X-ray binaries
X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor (usually a relatively normal star), to the other component, called the accretor, which is very compact: a neutron star or black hole. The infalling matter releases gravitational potential energy, up to several tenths of its rest mass, as X-rays. The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation. (Hydrogen fusion releases only about 0.7 percent of rest mass.) An estimated 1041positrons escape per second from a typical low-mass X-ray binary.
X-ray binaries are further subdivided into several (sometimes overlapping) subclasses, that perhaps reflect the underlying physics better. Note that the classification by mass (high, intermediate, low) refers to the optically visible donor, not to the compact X-ray emitting accretor.
A low-mass X-ray binary (LMXB) is a binary star system where one of the components is either a black hole or neutron star. The other component, a donor, usually fills its Roche lobe and therefore transfers mass to the compact star. The donor is less massive than the compact object, and can be on the main sequence, a degenerate dwarf (white dwarf), or an evolved star (red giant). Approximately two hundred LMXBs have been detected in the Milky Way, and of these, thirteen LMXBs have been discovered in globular clusters. The Chandra X-ray Observatory has revealed LMXBs in many distant galaxies.
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