These binaries, sometimes called x-ray novae, are known to consist of relatively normal stars dumping material on to massive, compact companions. As illustrated , the material swirls toward the companion in an accretion disk which itself glows in x-rays. But if it is indeed a black hole with a defining event horizon, then the x-ray hot material approaches the speed of light as it swirls past the surface of no return and is lost from view. Recent work describes observations of two classes of x-ray binaries , one class 100 times fainter than the other. The results imply the presence of an event horizon in the fainter class which causes the extreme difference in x-ray brightness.
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