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Compact Binaries

Unlike the Sun, most stars in our Galaxy orbit another star: these systems are called binary stars. Generally it takes many years for the two stars to orbit around each other. However, in recent years a small number of binary stars have been discovered which orbit on a timescale of less than 1 hour. The quickest is as fast as 5 minutes, with both stars being able to fit inside the volume of Saturn. These stars are called white dwarfs and have the same size as the Earth but weigh about the same mass as the Sun. Because the stars are so close together, gravity can pull material from one star which eventually lands on the other. As a result of this process, X-rays and optical light are emitted.

These binaries are interesting for many reasons. One is that they are predicted to emit lots of gravitational waves. A second is that their observed number is a good test of certain theoretical models. The Astrophysics group at Armagh Observatory has several project whose aim is to determine how many of these systems there are and secondly to determine what powers them, which in turn, determines how strong their gravitational waves are.

Currently, only 18 of them are known, but current theories predict that more than 30 million of them should exist in our Galaxy. Many of these will be too faint to detect, but more than 4000 should be easily detectable using current telescopes. It is not clear if this large discrepancy is due to the fact that we still have to find lots of them, or if it is due to the theories being incorrect.

Gavin Ramsay leads a project which aims to discover new `ultra compacts' using ground based optical telescopes and X-ray satellites currently orbiting the Earth. Full details of the optical strand can be found in the RApid Temporal Survey (RATS) homepage.

Gravitational Waves were predicted by Einstein in his General Theory of Relativity in 1916. While they have not been directly detected, their presence has been strongly supported by observations of objects including ultra compact binaries. Over time as gravitational waves are emitted, the time taken for the two stars to orbit one another changes by a small but measurable amount. We have shown that the white dwarfs in the two most compact binaries are actually getting closer over time.

One model, developed by Kinwah Wu at MSSL, suggests they are being powered a bit like an electrical bulb - this has been dubbed the `electric star' model. This model has been the subject of much debate and so far it is the model which comes closest to explaining their observational properties. If this mechanism is indeed driving these systems, then it has important consequences for their gravitational signal.

The European Space Agency and NASA plan to launch a space observatory called LISA in around 10 years time. It is predicted that these ultra compacts will be the first objects it detects. However, to properly interpret their gravitational signal we first have to know what is powering them. If they are powered like an electric star then their gravitational signal will be due to a combination of gravitational waves and electrical power.