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:
— low-mass X-ray binaries
— high-mass X-ray binaries
— Be X-ray binaries
— X-ray bursters
— X-ray pulsars
— Soft X-ray transients