X-ray binaries
X-ray binaries are a class of binary stars that are very luminous in
X-rays. The X-rays are produced by matter falling from one component (usually a relatively normal
star) to the other component, which is a
neutron star or a
black hole. The infalling matter releases gravitational
potential energy, up to several tens of per cent of its rest mass as X-rays. (Hydrogen
fusion releases about 0.7 per cent of rest mass)
X-ray binaries are further subdivided into several (sometimes overlapping) subclasses, that perhaps reflect the underlying physics better: