More on Cyg X-1 and its Companion Star
Cygnus X-1 is an X-ray binary in the constellation Cygnus, the swan, that
was one of the first X-ray sources discovered when it was detected in 1962.
It is called Cygnus X-1 because it was the first X-ray source discovered in
the constellation Cygnus. The visible object HDE226868 is a 9th magnitude
blue supergiant star whose radial velocity curve shows an orbital period of
a little less than a week. The fact that the object is a strong X-ray
emitter and that the optical and X-ray emission varies on very short time
scales (as short as one one-thousandth of a second) suggest that the
companion might be a black hole.