Astrophysics

Members: Fabio Frescura,

Pulsars are key objects in astrophysics. In them, matter exists under conditions so extreme that we cannot even begin to imagine how they might be realised in any laboratory. They are superdense remnants of stellar collapse which contain a mass of approximately 1.5 suns compressed into a ball approximately 15 km in diameter, and which spin at rates that range from once per second to as much as 750 times per second. Their surface is solid crystalline iron, and would ring like a bell were it to be hit with an hammer.

Descending into their interior, within tens of metres from the surface, the iron nuclei begin progressively to ooze neutrons until eventually, at a depth of several hundred metres, the nuclei dissolve completely. The free electrons combine with free protons to form neutrons. The bulk of the pulsar thus consists of neutrons. The density of matter in the pulsar has now risen from that of crystalline iron by 10 orders of magnitude to 10 to the power of 17 kilogram per meter cubed. The pressures are also fantastically high, and the neutrons are forced closer together than is possible in a nucleus. At this range, they form weak dynamical S-state bonds. The neutronic material thus becomes a boson fluid. Its temperature is below critical, so the entire interior of the pulsar becomes superfluid and flows without friction. Deeper in the star, as the pressures rise further, P-state bonds are formed, making the inner mantle a superfluid of a different type to that in the outer mantle. We don’t know what happens at the core, but the conditions are thought to be right for quark deconfinement. If so, the core is strange matter.

As you see, a pulsar is a very exotic object. But there is more. In stellar collapse, the total magnetic flux is conserved. So, in a pulsar, the magnetic field of a star larger than our sun is compressed into 15 km. This makes the magnetic field at the surface of a typical pulsar about 10 to the power of 10 gauss, which is fantastically huge – over 9 orders of magnitude larger than the most powerful laboratory field. The effect of this field is to produce a beaming by which pulsars were first detected. Since this field rotates with the pulsar, it induces an electric field at the surface which, at the polar caps, rips electrons out of the iron surface and hurls them into space along the magnetic field lines where they accelerate, cascade, and radiate. Furthermore, the rotation speeds near the pulsar surface are sufficiently close to the speed of light, and the mass of the pulsar so huge, as to make relativistic effects, general and special, important.

In short, pulsars are natural, cosmic laboratories, provided by nature herself, in which the properties of matter under extreme conditions may be observed. In fact, they are the only laboratories of this kind available. Black holes are denser, but apart from trace effects and radiation signatures, they cannot be seen. White dwarf stars are significantly less compact. The matter in them is not subject to conditions that are anywhere near as extreme. There is therefore no other astronomical object that provides the opportunities provided by pulsars for testing physical theories. And the range of theories that can be tested is large, ranging from elementary particle theory and the theory of strange matter, to nuclear physics, superfluidity, superconductivity, nuclear theory, solid state, and electrodynamics. And, interestingly, pulsars also provide a significant new testing ground for general relativity.

A sample of nearly 30 pulsars have been monitored at the Hartebeesthoek Radio Astronomy (HartRAO) since 1986. Claire Flanagan, who set up the pulsar monitoring programme, and her co-workers have accumulated some of the best long-term data sets on pulsars in the world. This data potentially holds the key to resolving some of the controversies surrounding pulsars and their properties. Many features of this data remain largely uninterpreted. Two years ago, Beate Woermann instigated the formation of a Pulsar Research Unit. This Unit brings together a wide range of expertise which includes observational and data analysis skills, technical and electronic know-how, and theoretical, computational and numerical knowledge and expertise. It currently consists of the following persons: Claire Flanagan, George Nicholson, Beate Woerman, Sarah Buchner, Adrian Tiplady, and Fabio Frescura. The unit is based at the University of the Witwatersrand, meets regularly at the Johannesburg Planetarium and at the School of Physics, and works closely with HartRAO, which runs the monitoring programme.

The HartRAO data is rich, and could be used in a wide range of programmes and projects. These range from the seriously theoretical to numerical and computational modelling and analysis. The expertise represented in the group is sufficiently broad to allow projects to be tailored to accommodate individual student preferences for each of these project types. Below we outline some projects that are currently in progress, and invite students to apply for participation in these. Should a student have a particular interest not represented in these projects but within the gambit of pulsar research, we invite that student to submit a project proposal to the Pulsar Research Unit for consideration. All of our projects are extendible to a PhD. Some of our projects involve aspects in which the current members of the Pulsar Research Unit have little or no expertise. Students willing to undertake research in those areas will thus have the opportunity to make a valuable contribution to the Unit and to establish themselves as the local experts in those areas. Students working with the Pulsar Research Unit would be registered for their Masters degree in the School of Physics at the University of the Witwatersrand.

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