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Binary star systems contain two stars that orbit around their common center of mass. Most of the stars in our Galaxy are part of a binary system.
A special class of binary stars is the X-ray binary, so-called because it is a source of X-rays. X-ray binaries are made up of a normal star and a collapsed star (a white dwarf, neutron star, or black hole). These pairs of stars produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star. The X-rays come from the area around the collapsed star where the material that is falling toward it is heated to very high temperatures (over a million degrees Centigrade!).
A sub-group of the X-ray binaries are the eclipsing X-ray binaries. These binaries orbit around one another such that, viewed from Earth, one star appears to pass in front of and behind the other star on a regular basis. The amount of time it takes the star to return to the same position in the orbit is called the orbital period. This is the same as the Earth orbiting around the Sun with an orbital period of 365.25 days.
In typical X-ray binaries, one star is very small compared to the other star. The collapsed star is the small one. Neutron stars are about 10 km in radius; white dwarfs are about 7,000 km in radius. Normal stars, like the Sun, have radii 100 to 1,000 times larger than a white dwarf. Thus, as the neutron star moves in its orbit around the collapsed star, it will (in part or in total, depending on our viewing angle) seem to disappear behind the central star for a period of time and then reappear on the other side. This event is called an eclipse.
An eclipse can have three parts to it: ingress, totality, and egress. Ingress occurs as the small star begins to move behind the larger central star. It ends when it is completely covered by the central star, at which time the period of totality begins. Egress begins when the small star begins to reappear from behind the central star and ends when it is once again completely revealed.
How we view the eclipsing binary system from Earth will determine what effect we detect in the radiation produced by the system as it eclipses. If one star is completely hidden by the other star (as seen from Earth), we call it a total eclipse. If the star is only partly obscured behind the central star during an orbit, we call it a partial eclipse.
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