The Observing Compact Objects Group

Observational Astronomy at the University of Newcastle

Compact objects

Most things in the Universe are tenuous, or are supported against gravity by motion – as with our Galaxy or our Solar System, or by the strength of ordinary matter – as with the Earth, or by the pressure due to heat – as with the Sun and most other stars. But in a few objects, gravity is able to overcome all of these and collapse the object to incredible densities. In some objects – white dwarfs – it is the quantum mechanical properties of electrons that support them. Neutron stars are too massive for this to work, and they are supported by the quantum mechanical properties of neutrons. Black holes are more massive, and cannot be supported by anything at all – they collapse into a sort of twist of spacetime beyond which we cannot see any details. These objects, supported by exotic mechanisms or not at all, are called “compact objects”, and their study can reveal how the Universe works in amazing and exotic circumstances. This group exists to support the study of such objects at the University of Newcastle.

Pulsar timing

Neutron stars are produced in the explosions of massive stars; these stellar cinders are left behind, often spinning rapidly and often with tremendous magnetic fields. In some, but not all, cases, these magnetic fields give us a way to see how the neutron star is rotating: we see a pulse of radiation every time the pulsar’s magnetic pole swings past us. Such neutron stars are called “pulsars”. Their study has proven a very productive field of research.

When we can observe the pulsations of a pulsar, they reveal the pulsar’s rotation. Just as with the Earth’s day, that rotation is generally quite regular, and just as with the Earth’s day, small deviations from that regularity can reveal subtle phenomena happening to the body: large earthquakes actually change the day length of the Earth, and pulsars likewise undergo sudden speed-ups called “glitches” that reveal something – no one is completely sure what – about what is going on in the interior of this object of incredible density and mysterious composition. What is more, many pulsars are in orbit with companion stars, and in that case their pulses can arrive early or late as a result of their motion through the sky – put another way, every pulse we measure provides a measurement of the pulsar’s orbit. Since these objects have incredible gravity, precise measurements of their orbits can probe how well Einstein’s theory of gravity matches the actual behaviour of bodies moving under gravity. Further, as LIGO has demonstrated, there are ripples in space-time itself – gravitational waves – passing over the Earth all the time. A gravitational wave with a very long wavelength passing over the Earth would stretch the space-time around the Earth, making pulses from pulsars in some directions arrive early and from pulsars in other directions arrive late. So there is an International Pulsar Timing Array project carrying out precision timing of many pulsars and looking for gravitational waves to show themselves in correlations between pulses from pulsars that are thousands of parsecs apart.

Precision pulsar timing offers a very powerful tool for measuring very unusual objects in very unusual systems.

Multiwavelength observations

Pulsars are tiny by astronomical standards – perhaps 20 km across, in spite of having masses greater than our Sun – and incredibly hot. They naturally glow in the X-rays. Many of them also have incredibly strong magnetic fields and are rapidly rotating, and that combination serves to accelerate particles to incredible energies. This leads to emissions all the way across the electromagnetic spectrum, from radio to gamma-rays. The Crab pulsar even produces these non-thermal pulsations in the optical bands, though sadly the pulsations are a little too fast to reliably see with the human eye. What’s more, these particles that are accelerated – and in fact synthesized out of the void – stream outward from the pulsar in the form of a wind of relativistic electrons and positrons and a magnetic field. This pulsar wind is what powers the Crab Nebula, which can be seen with a modest pair of binoculars. Many pulsars are also in an orbit with a companion star, and this star can generally be observed in many wavelengths. To develop a good understanding of a pulsar and its environment, we need to use observations across the electromagnetic spectrum – and perhaps also using other means like gravitational waves, cosmic rays, and neutrinos (this is called “multi-messenger astronomy”).

Unusual objects

While some of us find pulsars interesting in their own right, a few objects give a special view into the physics of pulsars, of accretion, or of the Universe as a whole. At the moment we pay particular attention to the millisecond pulsar triple system PSR J0337+1715 and to the class of transitional millisecond pulsars, in particular the defining example PSR J1023+0038.

Who works on this?

At the moment, this work is being done at Newcastle by me, Anne Archibald, in collaboration with fellow astronomers around the world. I am planning to take on undergraduate, Master’s, and PhD students in time, as well as hiring a postdoctoral fellow. I can be reached by email.