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 compact: a
white dwarf,
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 10
41 positrons escape per second from a typical hard
low-mass X-ray binary.