High Mass X-ray Binaries (HMXRB) contain large blue stars (spectral types O and B) and a neutron star companion (in some occasions a black hole, as is the case of Cyg X--1, shown in the figure). These companions are ``normal'' (non-degenerate) stars and they are very bright. When we observe the system in the infrared, optical or ultraviolet, we see only the normal star. The neutron star can only be seen in the X-ray range. Most HMXRB are X-ray pulsars.
When a star reaches the last stages of its life, after having consumed most of the hydrogen in its nucleus, the outer layers begin expanding and the star grows enormously. It becomes a supergiant. As it grows, the gravitational pull on its outer layers decreases. It is then when the gravitational attraction of an orbiting compact object in a close orbit can start distorting the shape of the supergiant and the dragging of matter towards the relativistic star begins. The technical name for these systems is Supergiant Roche-lobe Overflow X-ray Binaries.
An artist's impression of the High Mass X-ray Binary
Cygnus X-1. The supergiant star HD226868 (O9.7Iab), with a mass
about thirty times that of the Sun, is being distorted by the presence
of a black hole with a mass around 15 solar masses. There is an extremely hot
accretion disc around the black hole where the X-rays are being
produced. The name Cygnus X-1 means that it was the first X-ray source
discovered in the constellation Cygnus, the Swan.
As the supergiant expands, it increases in brightness. The outer layers of its atmosphere are continuously bombarded by very energetic photons. They push a stream of particles along with them. These particles dissipate into the empty space around the star at very high velocities. This is called a radiation-driven stellar wind. Sometimes, a neutron star in a rather distant orbit can pick up some of these particles and start the accretion process, becoming a weak X-ray source. These systems are known as Supergiant Stellar Wind X-ray Binaries.
However, if a neutron star is orbiting a star which has not reached the supergiant phase, it will not be able to steal any material from it and will not become an X-ray source.
The exception is the case when the normal star is surrounded by a disc of material. These are the Be stars.