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Cygnus X-1 is an x-ray source in the Cygnus constellation. It is one of the strongest x-ray sources seen from Earth, producing a peak x-ray flux density of 2.3×10-23Wm-2Hz-1
Cygnus X-1 is part of a high mass x-ray binary system 6,000 light years from the Sun. The other part of the system is a blue supergiant variable star known as HDE 226868 that orbits .2 astronomical units from the black hole. A stellar wind removes material from the star and provides the material in the accretion disk around Cygnus. The matter that has reached the inner disk is heated to several million degrees Kelvin. These high temperature generate the observable x-rays. Cygnus X-1 and its binary companion orbit around their center of mass every 5.5999 days. To an observer here on Earth the system does not eclipse, but the inclination of the orbital plane remains uncertain. The orbit is thought to be nearly circular with an eccentricity of .06.
The binary system shares a common motion with an association of massive stars named Cygnus OB3. This implies that the star, Cygnus X-1, and the OB3 association may have formed at the same time and location. This would make the age of the system about 5-6 million years. The motion of HDE 226868 with respect to Cygnus OB3 is 9 km/s; a typical value for random motion within a stellar association. The star is 60 parsecs from the center of the association. It would have taken around 7 million years to get there, which agrees with estimated age of the association. Since the system is located at galactic latitude 4 degrees and longitude 71 degrees, it lies along the same Orion Spur that the sun is in.
The compact object is thought to be orbited by a thin, flat accretion disk. This disk is intensely heated by friction between ionized gas in faster-moving inner orbits and that in slower outer ones. It is divided into a hot inner region with a relatively high level of ionization(plasma) and a cooler, less ionized outer region that extends to 500 times the Schwarzschild radius(15,000 km). The emitted x-rays are erratic and highly variable. The X-rays are produced as lower energy photons in the inner accretion disk, but their energy is increased by Compton scattering with very high temperature electrons in a nearly transparent corona.
Despite this being one of the most studied objects in the known universe, there are many things yet to be discovered.
We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?