ATCA) has been used to make the most detailed investigation yet of the shape and variability of a GHz-peaked spectrum (GPS) radio source. The source in question, PKS 1718-649, is one of the nearest GPS sources. This investigation attempts to distinguish between the two main competing physical models for the spectral turnover in GPS sources, synchrotron self-absorption (SSA) and free-free absorption (FFA). The interpretation of the ATCA data shows that while both SSA and FFA can explain aspects of the data, neither can convincingly explain the observed flux density variability below the turnover frequency.

This work has been accepted for publication in The Astronomical Journal, and has an associated table of flux densities.

New GHz-Peaked Spectrum (GPS) radio sources

The results of the ATCA monitoring observations of the VSOP AGN Survey sources (see above) have been used to identify a number of new GPS sources in the Southern Hemipshere, including the equal closest GPS source identified to date, PKS B2254-367 (Tingay, Edwards, & Tzioumis 2003, MNRS, in press). A number of other newly identified GPS and CSS sources in the Southern Hemisphere are described in Edwards & Tingay (2003, A&A, in press).

The high resolution study of free-free absorbed radio sources in NGC 253

Wide-field VLBI imaging techniques have been used with an array of Australian radio telescopes to produce the first high resolution image of NGC 253 at 1.4 GHz, comparable in resolution to 22 GHz imaging with the VLA. This image allows detection of compact sources of radio emission in NGC 253, a very nearby starburst galaxy. Many of these sources are supernova remnants. The image, in comparison to higher frequency images of NGC 253 allows measurements of the broadband radio spectra of the supernova remnants, showing that they lie behind a spatially variable screen of partially ionised gas that causes absorption of radio emission at low frequencies. This work is described in Tingay (2004, AJ, in press).