Tuorla Observatory News 18 Sep 2009

Probing the structure of an X-ray binary

The faint northern star UW CrB is probably what is known as a X-ray binary, consisting of a very dense compact object, such as a white dwarf, neutron star or even a black hole, onto which gas is being pulled from a relatively ordinary star. The temperatures generated by this so called accretion process are so high that the system can be a strong emitter of X-rays, and this is one of the main ways they are discovered.

Such was the case for UW CrB which was discovered in 1990 with the Einstein X-ray satellite. Exactly what the system consists of has been unclear since it was found. The most likely scenario at present is that it consists of a neutron star and a low mass main sequence star, an M dwarf. Most of the optical light from the system does not come from either of the two stars, but instead from the accretion disc of hot gas which forms around the neutron star, as material is transferred from the M dwarf.

Artist's impression of UW CrB. A low mass star shown in red, deposits material onto a thick accretion disc (shown in white with an orange edge) where the matter spirals in and eventually hits a compact neutron star in the center of the disc (not seen in this image). A halo of very hot gas also surrounds the disc and neutron star (fuzzy white light above and below the disc). Image: 'BinSim' program by R. Hynes.

The system rotates every 111 minutes from our line of sight, and shows a strong eclipse of the light each time it makes a full rotation. The eclipse is probably caused by the M dwarf regularly blocking light from the accretion disc, so that the system temporarily looks fainter.

Now, a team lead by Pasi Hakala at Tuorla Observatory have followed intensively the eclipsing behaviour of the system over a period of many months with the Nordic Optical Telescope. They have found that eclipsed light from the system changes in a regular manner with a period of about 5 days. The simplest explanation of this unusual behaviour is that the accretion disc is not symmetric around the neutron star. Furthermore, it appears that some part of the accretion disc is eclipsed at all times, so that the whole light curve is affected by the geometry of the disc and the secondary.

The changes in the light curve shape in these systems happen without any change in the overall optical or X-ray luminosity, so it is strongly suspected that the changes are caused by the geometry of the disc, rather than more mundane reasons such as changes in the rate at which material is being transferred between the two stars in the system.

There are one or two other systems with behaviour of this type, but UW CrB is the much the clearest case. Still, what geometry the accretion disc must have remains a puzzle: "We don't really know if the disc is physically thick and non-axisymmetric or heavily warped. However, the disc shape seems to repeat itself with this 5 day cycle..." says Pasi Hakala.

Although distance estimates are quite difficult for this type of system, the indications are that it is about 30000 light years away. The system is thus unusual in being very distant from the plane of the Milky Way's disc - in other words, it is part of the much more tenuous "halo" of the Milky Way. The halo contains only about 1 percent of the Milky Way's stars, stars which were amongst the first born in the Milky Way before it took its present disc shape.

Light curves for UWCrB on two different nights (June 30th and July 5th, 2002). The dip in the light profile is the eclipse of the accretion disc light caused by the secondary star passing in front of the line of sight, causing the system to become temporarily dimmer (left panel). These eclipses occur regularly with a period of 111 minutes. However, the shape of the eclipse profile changes over a longer period, as shown in the right panel. In these observations, the dip is much shallower and a little broader. The study has shown that the changing eclipse profile has a period of about 5 days before repeating itself again.

The team consists of Pasi Hakala (Tuorla Observatory), Linnea Hjalmarsdotter (Observatory, University of Helsinki), Diana Hannikainen (Metsähovi) and Panu Muhli (Observatory, University of Helsinki). The study has been published in the Monthly Notices of the Royal Astronomical Society, and a preprint is available here.

This page was last modified by  Chris Flynn  on  20/09/2009 15:30  astroweb@utu.fi