XMM-Newton examines a cataclysmic variable
19 October 2000Ominously-named "Cataclysmic Variables" - CVs for short - are not the kind of solar systems one would like to approach. End points of stellar evolution, they are binary systems in which one star is sucking material out of its partner. They revolve around each other very rapidly, typically every few hours. CVs can also exhibit outbursts on the time scale of weeks to months. XMM-Newton has been observing one such cataclysmic variable, named OY Car.
Situated in the Carina southern constellation, and 280 light years away, the first amazing aspect of OY Car, like all CVs, is the small scale of the binary: it is only 100 times the size of the Earth-Moon system.
CVs are composed of two stars, one a very dense and compact star, the other a star similar to our Sun. The more massive star is a white dwarf, a collapsed star with a mass similar to our Sun but compressed into the volume of the Earth! The less massive star is a red dwarf.
Because these two stars are very close together, material is lost from the red dwarf and spirals into the white dwarf. As it does so it forms a ring of material around the white dwarf - an accretion disc - which is bright in visible light. This accreting material has to slow down and merge with the white dwarf. It is this process which makes it radiate in X-rays.
The OY Car investigation took place during XMM-Newton's performance verification phase at the end of June, several days after an outburst, and again at the start of August when it was at a similar brightness. Each time it was observed with XMM-Newton's X-ray instruments and its Optical Monitor.
The unique nature of XMM-Newton is that both the X-ray and optical data are taken simultaneously. With previous X-ray satellites, it was difficult to schedule simultaneous earth-based, optical observations - and often there was the problem of clouds!
OY Car was was targeted because it is an eclipsing system: this means that once every orbit (90 min for OY Car) the red dwarf travels in front of the white dwarf and obscures it. By measuring the brightness of OY Car as it does so, astronomers can find out where the X-rays and optical light come from in the system.
Until these XMM observations astronomers were not able to pin-point the location of the X-ray source with any precision. With these high quality data they were able to accurately measure the time it took for the X-rays to be obscured: 20-30 seconds. This implies that the X-rays originate very close to the surface of the white dwarf, possibly in a band around the white dwarf.
The eclipse also allowed the masses of the two stars to be measured: until now, they were not accuratley known. The OY Car investigators measured that the white dwarf had a mass much like our Sun (0.9-1.1 solar masses), and the secondary star 0.08-0.11 solar masses.
Further analysis has shown a so far undetected periodicity of the X-ray emission which could be related to the rotation period of the white dwarf. If this can be confirmed then this would be the first time that the spin period of the white dwarf in a disc accreting CV has been measured.
Another aspect of the XMM-Newton mission is the high quality X-ray spectra that can be obtained. Although OY Car was rather faint in the RGS instument, various emission lines were detected. This was the first time these particular lines have been detected in a CV.
Using the EPIC detectors, very high quality X-ray spectra were obtained. These X-ray spectra imply that a range of temperatures are present in the X-ray emitting region.
The multi-temperature feature is interesting because material spiralling into the disk is accelerating. When it arrives at the white dwarf, which is relatively stationary compared to the infinitely faster rotation of the disc, material has to be dramatically brought to a stop in order to settle on the white dwarf's surface. How this occurs is still unclear.
There are many types of cataclysmic variables with accretion disks and theories about accretion mechanics are complex. Initially it was believed that the accretion disks were very flat structures, with all the matter in a single plane. Now there is mounting evidence - spurred on by XMM-Newton's observations of OY Car - that certain accretion disks are far more complicated, with the outside of the disk being flared and reaching very high above the orbital plane.
Acknowledgements to the first author of the papers on OY Car to be published in Astronomy and Astrophysics:
(Mullard Space Science Laboratory, UCL, UK),
"First XMM-Newton observations of a Cataclysmic Variable: Timing studies of OY Car" & "Spectral Studies of OY Car"