Cygnus X-1, also known as V1357 Cyg and HD 226868, was the first black hole candidate ever discovered.

The field of X-Ray astronomy grew rapidly in the 1960s. The brightest X-ray source besides the Sun, Scorpius X-1, was discovered first (in 1962), but much more sensitive detectors revealed many others in short order. Cygnus X-1 was discovered by Bowyer, Byram, Chubb and Friedman of the Naval Research Laboratory, using sounding rockets launched from the White Sands Missile Range in southern New Mexico in 1963 and 1964 (Science volume 147, p. 394). Their most successful flight was on June 16, 1964, in which they installed a sensitive geiger counter in the nose cone of an Aerobee rocket launched to a height of 127 miles. A collimator was placed in front of the detector to limit the field of view, and the rocket was then allowed to rotate and precess during the flight. In this way, the geiger counter was able to scan a large fraction of the sky. During this flight, they were able to detect eight new sources, including the brightest one dubbed Cygnus XR-1, the first X-ray source detected in the constellation Cygnus. Due to the limited resolution of the detector, they were unable to precisely determine its position and associate it with an optical counterpart.

Positive detection of this counterpart did not come until 1972. C.T. Bolton of the David Dunlap Observatory/University of Toronto found that the massive star HD 226868 was coincident with the best X-ray and radio positions of Cygnus X-1, and showed spectral evidence of binarity and variability (Nature volume 235, p. 271). Bolton also found something quite spectacular about this binary star. Based upon the optical spectrum, the visible star is very hot, and must have a mass of at least twelve times the mass of the Sun. No light was detected from a companion, suggesting it must be small and faint. However, the orbital period of 5.6 days and the orbital velocity amplitude of 66 kilometers per second requires a mass for the companion of at least three times the mass of the sun. Although the study of neutron stars was then very young, it was understood that neutron stars could not have masses greater than three solar masses. Thus the source powering the X-rays was most likely a black hole. Bolton's assessment of the system stands to this day, though the mass of the black hole is now believed to be at least seven times the mass of the Sun, far larger than a neutron star could be.

Like nearly all stellar X-ray sources, the high-energy radiation generated by the Cyg X-1 system comes from the accretion disk, mass that is being pulled off the companion star by the black hole. As the gas flows from the massive star to the black hole, it forms an accretion disk around the black hole. As it does so, viscosity within the disk converts the angular momentum of the gas into heat -- lots of heat, generating X-rays in the process. Most of the X-rays are generated via thermal bremsstrahlung and via inverse Compton scattering of lower energy photons off of hot electrons in the black hole's corona. The accretion done by black holes is slightly different from that done by white dwarf and neutron star binaries. Black holes have no surface (other than the event horizon) so when the gas reaches the horizon, it simply disappears, meaning that all the radiation has to come from the accretion disk. Stellar black hole accretion systems are known to have soft spectra, meaning that they are brighter in low-energy X-rays than in high-energy X-rays. However, despite the soft spectrum, Cygnus X-1 does emit some high energy photons, even gamma rays. Most of the gamma rays come from inverse Compton scattering, while some may also come from particle annihilations, for example from proton-proton collisions and pion decays.

Cygnus X-1 is a member of the high-mass X-ray binaries class or HMXBs, so named because the stellar companion is a high-mass star. (As a counterexample, Scorpius X-1 is a low-mass X-ray binary because its companion is a dwarf star.) Cygnus X-1 is also highly variable, with the variability caused by the orbit of the binary pair, precession of the accretion disk, and instabilities in the accretion stream. Furthermore, it undergoes what are called state changes where the shape of the X-ray spectrum changes over short time scales, briefly transitioning from a soft spectrum to a hard one and back again. These are also due to changes in the accretion disk.

The Cygnus X-1 system (both the black hole and companion star) are not visible with the naked eye, though a small telescope might let you see the stellar companion if you can point at the coordinates precisely enough (right ascension 19h 58m 21.68s, declination +35 12 05.8). The visible star is a O9I blue supergiant, with a visual magnitude of around 9, and would be hard to distinguish from the other stars in the Milky Way without a good star chart. However, Cygnus is a good summer constellation, and it lies directly along the Milky Way, so catch it while you can -- it should be directly overhead in the evenings in the northern hemisphere.

Sources: mainly the two papers noted above, as well as "Compact X-Ray Sources" by Blumenthal and Tucker (Annual Reviews of Astronomy and Astrophysics 1974) and "Evidence for Black Holes in Stellar Binary Systems" by Cowley (Annual Reviews... 1992). All papers and other info obtained from http://adsabs.harvard.edu, and coordinates from http://simbad.harvard.edu. Oh yeah, and Neil Peart too. :)

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