Introduction

In the 35 years since the first X-ray binary was optically identified (Sco X-1) the basic division of X-ray binaries into the high-mass (HMXBs) and low-mass (LMXBs) systems has become firmly established. The nomenclature refers to the nature of the mass donor, with HMXBs normally taken to be ≥10 M⊙, and LMXBs ≤1 M⊙. However, the past decade has seen the identification and measurement of a significant number of X-ray binaries whose masses are intermediate between these limits. Nevertheless, the nature of the mass-transfer process (stellar wind dominated in HMXBs, Roche lobe overflow in LMXBs) produces quite different properties in the two groups and so this chapter will be divided into two main sections on HMXBs and LMXBs. A more complete introduction can be found in Chapter 1.

While the nature of the compact object and its properties are largely determined from X-ray studies, longer-wavelength observations allow detailed studies of the properties of the mass donor. This is most straightforward for the intrinsically luminous early-type companions of HMXBs, which provide the potential for a full solution of the binary parameters for those systems containing X-ray pulsars. This is particularly important for HMXB evolution in that it allows a comparison of the derived masses with those obtained for neutron stars in the much older binary radio pulsar systems (Thorsett & Chakrabarty 1999).

However, when HMXBs are suspected of harboring black holes (e.g., Cyg X-1), the mass measurement process runs into difficulties.