Cambridge X-Ray Astronomy Group
Black Holes and X-ray binaries

Black Holes

Stellar mass black holes are formed when a massive star explodes in a Supernova. A black hole is something that is so massive that even light cannot escape its surface. When astronomers talk about a black hole, they usually mean the Schwarzschild Radius, beyond which nothing can escape. As no light can escape from a black hole we cannot see them directly; however they have an effect on anything that comes too close. If the black hole formed in a binary star system, then the other companion star will still be in orbit around the black hole (technically their common centre of mass). What is then observed is the companion star "wobbling" around some point in space. The black hole will also "suck" in matter from the region around it. As it is very massive, its gravity pulls things in. Most things in space spin - for example galaxies, stars and planets - and so when things are falling towards the black hole they begin to swirl around it, like bath water around a plug-hole. Different parts of the material orbit around the black hole a different speeds and so they rub against one-another and become hot from friction. As the material is moving very fast by the time it is close to the black hole it is very hot and so emits X-rays.

Larger black holes are found at the centre of most galaxies, these as millions if not billions of times as massive as our Sun. They are too far away to be seen directly, but they have to be there because they are the only things that are known about that can explain what is seen. There is good evidence for a black hole at the centre of the Milky Way as stars are orbiting around something that has to be very massive in a small volume. The only thing that can form with this density is a black hole. Using the Doppler effect at the centres of other galaxies it seems as if black holes are there as well - the stars and gas are moving too fast for it to be anything else. Some of the black holes at the centres of galaxies are real monsters and are performing some of the most extreme physics known to man, see the section on Active Galactic Nuclei.

X-ray Binaries

As the name suggests these are binary systems which emit large amounts of X-rays. These were among the first X-ray sources to be discovered (apart from the Sun and other Solar System sources) as they are relatively close as there are many in the Milky Way. Sco X-1 and Cygnus X-1 were the first X-ray sources to be discovered in the constellations of Scorpius and Cygnus respectively and they are both X-ray binaries.

There are two different types of X-ray Binaries - High Mass (HMXB) and Low Mass (LMXB), and they have different properties

High Mass X-ray Binaries form from two stars of different mass which are in orbit around each other. The more massive one evolves faster and reaches the end of its life first, after a few million years or so. It becomes a giant and the outer layers are lost to its companion. Then it explodes in a supernova leaving behind either a neutron star or a black hole. This can disrupt the binary system, but if the star that exploded was less massive than its companion when it exploded they the systems will remain in tact, though the orbits may be more eccentric. The companion star then comes to the end of its life and swells to form a giant. It then looses its outer layers onto the neutron star or black hole. This is the HMXB phase. The material forms an accretion disc around the compact object, which heats up because of friction. This heating, combined with jets that can be formed by the black hole, cause the X-ray emission. Eventually the companion star comes to the end of its life, leaving a neutron star/black hole - white dwarf/neutron star/black hole binary, depending on the initial masses of the stars. Cygnus X-1 is this type of X-ray Binary. They are bright in X-rays not only because of the accretion disc, but also because there is a corona which is much more powerful than the Sun's corona. Cygnus X-1 is 10,000 times more powerful than the Sun, and most of it is powered by the gravity caused by the black hole.

GRS 1915 impression
An artists impression of GRS 1915 which is thought to be an X-ray binary. The black hole sucks material off the companion star, which is heated by friction, emitting X-rays. Picture by Rob Hynes.

Low Mass X-ray Binaries have a less clear origin. The most likely explanation is that they form by capture, the lone compact object, the remnant of a massive star, has a close interaction in a cluster and picks up a companion. The mass transfer on to the compact object is much slower and more controlled. This mass transfer can spin up a neutron star so that it is a millisecond pulsar, spinning thousands of times a second. Low Mass X-ray Binaries tend to emit X-rays in bursts and transients and there could be many more present in our galaxy than we see, but which are currently switched off. They also tend to have softer spectra (they emit lower energy X-rays), whereas the HMXB's have harder spectra (more energetic X-rays).

Images of the globular clusters and the centre of our galaxy show that there are many X-ray binaries in our galaxy. They appear as the point sources in the image below, which is of the centre of our galaxy. This image of the centre of our galaxy also shows the source Sagittarius A*, which is the super massive black hole at the centre of the Milky Way.

In X-ray binaries where the compact object is a Neutron Star, rather than a black hole, the material which falls from the giant star onto the neutron star builds up on the surface. Eventually there is enough material there for it to "burn" like at the centre of a star, and this causes the largest visible thermonuclear flash. The X-ray emission from the X-ray binary can go up by a factor of ten, and then it decays back down again.

CHANDRA image of the
centre of the Milky Way
CHANDRA image of the centre of the Milky Way. The small point sources are mostly X-ray binaries in our own galaxy. The super massive black hole at the centre of the Galaxy is located inside the bright white patch in the centre of the image. The colours indicate X-ray energy bands - red (low), green (medium), and blue (high). Click for a labelled version. (Image courtesy CHANDRA)

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