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Issue 3.12

The Astrophysics Spectator

July 19, 2006

This week we consider the third place in our universe that yields black-hole candidates to astronomers: x-ray binary star systems. This page appears under the “General Relativity” survey path.

X-ray binary stars are among the brightest objects in the x-ray and gamma-ray sky. As with the visible stars, the brightest x-ray systems are intrinsically luminous, which allows us to see them despite their great distant. We see x-ray binaries from all parts of our own Galaxy and from the Magellanic Clouds, two nearby dwarf galaxies.

Stars are commonly found in binary systems. Usually these stars are very far apart, and both stars in the system are common fusion-powered stars. X-ray binaries, on the other hand, are very rare, numbering at most only several thousand in our own galaxy. Only one of the two objects in these systems is a fusion-powered star; the second object, a compact object, is either a neutron star or a black hole candidate. The separation between the fusion star and the compact object in these systems is approximately the diameter of the fusion star, so that the center of gravity of the system fall within the atmosphere of the fusion star. This enables the compact object in these systems to pull gas from the fusion star onto itself, creating a brilliant glow of x-rays and gamma-rays as the gas converts gravitational potential energy into radiation.

This is the perfect condition to find a black hole candidate. Unlike the unassuming solitary black hole, which invisibly travels through the galaxy, leaving only the slightest hint of its existence in the brightening of background stars, the black hole in a compact binary systems announces itself with fireworks, making itself visible over thousands, and sometimes hundreds of thousands, of parsecs. But beyond lighting up a black hole, an x-ray binary provides us with a second tool: the means of setting a lower value to the mass of the compact object in the system. A compact object with a lower mass limit larger than three times the Sun's mass must be a black hole under our current theories. A handful of x-ray binary systems are now known to contain compact objects with masses far above this limit.

Next Issue: The next issue of The Astrophysics Spectator is planned for release on August 2.

Jim Brainerd

General Relativity

Black Holes in X-ray Binary Systems. The best place to look for black holes is in x-ray binaries systems. These systems, which dominate the x-ray sky, are composed of a fusion-powered star in orbit with a compact object, either a neutron star or black hole. We know from Dopper-shift measurements that the compact objects in a handful of systems are more massive than three solar masses, which implies under current theory that they are black holes. Generating neither thermonuclear bursts nor black-body radiation, these massive compact objects behave like objects without a surface, a characteristic expected for black holes. Positive evidence to test general relativity in these systems, however, is absent. (continue)

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