<<Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus, and the first such source widely accepted to be a black hole. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 2,300 Jansky. It remains among the most studied astronomical objects in its class. Cygnus X-1 belongs to a high-mass X-ray binary system, located about 6,070 light-years from the Sun, that includes a blue supergiant variable star designated HDE 226868. The compact object is now estimated to have a mass about 14.8 times the mass of the Sun and has been shown to be too small to be any known kind of normal star, or other likely object besides a black hole. If so, the radius of its event horizon has 300 km "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1 ms. Cygnus X-1 is about five million years old and formed from a progenitor star that had more than 40 solar masses. The majority of the star's mass was shed, most likely as a stellar wind. If this star had then exploded as a supernova, the resulting force would most likely have ejected the remnant from the system. Hence the star may have instead collapsed directly into a black hole.
The compact object and blue supergiant star form a binary system in which they orbit around their center of mass every 5.599829 days. From the perspective of the Earth, the compact object never goes behind the other star; however, the inclination of the orbital plane to the line of sight from the Earth remains uncertain, with predictions ranging from 27–65°. A 2007 study estimated the inclination is 48.0±6.8°, which would mean that the semi-major axis is about 0.2 AU, or 20% of the distance from the Earth to the Sun. The orbital eccentricity is thought to be only 0.0018±0.002; a nearly circular orbit. A stellar wind from the star provides material for an accretion disk around the X-ray source. Matter in the inner disk is heated to millions of degrees, generating the observed X-rays. A pair of jets, arranged perpendicularly to the disk, are carrying part of the energy of the infalling material away into interstellar space.
As accreted matter falls toward the compact object, it loses gravitational potential energy. Part of this released energy is dissipated by jets of particles, aligned perpendicular to the accretion disk, that flow outward with relativistic velocities. (That is, the particles are moving at a significant fraction of the speed of light.) This pair of jets provide a means for an accretion disk to shed excess energy and angular momentum. They may be created by magnetic fields within the gas that surrounds the compact object.
The Cygnus X-1 jets are inefficient radiators and so release only a small proportion of their energy in the electromagnetic spectrum. That is, they appear "dark". The estimated angle of the jets to the line of sight is 30° and they may be precessing. One of the jets is colliding with a relatively dense part of the interstellar medium (ISM), forming an energized ring that can be detected by its radio emission. This collision appears to be forming a nebula that has been observed in the optical wavelengths. To produce this nebula, the jet must have an estimated average power of 9×1029
W. This is more than 1,000 times the power emitted by the Sun.] There is no corresponding ring in the opposite direction because that jet is facing a lower density region of the ISM.
In 2006, Cygnus X-1 became the first stellar mass black hole found to display evidence of gamma ray emission in the very high energy band, above 100 GeV. The signal was observed at the same time as a flare of hard X-rays, suggesting a link between the events. The X-ray flare may have been produced at the base of the jet while the gamma rays could have been generated where the jet interacts with the stellar wind of HDE 226868.
Cygnus X-1 was the subject of a friendly scientific wager between physicists Stephen Hawking and Kip Thorne in 1974, with Hawking betting that it was not a black hole. He conceded the bet in 1990 after observational data had strengthened the case that there was indeed a black hole in the system. This hypothesis lacks direct empirical evidence but has generally been accepted from indirect evidence.>>