|MadSci Network: Astronomy|
An X-ray binary occurs when a star and a compact object (a black hole or a neutron star) are in orbit around one another and are so close that material can flow from the star onto the black hole or neutron star. This produces a strong X-ray source, which can be observed with an X-ray telescope. There are hundreds of such x-ray binaries in our Galaxy.
X-rays are photons of light with extremely large energies, which gives us a hint to how energetic these events are. For example, if you drop material directly onto the surface of a neutron star, by the time it hits the surface it will be travelling close to the speed of light. When it hits the surface, a vast amount of energy is released and and x-ray flash is produced.
You can generate even more energy by dropping that material onto a black hole, but of course the energy has to be released somehow before it enters the black hole and disappears forever. In a black hole x-ray binary, this usually occurs in what's called an `accretion disk': so much material is falling onto the black hole that it collides, heats up, and forms a hot disk around the black hole. The material orbits the black hole, and can only move to a lower orbit by releasing energy (usually by friction within the disk). This can release up to 10% of the rest mass energy of the material -- which is way more efficient way of turning matter into energy than even the fusion reactions that power the Sun!
It's an exciting time for x-ray astronomy at the moment because of the launch last year of two new X-ray telescopes: the NASA Chandra telescope and the ESA XMM-Newton telescope. You can find out more about X-ray binaries (and X-ray astronomy in general) at the Chandra website.
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