X-ray emission from this region suggests the presence of a high-temperature gas.
Rapid time variations in the X-ray radiation of Cygnus X-1 imply that the size of the X-ray emitting region is less than a few hundred kilometers across.
Spectroscopic observation of visible light show that the binary-star system has an orbital period of less than six days.
Its visible companion star, if on the main sequence, should have a mass about 25 times that of the Sun.
A small, dark region in the space near the visible companion suggests light cannot escape from that region, which is the characteristic feature of a black hole.