Mark A. Walker

Science with the Australian Square Kilometre Array Pathfinder

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

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 properties of extragalactic radio sources selected at 20 GHz

We present some first results on the variability, polarization and general properties of radio sources selected at 20 GHz, the highest frequency at which a sensitive radio survey has been carried out over a large area of sky. Sources with flux densities above 100 mJy in the Australia

The Australia Telescope 20-GHz (AT20G) Survey: the Bright Source Sample

The Australia Telescope 20-GHz (AT20G) Survey is a blind survey of the whole southern sky at 20GHz (with follow-up observations at 4.8 and 8.6 GHz) carried out with the Australia Telescope Compact Array from 2004 to 2007. The Bright Source Sample (BSS) is a complete flux-limited subsample of the AT20G Survey catalogue comprising 320 extragalactic (|b| > 1°.5) radio sources south of δ = -15° with S 20GHz > 0.50 Jy. Of these, 218 have near simultaneous observations at 8 and 5 GHz. In this paper we present an analysis of radio spectral properties in total intensity and polarization, size, optical identifications and redshift distribution of the BSS sources. The analysis of the spectral behaviour shows spectral curvature in most sources with spectral steepening that increases at higher frequencies (the median spectral index a, assuming S oc v α , decreases from α 8.6 4.8 = 0.11 between 4.8 and 8.6 GHz to α 20 8.6 = -0.16 between 8.6 and 20 GHz), even if the sample is dominated by flat spectra sources (85 per cent of the sample has α 20 8.6 > -0.5). The almost simultaneous spectra in total intensity and polarization allowed us a comparison of the polarized and total intensity spectra: polarized fraction slightly increases with frequency, but the shapes of the spectra have little correlation. Optical identifications provided an estimation of redshift for 186 sources with a median value of 1.20 and 0.13, respectively, for QSO and galaxies.

VAST: An ASKAP survey for variables and slow transients

The Australian Square Kilometre Array Pathfinder (ASKAP) will give us an unprecedented opportunity to investigate the transient sky at radio wavelengths. In this paper we present VAST, an ASKAP survey for Variables and Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP to enable the discovery and investigation of variable and transient phenomena from the local to the cosmological, including flare stars, intermittent pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar scintillation, radio supernovae, and orphan afterglows of gamma-ray bursts. In addition, it will allow us to probe unexplored regions of parameter space where new classes of transient sources may be detected. In this paper we review the known radio transient and variable populations and the current results from blind radio surveys. We outline a comprehensive program based on a multi-tiered survey strategy to characterise the radio transient sky through detection and monitoring of transient and variable sources on the ASKAP imaging timescales of 5 s and greater. We also present an analysis of the expected source populations that we will be able to detect with VAST.

Interpretation of parabolic arcs in pulsar secondary spectra

Pulsar dynamic spectra sometimes show organized interference patterns: these patterns have been shown to have power spectra that often take the form of parabolic arcs, or sequences of inverted parabolic arclets whose apexes themselves follow a parabolic locus. Here, we consider the interpretation of these arc and arclet features. We give a statistical formulation for the appearance of the power spectra, based on the stationary phase approximation to the Fresnel-Kirchoff integral. We present a simple analytic result for the power spectrum expected in the case of highly elongated images and a single-integral analytic formulation appropriate to the case of axisymmetric images. Our results are illustrated in both the ensemble-average and snapshot regimes. Highly anisotropic scattering appears to be an important ingredient in the formation of the observed arclets.

Synchrotron Aging in Filamented Magnetic Fields

Magnetic fields in synchrotron sources are almost certainly inhomogeneous, mixing high-field and low-field regions. This inhomogeneity affects the evolution of a relativistic electron distribution function due to the rate of energy loss of the electrons changing as they move between the two regions. We present two models for the evolution of the distribution function, and discuss the results of these models in terms of the critical energies, or synchrotron frequencies, where the particle and photon spectra steepen. We find these critical frequencies are higher than would be the case if the electrons were confined to a homogeneous high-field region. We apply our results to the interpretation of extragalactic radio sources whose dynamical ages are known to be significantly greater than the ages inferred from their high-frequency spectral breaks.

Thermal Stability of Cold Clouds in Galaxy Halos.

We consider the thermal properties of cold, dense clouds of molecular hydrogen and atomic helium. For cloud masses below 10-1.7 M middle dot in circle, the internal pressure is sufficient to permit the existence of particles of solid or liquid hydrogen at temperatures above the cosmic microwave background temperature. Optically thin thermal continuum emission by these particles can balance cosmic-ray heating of the cloud, leading to equilibria that are thermally stable even though the heating rate is independent of cloud temperature. For the Galaxy, the known heating rate in the disk sets a minimum mass of order 10-6 M middle dot in circle necessary for survival. Clouds of this type may in principle comprise most of the dark matter in the Galactic halo. However, we caution that the equilibria do not exist at redshifts z greater, similar1 when the temperature of the microwave background was substantially larger than its current value; therefore, the formation and the survival of such clouds to the present epoch remain open questions.

The dynamical influence of radiation in type 1 X-ray bursts

Consideration is given to the dynamical effects upon an accretion disk of incident radiation generated by thermonuclear burning on the surface of a nonrotating, nonmagnetic neutron star - as exemplified in type 1 X-ray burst sources. Under these conditions, it is found that the torque applied by the radiation field leads to enhanced mass transfer, and the associated accretion power contributes substantially to the total luminosity of the burst. However, this accretion will provide a smaller fraction of the total burst energy if the neutron star possesses a magnetosphere or is in rapid rotation. 16 refs.

Multi-epoch H i line measurements of four southern pulsars

We have measured 21-cm absorption spectra in the direction of three southern pulsars, PSRs B0736-40, B1451-68 and B1641-45, in three separate epochs spread over 2.5 yr. We see no evidence for any changes in the absorption spectra over this time span in spite of good velocity resolution and sensitivity. Towards PSR B1641-45 we place an upper limit of 10 1 9 cm - 2 on the change in the column density of the cold, neutral gas. In addition, we observed PSR B 1557-50 and compared its HI absorption spectrum with spectra taken in 1980 and 1994. A prominent deep absorption feature seen at -110 km s - 1 in 1994 is weaker in 2000 and likely was not present at all in 1980. Using the standard interpretation which links the distance traversed by the pulsar to a physical cloud size, this results in a cloud size of ∼1000 au and density of 2 x 10 4 cm - 3 , parameters typical of those seen in other observations. These results are also consistent with an alternative model by Deshpande who expects the largest variations in optical depth to be seen against the longest time intervals.