In this chapter neutron stars (NS) are discussed, which are powered through accretion from a companion star in a binary system. The companion is either a "normal star" or a White Dwarf. X-ray binaries where the mass receiving compact object is a White Dwarf or Black Hole, as well as isolated NS, which are powered by a different kind of energy supply (spin energy, magnetic field energy, thermal energy content or accretion from the interstellar medium), are discussed in other chapters of this book.
The transfer of material from the companion star, finally ending up on the surface of the NS where most of the gravitational energy is released, is by stellar wind or by Roche lobe overflow (see Fig. 15.1), or by the NS crossing the circumstellar disk of an OB or Be star.
The first extra-solar X-ray source, ScoX-1, discovered in 1962 by a rocket experiment  was later found to be a NS binary. Only the first X-ray satellite, Uhuru, with its first all sky X-ray survey in 1970, allowed to identify these objects as a class: of the 339 sources listed in the 4th Uhuru catalog  one third are associated with accreting NS in a binary system. Today we know a few hundred sources of this kind in our galaxy and the nearest neighbour galaxies [29,30]. These sources are characterized by a high X-ray luminosity (1034 to 1038 erg s^1), hard spectra, and a high degree of variability of different nature. They are important laboratories for the study of material under extreme physical conditions, such as high densities, high temperatures, high photon densities, or strong gravitational and magnetic fields, as well as for stellar evolution. The research on NS deals with fundamental physics, e.g., the nature of ultra-dense matter and its equation of state or general relativity in the immediate vicinity of the compact object where particles in close orbits move in curved space time.
Important books dealing with accreting neutron stars are: X-ray Timing 2003 , X-Ray Binaries , Accretion driven stellar X-ray sources , X-ray astronomy , Compact stellar X-ray sources . The textbook Exploring the X-ray Universe  contains three chapters on X-ray binaries. More specialized reviews and original references are cited later. An introduction to Accretion Power in Astrophysics is given by . A catalogue of low and high mass X-ray binaries compiled by van Paradijs can be found in X-Ray Binaries  and in updated form by [29,30].
Porb= 3.1 days
Fig. 15.1 Wind fed HMXB and Roche lobe overflow fed LMXB
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