In the previous chapter we met the black hole X-ray binary Cyg X-1. Currently there are about twenty strong black hole candidates in our galaxy similar to Cyg X-1. But in the whole of our galaxy there could be more than a thousand black hole X-ray binaries, although at any one time only a small fraction might be "switched on," in other words, X-ray bright.
Most black hole X-ray binaries work in a slightly different way to Cyg
X-1. Technically speaking Cyg X-1 is a high mass X-ray binary
because it's companion is a massive supergiant star. Most probably
have less massive companion stars - they are low mass X-ray
binary systems.
In high mass X-ray binaries the supergiant companion spews out material in the form of a wind, which the black hole then accretes ("feeds on"). But in low mass X-ray binaries there is no strong wind, instead the stars are close enough together that the black hole's gravity actually pulls material from the surface of the companion. A nice illustration of this (by Rob Hynes) is shown to the left.
The black hole is in the centre of the accretion disc. The companion star is being visibly stretched towards the black hole, and a small stream of matter is flowing from it onto the disc.
Maintained by Simon Vaughan
(sav2 at star. le. ac. uk)
Last updated: 16/9/2003
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