X-Ray Sources and Stellar Evolution

Current ideas concerning the nature ad origin of Galactic X-ray sources are discussed in light of Einstein Observatory observations of point X-ray sources in M 31. Strong Galactic point X-ray sources, with the exception of young supernova remnants, are divided into two types, with group I sources displaying hard spectra, usually pulsating, having a luminous optical counterpart and being concentrated near the galactic plane as extreme population I. Group II sources on the other hand do not show pulsation, always have intrinsically faint optical companions, and are concentrated in the galactic bulge and globular clusters. Group I sources are explained as accreting neutron stars in massive binary systems which represent a brief stage in the evolution of all normal massive close binaries. A low-mass close binary model has been developed to explain the group II sources which allows binary periods and mass transfer rates to be calculated as a function of the mass of the companion. Mechanisms involving a collision or capture between a normal and a neutron star have been invoked to explain the presence of group II sources in globular clusters and the galactic bulge.