Black Holes, Pulsars & Neutron Stars

Radio pulsars are the collapsed cores of once-massive stars which ended their lives in cataclysmic events known as supernovae. They are highly magnetized and rotate very rapidly, from once every 1.5 milliseconds to once every 8.5 seconds. Their phenomenal rotational stability coupled with the emission of radio pulses make them unique in their own right and as probes of the Universe.

Australia has been at the forefront of pulsar research since their discovery in 1968. Australian researchers have found nearly two thirds of the 1500 known radio pulsars, including 750 new discoveries in the past 5 years using the multibeam instrument on the Parkes radio telescope, and have made important advances in the theoretical understanding of the pulsar emission mechanism.

Pulsar research is extremely active at the present time, with about 30 academics, postdoctoral fellows and students working directly on a myriad of challenging problems. Pulsar groups are located at the ATNF, the University of Sydney, Swinburne University, the University of Tasmania, the University of Melbourne, Macquarie University, and the University of Western Australia with significant collaborations between the national groups and with other overseas institutions. Postdocs and students interested in pursuing pulsar research should follow the relevant links.

The ATNF Pulsar Group lists among their interests: pulsar timing, the origin and evolution of pulsars, pulsar emission properties and mechanisms, scintillation studies and the structure and evolution of supernova remnants. The group has played a major role in very successful pulsar surveys using the multibeam receiver on the Parkes 64-m radio telescope which have resulted in the discovery of more than 750 pulsars, doubling the number known. The group is also working toward setting up an international collaboration to form a “Pulsar Timing Array” with the objective of detecting the effects of gravitational radiation on the Earth and establishing a long-term standard of time based on pulsar observations. A major recent initiative of the group has been to construct the ATNF Pulsar Catalogue. This catalogue contains information on all published pulsars, with complete bibliographic information and is intended to serve both professional astronomers and the wider community. A versatile web interface to the database allowing both tabular and graphical outputs has been developed.

The Institute of Astronomy in the School of Physics at the University of Sydney has a vibrant group working on observational and theoretical pulsar research. The pulsar group has a strong theoretical effort in study of pulsar physics. The active areas include radio and high energy emission mechanisms, radio wave propagation in pulsar magnetospheres, scintillation effects, pulsar electrodynamics, and electromagnetic processes in superstrong magnetic fields with application to magnetars. There is also an active interest in the theory and observation of scintillations, particularly in connection with pulsars, Intra-Day Variability in Active Galactic Nuclei and in gravitational lensing. For pulsars, which are exceptionally compact sources, refraction and scattering lead to diverse and evolving interference patterns in their radio spectra. The group uses spectroscopic data on pulsars to investigate the structure of the ionised interstellar medium. Observationally, the research topics include multi-frequency observations of single pulses from bright pulsars with emphasis on their polarization, searching for giant pulses and other extreme manifestations of the pulsar weather, the study of the unique binary pulsar PSR B1259-63, pulsar scintillation and determining the distances to pulsars via HI.

The Centre for Astrophysics and Supercomputing at Swinburne University of Technology specialises in areas of pulsar research that benefit from its unique access to its large array of powerful computers. These include pulsar searches, precision timing using baseband techniques with coherent dedispersion and giant pulse searches. Swinburne has developed a very large baseband recorder (4×64 MHz) in collaboration with Caltech. The system, known as CPSR2, has an in-built supercomputer that allows coherent dedispersion and searches for fast pulsars. Over the past few years, Swinburne has been involved in very successful searches for millisecond pulsars at intermediate latitudes and its precision timing of PSR J0437-4715 has set the benchmark in this area with residuals of just 130 nanoseconds.

The University of Tasmania Radio Astronomy Group, who are making careful timing measurements of the Vela pulsar, and the University of Tasmania optical astronomy group, who use the Mt Canopus 1 m telescope for optical identification and photometry of X-ray transients and pulsating X-ray sources. The UTas group have played an important role in the identification of two of the recently discovered millisecond X-ray pulsars: XTE J0929-314 and SAX J1808.4-3568. Measurements of two transient millisecond X-ray pulsators (4U 1735-44 and 4U 1636-53) are continuing.

The Macquarie University Pulsar Group is investigating the pulsar radio emission mechanism and pulsar population statistics.

The University of Melbourne is engaged in three main areas of theoretical research related to the physics of neutron stars. (i) The group is studying the electrodynamics of relativistic pulsar winds, in an effort to answer fundamental questions such as how energy is transported in the (wave-like) wind as well as understand the intricate, variable shock structures observed by HST and Chandra at the wind-nebula interface in pulsar-driven supernova remnants like the Crab. (ii) The group is undertaking analytic and numerical modelling of accreting neutron stars to understand surface processes, such as magnetic burial (and its implications for gravitational radiation), as well as global magnetospheric phenomena, such as the dynamics of the disk-magnetosphere boundary layer and its signature in the torques of X-ray pulsars measured by RXTE and other satellites. (iii) The group is investigating the global dynamics of the superfluid in neutron star interiors by applying novel numerical techniques to elucidate the phenomenon of pulsar glitches.

The pulsar group at the University of Western Australia currently has two main areas of research. Firstly a new model of the pulsar radio emission mechanism is being developed that promises to explain many of the observed features of the emission including mode changing, drifting subpulses and orthogonal polarization modes. Observational studies of single pulses from pulsars are tied in with this theoretical work. Secondly, we are undertaking a study of pulsar population statistics with a particular view to testing whether the pulsar magnetic and rotation axes align over time, and to estimating the birth rate of pulsars. The group has links with other pulsar groups in Australia and with the Australian Consortium for Interferometric Gravitational Astronomy

PASA Review Paper: Finding Pulsars at Parkes

Black Hole Accretion Disks:

The Astrophysics Group of the Mathematical Sciences Institute at ANU were among the first groups in the country to systematically study the physics of accretion discs around compact stars. Their studies led to the development of the idea that X-ray heating could play a crucial role in disk structure and stability, and of evaporative disc models. Their models have been applied to Black Hole X-ray Transients, and the group is currently actively involved in predicting the theoretical spectra of accretion discs around black holes allowing for relativistic effects.