Cataclysmic Variables

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Schematic diagram of a non-magnetic cataclysmic variable system, showing a white dwarf surrounded by an accretion disk, with the Sun-like star feeding the disk in the background. Graphic by Moran & Baskill using Celestia.

 

Cataclysmic Variable stars are binary star systems where a white dwarf and a Sun-like star orbit each other so closely that the white dwarf star drags gas away from its Sun-like companion, at a rate of over a thousand million tonnes per second. 

If the white dwarf has a strong magnetic field, the material falls directly onto its magnetic poles - these stars are known as polars. However, if the magnetic field is sufficiently weak, the gas forms an accretion disk around the white dwarf star before falling onto the white dwarf. At the point where the gas strikes the white dwarf, the gas heats up to millions of degrees - hot enough to emit high-energy X-rays. 

The optical brightness of the accretion disk depends on the rate at which gas falls through the disk. Cataclysmic variables can suddenly, and without warning, brighten by a factor of 100 in just 6 hours, before fading away over a week, repeating over timescales of weeks to many years. This was how the objects were originally discovered over 100 years ago, and why they have remained popular targets for amateur astronomers to observe, looking to catch an outburst. The outbursts are caused by gas building up in the outer disk, before falling rapidly through the disk. 

Research in the department covers two groups that work closely together: the X-ray & Observational Astronomy group, and the Theoretical Astrophysics group. 

Using the UKAFF supercomputer based within the department, members of the Theoretical Astrophysics group use advance mathematical models to recreate cataclysmic variables 'inside' UKAFF. By seeing how the simulations behave with time, and then comparing the results with actual observations, we can tell whether the mathematical models are accurate or not, leading to a further understanding of how these systems work.

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With a strongly magnetic white dwarf, an accretion disk is unable to form around the white dwarf. Graphic by West & Wynn using UKAFF; the movie is available at http://www.ukaff.ac.uk/movies.shtml

 

 

 

In the X-ray astronomy group, researchers use space-borne X-ray telescopes (including XMM-Newton, and Swift) to study in detail the region where the gas smashes into the surface of the white dwarf star, in polars, intermediate polars and non-magnetic systems. Such observations are used to help select the best theoretical models.

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