Cygnus X-1 takes approximately 5.6 days to orbit its companion star.
Cygnus X-1 is a binary system containing a stellar-mass black hole and a blue supergiant star. The two objects orbit one another closely, with the black hole pulling gas from the companion. That gas forms a hot accretion flow and produces intense X-rays.
The 5.6-day orbital period is measured from the repeating motion of the companion star and changes in the system’s radiation. Cygnus X-1 lies in the constellation Cygnus, roughly 6,000 light-years from Earth.
The system became one of the strongest early candidates for a black hole because its unseen companion is too massive to be a neutron star. The orbital period alone does not prove the object is a black hole; the mass estimate and absence of a visible surface are also important evidence.