Forty Years of X-Ray Binaries

In 2012 it was forty years ago that the discovery of the first X-ray binary Centaurus X-3 became known. That same year it was discovered that apart from the High-Mass X-ray Binaries (HMXBs) there are also Low-Mass X-ray Binaries (LMXBs), and that Cygnus X-1 is most probably a black hole. By 1975 also the new class of Be/X-ray binaries was discovered. After this it took 28 years before ESAs INTEGRAL satellite team discovered two new classes of High-Mass X-ray Binaries: the highly obscured supergiant systems and the Supergiant Fast X-ray Transients (SFXT). In most HMXBs the neutron stars have very long spin periods. The causes of these long periods and of the outbursts of SFXTs are discussed. Furthermore, the formation rate, duration of the X-ray phase and the later evolution after the X-ray phase of the HMXBs are discussed. Many systems will later in life merge to form Thorne-Zytkow Objects. The fate of such objects is still unclear, but the relatively high formation rate of supergiant HMXBs in the Galaxy (of order 6×10-4yr-1) implies that their remnants must be all around us. Finally, the evidence for the existence of three groups of neutron star masses, derived from the study of X-ray binaries and binary radio pulsars, is briefly discussed, as well as the origins of these three types of neutron stars.