X-ray binaries are among the brightest X-ray objects in the sky. As shown in the picture above, an X-ray binary consists of a normal star (like the Sun) and a compact object which can be either a black hole or a neutron star, orbiting around each other. At certain stage of the binary evolution, the separation of the two stars becomes so close that material from the upper atmosphere of the companion star begins to flow toward the compact object under the influence of the latter's gravity. This process is known as mass accretion due to ``Roche-lobe overflow''. The accreted matter, carrying large amount of angular momentum from the orbital motion, circulates around the compact object and forms an accretion disk. As the matter spirals in toward the compact object, due to angular momentum losses from viscous processes, its gravitational energy is converted into heat. Depending upon the mass accretion rate, the temperature of the inner accretion disk can reach more than one million degrees so that X-rays are produced. This process is very efficient (compared to, e.g., nuclear fusion that powers the Sun) in converting gravitational energy to radiation, which is why X-ray binaries are such bright X-ray sources. X-ray observation of such objects, therefore, provides a valuable tool to probe regions very close to the compact object, where relativistic effects are strong thus important. It should be noted, however, that Roche-lobe overflow does not always occur. For some sources (especially those with a massive companion star), the accretion process may simply involve the capture of stellar wind from the companion star by the compact object. A small accretion disk can still form in such ``wind-fed'' systems, due to any residual angular momentum of the captured matter. The details of the mass accretion process are still poorly understood. In some cases, spectacular jets are produced, presumably in connection with the accretion process. The jets can travel at nearly the speed of light, which then produce all kinds of interesting observable phenomena. One example is "superluminal motion" which describes the fact that the apparent velocity of such jets across the sky, as measured by an observer, can be greater than the speed of light, if the jets point roughly toward the observer. The phenomenon has been observed in quasars that harbor a massive black hole (of tens of millions of solar mass), as well as in systems that contain a much "lighter" black hole (several times the mass of the Sun). The latter is often referred to as microquasars.

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