SALT - Southern African Large Telescope

Studies of binary stars

An artist's impression of an X-ray binary system showing the distorted companion star on the left and the compact object and its accretion disk to the right.

Did you know that unlike our Sun, most stars are actually found in pairs or multiple systems? Stars orbiting in pairs are called binary stars and they orbit around each other about a common centre of mass. There are many types of binary systems, for example red stars orbiting blue stars, huge stars orbiting tiny stars, red stars orbiting neutron stars and even stars orbiting black holes.

X-ray binaries are one type of binary system so called because they emit most of their light in X-rays. In this case one star is a collapsed star, either a neutron star or even a black hole which we call a "compact object". There are two types of X-ray binary systems; low mass X-ray binaries (LMXB) and high mass X-ray binaries (HMXB). In the case of LMXBs the other star in the system is a low mass star (similar or less than the mass of our Sun) and is either a normal star, a swollen star or a white dwarf. In the case of HMXBs the other star is massive blue giant star at least 10x the mass of our Sun. The second star is commonly referred to as the companion or donor star.

The two stars orbit each other once every few hours and they are so close together that the average X-ray binary system would fit comfortably into our Sun. Because they are so close together we can't resolve the two stars, they appear on the sky as a point source.

The companion star in an X-ray binary is so close to the neutron star or black hole that it becomes tidally distorted and no longer remains spherical. Gas is ripped off the companion star and falls towards the compact object. Because of conservation of angular momentum, the infalling gas can't plunge directly onto the surface of the compact object and the infalling gas forms a disc called an accretion disc with the compact object at its centre. As the gas is accelerated towards the compact object it is heated to more than 1 million degrees, which is why it emits such energetic X-rays.

The gas in the disc spirals down towards the compact object, radiating its gravitational potential energy away as it goes. The radiation is emitted at ultra-violet and X-ray wavelengths. Some of this radiation is also reprocessed in the accretion disc and emitted at visible wavelengths.

X-ray binaries provide excellent laboratories for the observation of accretion discs. The accretion disc may have an instability caused by the change in mass transfer rate in the disc (changes in the rate at which matter spirals down to the compact object), that leads to outbursts or temporary brightening in the X-ray radiation which we call flares. Because the X-ray light is reprocessed in the accretion disc to produce optical light we will observe an optical flare as well. The X-ray light is emitted from the inner part of the accretion disc, however, the optical light is emitted from the outer edge of the accretion disc. We can therefore use observations of the times difference between the X-ray and optical flares to determine the size of the accretion disc.

Observation of these flares, requires very rapid timing studies at optical and X-ray wavelengths. Simultaneous observations are conducted with X-ray satellites such as SWIFT or RXTE and optical telescopes such as the South African Large Telescope (SALT) or the Very Large Telescope (VLT). Such observations allow determination of the lag between the optical and X-ray flares and the lag found is typically < 1 second!

Astronomers at the South African Astronomical Observatory led by Prof. Phil Charles and graduate student Marissa Kotze have studied a low mass X-ray binary system called GX339-4. The graphs below show lightcurves of the GC339-4 X-ray binary system observed with SALT and the RXTE X-ray satellite.

The x-axis shows the time in seconds and the y-axis shows how bright the accretion disc radiation was during the observation.  The spikes in the graphs where the flares occur are apparent, as is the correlation between the optical and X-ray curves clearly from the two graphs. The time lag between the two is around 0.15 s. As light travels at 300000000 m/s, this implies a distance of 45000000 m = 450000 km (~7 Earth radii)  between the areas where the X-ray and the optical flares originate. The Sun has radius 695000 km, so the black hole and its surrounding accretion disc would fit comfortably inside the Sun!