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A comparison of theory and experiment for the binary pulsar.The plotted quantity keeps track of energy lost in the orbit over the years that it has been observed.The predictions from general relativity include gravitational radiation as the mechanism for energy loss. [Joseph Taylor, Princeton University]

in would also emit X-rays (before crossing the event horizon). We could start searching for stellar black holes by looking for irregular X-ray sources.

One interesting possibility is known as Cyg X-1 (Fig. 12.10), the brightest X-ray source in Cygnus. The Uhuru satellite showed this to have both short and long term variability. Until the Einstein observatory was launched in 1978, the positions of X-ray sources were not accurately determined. However, there is also a radio source associated with the X-ray source. We know that the X-ray and radio sources are associated because they have the same pattern of variability. The position of

12.4 I Systems with black holes

In Chapter 11, we saw that neutron stars are supported by neutron degeneracy pressure. However, if the neutron star is too massive, neutron degeneracy pressure is insufficient to support the star. We think this limit is about 3 M0. We know of no other source of pressure that will stop the collapse of the star. It will collapse right through the Schwarzschild radius for its mass, and will become a black hole. Black holes would be a normal state of evolution for some stars.

How would we detect a stellar black hole? We obviously could not see it directly. We could not even see it in silhouette against a bright source since the area blocked would be only a few kilometers across. We have to detect stellar black holes indirectly. We hope to see their gravitational effects on the surrounding environment. This is not a hopeless task, since we might expect to find a reasonable number of binary systems with black holes. By studying a binary with a suspected black hole, the problem would be to show the existence of a very small object (as inferred from orbits) with a mass in excess of 3 M0.

How do we find a candidate binary to study? In Section 12.3, we saw that a neutron star in a binary system can give rise to strong X-ray sources. The importance of the neutron star is that its radius is so small that infalling material acquires enough energy to give off X-rays. A stellar black hole would be smaller than a neutron star, so material falling

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