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
