Soon after the opening of X-ray astronomy in 1962, the concept of an accreting compact object emerged to account for the large X-ray luminosity. It is well established that bright X-ray sources with X-ray luminosity Lx ^ 1036 erg s-1in the range 1-10 keV, excluding supernova remnants, are binary systems containing a gravi-tationally collapsed object, either a neutron star or a black hole. As explained in Sect. 16.4.1, they are powered by mass accretion from a companion star, and the key to the high X-ray luminosity is an extremely deep gravitational potential well of the collapsed object.
X-ray binaries (XBs) are divided into two distinct groups according to the type of the companion star. Those with high-mass O or B stars are called high-mass XBs
(HMXBs), and the others with low-mass (<M0) stars, mostly K, M stars, are called low-mass XBs (LMXBs).
Historically, the first discovery of the binary nature was made for Cyg X-1 in 1972. When its optical counterpart was identified with the O-type super-giant HDE226868, Webster & Murdin  and Bolton  discovered a sinusoidal Doppler motion of the O-type star, which is a clear evidence for a binary system. The invisible companion must be an X-ray emitting compact object. Thus, Cyg X-1 is a HMXB.
Their results contained another big surprise. The mass of the compact object estimated from the mass function (see Sect. 16.3) most likely exceeded 3M0. They independently considered that the compact object was a black hole! This discovery of great impact excited various critical discussions. However, none of the alternative possibilities survived. Cyg X-1 still remains as one of the secure black-hole binaries.
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