X-ray observations of Be stars

Be star binaries with neutron star companions are shown to constitute a major class of X-ray sources. Some general observational and interpretive techniques of X-ray astronomy are reviewed. Data for 12 Be/X-ray binary systems are summarized. The Be/X-ray binaries are found to be systematically wider systems, with lower-mass primaries, and with significantly more transient behavior than the 'standard' massive X-ray binaries such as Cen X-3 and SMC X-1. The difference between the two types of X-ray binaries is explained in the context of slightly different evolutionary scenarios for the progenitor binaries. The 'standard' massive X-ray binaries result from wind-mass-loss dominated evolution of very massive close binaries, while Be/X-ray binaries probably result from mass-transfer dominated evolution of systems with primary masses less than approximately 20 solar masses. The implications of the X-ray observations of Be/X-ray binaries for Be stars in general are discussed.