Epoch J2000 Equinox J2000
|Right ascension||19h 58m 21.67595s|
|Declination||+35° 12′ 05.7783″|
|Apparent magnitude (V)||8.95|
|U−B color index||−0.30|
|B−V color index||+0.81|
|Variable type||Ellipsoidal variable|
|Radial velocity (Rv)||−13 km/s|
|Proper motion (μ)|| RA: −3.37 mas/yr |
Dec.: −7.15 mas/yr
|Parallax (π)||0.539 ± 0.033 mas|
|Distance||6,100 ± 400 ly |
(1,900 ± 100 pc)
|Absolute magnitude (MV)||−6.5±0.2|
|Surface gravity (log g)||3.31±0.07 cgs|
|Rotation||every 5.6 days|
Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was 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 2.3×10−23 Wm−2 Hz−1 (2.3×103 Jansky). It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 21.2 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 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 which it orbits at about 0.2 AU, or 20% of the distance from the Earth to the Sun. 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.
This system may belong to a stellar association called Cygnus OB3, which would mean that 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.
Cygnus X-1 was the subject of a friendly scientific wager between physicists Stephen Hawking and Kip Thorne in 1975, 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.
Discovery and observation
Observation of X-ray emissions allows astronomers to study celestial phenomena involving gas with temperatures in the millions of degrees. However, because X-ray emissions are blocked by the Earth's atmosphere, observation of celestial X-ray sources is not possible without lifting instruments to altitudes where the X-rays can penetrate. Cygnus X-1 was discovered using X-ray instruments that were carried aloft by a sounding rocket launched from White Sands Missile Range in New Mexico. As part of an ongoing effort to map these sources, a survey was conducted in 1964 using two Aerobee suborbital rockets. The rockets carried Geiger counters to measure X-ray emission in wavelength range 1–15 Å across an 8.4° section of the sky. These instruments swept across the sky as the rockets rotated, producing a map of closely spaced scans.
As a result of these surveys, eight new sources of cosmic X-rays were discovered, including Cyg XR-1 (later Cyg X-1) in the constellation Cygnus. The celestial coordinates of this source were estimated as right ascension 19h53m and declination 34.6°. It was not associated with any especially prominent radio or optical source at that position.
Seeing a need for longer duration studies, in 1963 Riccardo Giacconi and Herb Gursky proposed the first orbital satellite to study X-ray sources. NASA launched their Uhuru Satellite in 1970, which led to the discovery of 300 new X-ray sources. Extended Uhuru observations of Cygnus X-1 showed fluctuations in the X-ray intensity that occurs several times a second. This rapid variation meant that the energy generation must take place over a relatively small region of roughly 105 km, as the speed of light restricts communication between more distant regions. For a size comparison, the diameter of the Sun is about 1.4×106 km.
In April–May 1971, Luc Braes and George K. Miley from Leiden Observatory, and independently Robert M. Hjellming and Campbell Wade at the National Radio Astronomy Observatory, detected radio emission from Cygnus X-1, and their accurate radio position pinpointed the X-ray source to the star AGK2 +35 1910 = HDE 226868. On the celestial sphere, this star lies about half a degree from the 4th-magnitude star Eta Cygni. It is a supergiant star that is, by itself, incapable of emitting the observed quantities of X-rays. Hence, the star must have a companion that could heat gas to the millions of degrees needed to produce the radiation source for Cygnus X-1.
Louise Webster and Paul Murdin, at the Royal Greenwich Observatory, and Charles Thomas Bolton, working independently at the University of Toronto's David Dunlap Observatory, announced the discovery of a massive hidden companion to HDE 226868 in 1971. Measurements of the Doppler shift of the star's spectrum demonstrated the companion's presence and allowed its mass to be estimated from the orbital parameters. Based on the high predicted mass of the object, they surmised that it may be a black hole as the largest possible neutron star cannot exceed three times the mass of the Sun.
With further observations strengthening the evidence, by the end of 1973 the astronomical community generally conceded that Cygnus X-1 was most likely a black hole. More precise measurements of Cygnus X-1 demonstrated variability down to a single millisecond. This interval is consistent with turbulence in a disk of accreted matter surrounding a black hole—the accretion disk. X-ray bursts that last for about a third of a second match the expected time frame of matter falling toward a black hole.
Cygnus X-1 has since been studied extensively using observations by orbiting and ground-based instruments. The similarities between the emissions of X-ray binaries such as HDE 226868/Cygnus X-1 and active galactic nuclei suggests a common mechanism of energy generation involving a black hole, an orbiting accretion disk and associated jets. For this reason, Cygnus X-1 is identified among a class of objects called microquasars; an analog of the quasars, or quasi-stellar radio sources, now known to be distant active galactic nuclei. Scientific studies of binary systems such as HDE 226868/Cygnus X-1 may lead to further insights into the mechanics of active galaxies.
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; in other words, the system does not eclipse. 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. Earth's distance to this system is about 1,860 ± 120 parsecs (6,070 ± 390 light-years).
The HDE 226868/Cygnus X-1 system shares a common motion through space with an association of massive stars named Cygnus OB3, which is located at roughly 2,000 parsecs from the Sun. This implies that HDE 226868, Cygnus X-1 and this OB association may have formed at the same time and location. If so, then the age of the system is about 5±1.5 Ma. The motion of HDE 226868 with respect to Cygnus OB3 is 9±3 km/s; a typical value for random motion within a stellar association. HDE 226868 is about 60 parsecs from the center of the association, and could have reached that separation in about 7±2 Ma—which roughly agrees with estimated age of the association.
With a galactic latitude of 4 degrees and galactic longitude 71 degrees, this system lies inward along the same Orion Spur in which the Sun is located within the Milky Way, near where the spur approaches the Sagittarius Arm. Cygnus X-1 has been described as belonging to the Sagittarius Arm, though the structure of the Milky Way is not well established.
From various techniques, the mass of the compact object appears to be greater than the maximum mass for a neutron star. Stellar evolutionary models suggest a mass of 20±5 solar masses, while other techniques resulted in 10 solar masses. Measuring periodicities in the X-ray emission near the object yielded a more precise value of 14.8±1 solar masses. In all cases, the object is most likely a black hole—a region of space with a gravitational field that is strong enough to prevent the escape of electromagnetic radiation from the interior. The boundary of this region is called the event horizon and has an effective radius called the Schwarzschild radius, which is about 44 km for Cygnus X-1. Anything (including matter and photons) that passes through this boundary is unable to escape. New measurements published in 2021 yielded an estimated mass of 21.2±2.2 solar masses.
Evidence of just such an event horizon may have been detected in 1992 using ultraviolet (UV) observations with the High Speed Photometer on the Hubble Space Telescope. As self-luminous clumps of matter spiral into a black hole, their radiation will be emitted in a series of pulses that are subject to gravitational redshift as the material approaches the horizon. That is, the wavelengths of the radiation will steadily increase, as predicted by general relativity. Matter hitting a solid, compact object would emit a final burst of energy, whereas material passing through an event horizon would not. Two such "dying pulse trains" were observed, which is consistent with the existence of a black hole.
The spin of the compact object is not yet well determined. Past analysis of data from the space-based Chandra X-ray Observatory suggested that Cygnus X-1 was not rotating to any significant degree. However, evidence announced in 2011 suggests it is rotating extremely rapidly, approximately 790 times per second.
The largest star in the Cygnus OB3 association has a mass 40 times that of the Sun. As more massive stars evolve more rapidly, this implies that the progenitor star for Cygnus X-1 had more than 40 solar masses. Given the current estimated mass of the black hole, the progenitor star must have lost over 30 solar masses of material. Part of this mass may have been lost to HDE 226868, while the remainder was most likely expelled by a strong stellar wind. The helium enrichment of HDE 226868's outer atmosphere may be evidence for this mass transfer. Possibly the progenitor may have evolved into a Wolf–Rayet star, which ejects a substantial proportion of its atmosphere using just such a powerful stellar wind.
If the progenitor star had exploded as a supernova, then observations of similar objects show that the remnant would most likely have been ejected from the system at a relatively high velocity. As the object remained in orbit, this indicates that the progenitor may have collapsed directly into a black hole without exploding (or at most produced only a relatively modest explosion).
The compact object is thought to be orbited by a thin, flat disk of accreting matter known as an 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—forming a plasma—and a cooler, less ionized outer region that extends to an estimated 500 times the Schwarzschild radius, or about 15,000 km.
Though highly and erratically variable, Cygnus X-1 is typically the brightest persistent source of hard X-rays—those with energies from about 30 up to several hundred keV—in the sky. The X-rays are produced as lower-energy photons in the thin inner accretion disk, then given more energy through Compton scattering with very high-temperature electrons in a geometrically thicker, but nearly transparent corona enveloping it, as well as by some further reflection from the surface of the thin disk. An alternative possibility is that the X-rays may be Compton scattered by the base of a jet instead of a disk corona.
The X-ray emission from Cygnus X-1 can vary in a somewhat repetitive pattern called quasi-periodic oscillations (QPO). The mass of the compact object appears to determine the distance at which the surrounding plasma begins to emit these QPOs, with the emission radius decreasing as the mass decreases. This technique has been used to estimate the mass of Cygnus X-1, providing a cross-check with other mass derivations.
Pulsations with a stable period, similar to those resulting from the spin of a neutron star, have never been seen from Cygnus X-1. The pulsations from neutron stars are caused by the neutron star's magnetic field; however, the no-hair theorem guarantees that black holes do not have magnetic poles. For example, the X-ray binary V 0332+53 was thought to be a possible black hole until pulsations were found. Cygnus X-1 has also never displayed X-ray bursts similar to those seen from neutron stars. Cygnus X-1 unpredictably changes between two X-ray states, although the X-rays may vary continuously between those states as well. In the most common state, the X-rays are "hard", which means that more of the X-rays have high energy. In the less common state, the X-rays are "soft", with more of the X-rays having lower energy. The soft state also shows greater variability. The hard state is believed to originate in a corona surrounding the inner part of the more opaque accretion disk. The soft state occurs when the disk draws closer to the compact object (possibly as close as 150 km), accompanied by cooling or ejection of the corona. When a new corona is generated, Cygnus X-1 transitions back to the hard state.
The spectral transition of Cygnus X-1 can be explained using a two component advective flow solution, as proposed by Chakrabarti and Titarchuk. A hard state is generated by the inverse Comptonisation of seed photons from the Keplarian disk and likewise synchrotron photons produced by the hot electrons in the Centrifugal Pressure-supported Boundary Layer (CENBOL).
The X-ray flux from Cygnus X-1 varies periodically every 5.6 d, especially during superior conjunction when the orbiting objects are most closely aligned with the Earth and the compact source is the more distant. This indicates that the emissions are being partially blocked by circumstellar matter, which may be the stellar wind from the star HDE 226868. There is a roughly 300 d periodicity in the emission that could be caused by the precession of the accretion disk.
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 4–14×1036 erg/s, or (9±5)×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.
HDE 226868 is a supergiant star with a spectral class of O9.7 Iab, which is on the borderline between class O and class B stars. It has an estimated surface temperature of 31,000 K and mass approximately 20–40 times the mass of the Sun. Based on a stellar evolutionary model, at the estimated distance of 2,000 parsecs this star may have a radius equal to about 15–17 times the solar radius and is approximately 300,000–400,000 times the luminosity of the Sun. For comparison, the compact object is estimated to be orbiting HDE 226868 at a distance of about 40 solar radii, or twice the radius of this star.
The surface of HDE 226868 is being tidally distorted by the gravity of the massive companion, forming a tear-drop shape that is further distorted by rotation. This causes the optical brightness of the star to vary by 0.06 magnitudes during each 5.6-day binary orbit, with the minimum magnitude occurring when the system is aligned with the line of sight. The "ellipsoidal" pattern of light variation results from the limb darkening and gravity darkening of the star's surface.
When the spectrum of HDE 226868 is compared to the similar star Epsilon Orionis, the former shows an overabundance of helium and an underabundance of carbon in its atmosphere. The ultraviolet and hydrogen alpha spectral lines of HDE 226868 show profiles similar to the star P Cygni, which indicates that the star is surrounded by a gaseous envelope that is being accelerated away from the star at speeds of about 1,500 km/s.
Like other stars of its spectral type, HDE 226868 is thought to be shedding mass in a stellar wind at an estimated rate of 2.5×10−6 solar masses per year. This is the equivalent of losing a mass equal to the Sun's every 400,000 years. The gravitational influence of the compact object appears to be reshaping this stellar wind, producing a focused wind geometry rather than a spherically symmetrical wind. X-rays from the region surrounding the compact object heat and ionize this stellar wind. As the object moves through different regions of the stellar wind during its 5.6-day orbit, the UV lines, the radio emission, and the X-rays themselves all vary.
The Roche lobe of HDE 226868 defines the region of space around the star where orbiting material remains gravitationally bound. Material that passes beyond this lobe may fall toward the orbiting companion. This Roche lobe is believed to be close to the surface of HDE 226868 but not overflowing, so the material at the stellar surface is not being stripped away by its companion. However, a significant proportion of the stellar wind emitted by the star is being drawn onto the compact object's accretion disk after passing beyond this lobe.
The gas and dust between the Sun and HDE 226868 results in a reduction in the apparent magnitude of the star as well as a reddening of the hue—red light can more effectively penetrate the dust in the interstellar medium. The estimated value of the interstellar extinction (AV) is 3.3 magnitudes. Without the intervening matter, HDE 226868 would be a fifth-magnitude star and, thus, visible to the unaided eye.
Stephen Hawking and Kip Thorne
Cygnus X-1 was the subject of a bet between physicists Stephen Hawking and Kip Thorne, in which Hawking bet against the existence of black holes in the region. Hawking later described this as an "insurance policy" of sorts. In his book A Brief History of Time he wrote:
This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would win me four years of the magazine Private Eye. If black holes do exist, Kip will get one year of Penthouse. When we made the bet in 1975, we were 80% certain that Cygnus X-1 was a black hole. By now , I would say that we are about 95% certain, but the bet has yet to be settled.
According to the updated tenth-anniversary edition of A Brief History of Time, Hawking has conceded the bet due to subsequent observational data in favor of black holes. In his own book, Black Holes and Time Warps, Thorne reports that Hawking conceded the bet by breaking into Thorne's office while he was in Russia, finding the framed bet, and signing it. (While Hawking referred to the bet as taking place in 1975, the written bet itself (in Thorne's handwriting, with his and Hawking's signatures) bears additional witness signatures under a legend stating "Witnessed this tenth day of December 1974". This date was confirmed by Kip Thorne on the January 10, 2018 episode of Nova on PBS.)
- van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600
- Staff (March 3, 2003), V* V1357 Cyg -- High Mass X-ray Binary, Centre de Données astronomiques de Strasbourg, retrieved 2008-03-03
- Bregman, J.; et al. (1973), "Colors, magnitudes, spectral types and distances for stars in the field of the X-ray source Cyg X-1", Lick Observatory Bulletin, 647: 1, Bibcode:1973LicOB..24....1B
- Reid, Mark J.; et al. (December 2011), "The Trigonometric Parallax of Cygnus X-1", The Astrophysical Journal, 742 (2): 83, arXiv:1106.3688, Bibcode:2011ApJ...742...83R, doi:10.1088/0004-637X/742/2/83, S2CID 96429771
- Ninkov, Z.; Walker, G. A. H.; Yang, S. (1987), "The primary orbit and the absorption lines of HDE 226868 (Cygnus X-1)", Astrophysical Journal, 321: 425–437, Bibcode:1987ApJ...321..425N, doi:10.1086/165641, archived from the original on 2017-09-22, retrieved 2018-11-04
- Miller-Jones, James C. A.; et al. (18 February 2021). "Cygnus X-1 contains a 21–solar mass black hole—Implications for massive star winds". Science. arXiv:2102.09091. doi:10.1126/science.abb3363. Retrieved 21 February 2021. CS1 maint: discouraged parameter (link)
- Overbye, Dennis (18 February 2021). "A Famous Black Hole Gets a Massive Update - Cygnus X-1, one of the first identified black holes, is much weightier than expected, raising new questions about how such objects form". The New York Times. Retrieved 21 February 2021. CS1 maint: discouraged parameter (link)
- Ziółkowski, J. (2005), "Evolutionary constraints on the masses of the components of HDE 226868/Cyg X-1 binary system", Monthly Notices of the Royal Astronomical Society, 358 (3): 851–859, arXiv:astro-ph/0501102, Bibcode:2005MNRAS.358..851Z, doi:10.1111/j.1365-2966.2005.08796.x, S2CID 119334761 Note: for radius and luminosity, see Table 1 with d=2 kpc.
- Hadrava, Petr (September 15–21, 2007), "Optical spectroscopy of Cyg X-1", Proceedings of RAGtime 8/9: Workshops on Black Holes and Neutron Stars, Opava, Czech Republic: 71, arXiv:0710.0758, Bibcode:2007ragt.meet...71H
- Staff (June 10, 2003), Integral's view of Cygnus X-1, ESA, retrieved 2008-03-20
- Mirabel, I. Félix; Rodrigues, Irapuan (2003), "Formation of a Black Hole in the Dark", Science, 300 (5622): 1119–1120, arXiv:astro-ph/0305205, Bibcode:2003Sci...300.1119M, doi:10.1126/science.1083451, PMID 12714674, S2CID 45544180
- Bowyer, S.; et al. (1965), "Cosmic X-ray Sources", Science, 147 (3656): 394–398, Bibcode:1965Sci...147..394B, doi:10.1126/science.147.3656.394, PMID 17832788, S2CID 206565068
- Staff (2004-11-05), Observations: Seeing in X-ray wavelengths, ESA, retrieved 2008-08-12
- Glister, Paul (2011), "Cygnus X-1: A Black Hole Confirmed." Centauri Dreams: Imagining and Planning Interstellar Exploration, 2011-11-29. Accessed 2016-09-16.
- Lewin, Walter; Van Der Klis, Michiel (2006), Compact Stellar X-ray Sources, Cambridge University Press, p. 159, ISBN 0-521-82659-4
- "2010 X-Ray Sources", The Astronomical Almanac, U.S. Naval Observatory, retrieved 2009-08-04 gives a range of 235–1320 μJy at energies of 2–10 kEv, where a Jansky (Jy) is 10−26 Wm−2 Hz−1.
- The Illustrated Encyclopedia of the Universe. New York, NY: Watson-Guptill. 2001. p. 175. ISBN 0-8230-2512-8.
- Harko, T. (June 28, 2006), Black Holes, University of Hong Kong, archived from the original on February 10, 2009, retrieved 2008-03-28
- Ziolkowski, Janusz (2014). "Masses of the components of the HDE 226868/Cyg X-1 binary system". Monthly Notices of the Royal Astronomical Society: Letters. 440: L61. arXiv:1401.1035. Bibcode:2014MNRAS.440L..61Z. doi:10.1093/mnrasl/slu002. S2CID 54841624.
- Gies, D. R.; Bolton, C. T. (1986), "The optical spectrum of HDE 226868 = Cygnus X-1. II — Spectrophotometry and mass estimates", The Astrophysical Journal, 304: 371–393, Bibcode:1986ApJ...304..371G, doi:10.1086/164171
- Nayakshin, Sergei; Dove, James B. (November 3, 1998), "X-rays From Magnetic Flares In Cygnus X-1: The Role Of A Transition Layer", arXiv:astro-ph/9811059
- Young, A. J.; et al. (2001), "A Complete Relativistic Ionized Accretion Disc in Cygnus X-1", Monthly Notices of the Royal Astronomical Society, 325 (3): 1045–1052, arXiv:astro-ph/0103214, Bibcode:2001MNRAS.325.1045Y, doi:10.1046/j.1365-8711.2001.04498.x, S2CID 14226526
- Gallo, Elena; Fender, Rob (2005), "Accretion modes and jet production in black hole X-ray binaries", Memorie della Società Astronomica Italiana, 76: 600–607, arXiv:astro-ph/0509172, Bibcode:2005MmSAI..76..600G
- Staff (February 27, 2004), Galaxy Entree or Main Course?, Swinburne University, retrieved 2008-03-31
- Herbert, Friedman (2002), "From the ionosphere to high energy astronomy – a personal experience", The Century of Space Science, Springer, ISBN 0-7923-7196-8
- Liu, C. Z.; Li, T. P. (2004), "X-Ray Spectral Variability in Cygnus X-1", The Astrophysical Journal, 611 (2): 1084–1090, arXiv:astro-ph/0405246, Bibcode:2004ApJ...611.1084L, doi:10.1086/422209
- Staff (June 26, 2003), The Uhuru Satellite, NASA, retrieved 2008-05-09
- Giacconi, Riccardo (December 8, 2002), The Dawn of X-Ray Astronomy, The Nobel Foundation, retrieved 2008-03-24
- Oda, M.; et al. (1999), "X-Ray Pulsations from Cygnus X-1 Observed from UHURU", The Astrophysical Journal, 166: L1–L7, Bibcode:1971ApJ...166L...1O, doi:10.1086/180726
- This is the distance light can travel in a third of a second.
- Kristian, J.; et al. (1971), "On the Optical Identification of Cygnus X-1", The Astrophysical Journal, 168: L91–L93, Bibcode:1971ApJ...168L..91K, doi:10.1086/180790
- Braes, L.L.E.; Miley, G.K. (July 23, 1971), "Physical Sciences: Detection of Radio Emission from Cygnus X-1", Nature, 232 (5308): 246, Bibcode:1971Natur.232Q.246B, doi:10.1038/232246a0, PMID 16062947, S2CID 33340308
- Braes, L.L.E.; Miley, G.K. (1971), "Variable Radio Emission from X-Ray Sources", Veröffentlichungen Remeis-Sternwarte Bamberg, 9 (100): 173, Bibcode:1972VeBam.100......
- Abrams, Bernard; Stecker, Michael (1999), Structures in Space: Hidden Secrets of the Deep Sky, Springer, p. 91, ISBN 1-85233-165-8,
Eta Cygni is 25 arc minutes to the west-south-west of this star.
- Webster, B. Louise; Murdin, Paul (1972), "Cygnus X-1—a Spectroscopic Binary with a Heavy Companion?", Nature, 235 (5332): 37–38, Bibcode:1972Natur.235...37W, doi:10.1038/235037a0, S2CID 4195462
- Bolton, C. T. (1972), "Identification of Cygnus X-1 with HDE 226868", Nature, 235 (5336): 271–273, Bibcode:1972Natur.235..271B, doi:10.1038/235271b0, S2CID 4222070
- Luminet, Jean-Pierre (1992), Black Holes, Cambridge University Press, ISBN 0-521-40906-3
- Bombaci, I. (1996), "The maximum mass of a neutron star", Astronomy and Astrophysics, 305: 871–877, arXiv:astro-ph/9608059, Bibcode:1996A&A...305..871B, doi:10.1086/310296, S2CID 119085893
- Rolston, Bruce (November 10, 1997), The First Black Hole, University of Toronto, archived from the original on March 7, 2008, retrieved 2008-03-11
- Shipman, H. L.; Yu, Z; Du, Y.W (1975), "The implausible history of triple star models for Cygnus X-1 Evidence for a black hole", Astrophysical Letters, 16 (1): 9–12, Bibcode:1975ApL....16....9S, doi:10.1016/S0304-8853(99)00384-4
- Rothschild, R. E.; et al. (1974), "Millisecond Temporal Structure in Cygnus X-1", The Astrophysical Journal, 189: 77–115, Bibcode:1974ApJ...189L..13R, doi:10.1086/181452
- Koerding, Elmar; Jester, Sebastian; Fender, Rob (2006), "Accretion states and radio loudness in Active Galactic Nuclei: analogies with X-ray binaries", Monthly Notices of the Royal Astronomical Society, 372 (3): 1366–1378, arXiv:astro-ph/0608628, Bibcode:2006MNRAS.372.1366K, doi:10.1111/j.1365-2966.2006.10954.x, S2CID 14833297
- Brainerd, Jim (July 20, 2005), X-rays from AGNs, The Astrophysics Spectator, retrieved 2008-03-24
- Brocksopp, C.; et al. (1999), "An Improved Orbital Ephemeris for Cygnus X-1", Astronomy & Astrophysics, 343: 861–864, arXiv:astro-ph/9812077, Bibcode:1999A&A...343..861B
- Orosz, Jerome (December 1, 2011), "The Mass of the Black Hole In Cygnus X-1", The Astrophysical Journal, 742 (2): 84, arXiv:1106.3689, Bibcode:2011ApJ...742...84O, doi:10.1088/0004-637X/742/2/84, S2CID 18732012
- Bolton, C. T. (1975), "Optical observations and model for Cygnus X-1", The Astrophysical Journal, 200: 269–277, Bibcode:1975ApJ...200..269B, doi:10.1086/153785
- Gursky, H.; et al. (1971), "The Estimated Distance to Cygnus X-1 Based on its Low-Energy X-Ray Spectrum", Astrophysical Journal, 167: L15, Bibcode:1971ApJ...167L..15G, doi:10.1086/180751
- Goebel, Greg, 7.0 The Milky Way Galaxy, In The Public Domain, archived from the original on 2008-06-12, retrieved 2008-06-29
- Strohmayer, Tod; Shaposhnikov, Nikolai; Schartel, Norbert (May 16, 2007), New technique for 'weighing' black holes, ESA, retrieved 2008-03-10
- Staff (January 9, 2006), Scientists find black hole's 'point of no return', Massachusetts Institute of Technology, archived from the original on 13 January 2006, retrieved 2008-03-28
- Dolan, Joseph F. (2001), "Dying Pulse Trains in Cygnus XR-1: Evidence for an Event Horizon?", The Publications of the Astronomical Society of the Pacific, 113 (786): 974–982, Bibcode:2001PASP..113..974D, doi:10.1086/322917
- Miller, J. M.; et al. (July 20–26, 2003), "Relativistic Iron Lines in Galactic Black Holes: Recent Results and Lines in the ASCA Archive", Proceedings of the 10th Annual Marcel Grossmann Meeting on General Relativity, Rio de Janeiro, Brazil, p. 1296, arXiv:astro-ph/0402101, Bibcode:2006tmgm.meet.1296M, doi:10.1142/9789812704030_0093, ISBN 9789812566676, S2CID 119336501
- Roy, Steve; Watzke, Megan (September 17, 2003), "Iron-Clad" Evidence For Spinning Black Hole, Chandra press Room, retrieved 2008-03-11
- Gou, Lijun; et al. (November 9, 2011), "The Extreme Spin of the Black Hole in Cygnus X-1", The Astrophysical Journal, American Astronomical Society, 742 (85): 85, arXiv:1106.3690, Bibcode:2011ApJ...742...85G, doi:10.1088/0004-637X/742/2/85, S2CID 16525257
- Podsiadlowski, Philipp; Saul, Rappaport; Han, Zhanwen (2003), "On the formation and evolution of black-hole binaries", Monthly Notices of the Royal Astronomical Society, 341 (2): 385–404, arXiv:astro-ph/0207153, Bibcode:2003MNRAS.341..385P, doi:10.1046/j.1365-8711.2003.06464.x, S2CID 119476943
- Staff (August 30, 2006), More Images of Cygnus X-1, XTE J1650-500 & GX 339-4, Harvard-Smithsonian Center for Astrophysics/Chandra X-ray Center, retrieved 2008-03-30
- Ling, J. C.; et al. (1997), "Gamma-Ray Spectra and Variability of Cygnus X-1 Observed by BATSE", The Astrophysical Journal, 484 (1): 375–382, Bibcode:1997ApJ...484..375L, doi:10.1086/304323
- Kylafis, N.; Giannios, D.; Psaltis, D. (2006), "Spectra and time variability of black-hole binaries in the low/hard state", Advances in Space Research, 38 (12): 2810–2812, Bibcode:2006AdSpR..38.2810K, doi:10.1016/j.asr.2005.09.045
- Titarchuk, Lev; Shaposhnikov, Nikolai (February 9, 2008), "On the nature of the variability power decay towards soft spectral states in X-ray binaries. Case study in Cyg X-1", The Astrophysical Journal, 678 (2): 1230–1236, arXiv:0802.1278, Bibcode:2008ApJ...678.1230T, doi:10.1086/587124, S2CID 5195999
- Fabian, A. C.; Miller, J. M. (August 9, 2002), "Black Holes Reveal Their Innermost Secrets", Science, 297 (5583): 947–948, doi:10.1126/science.1074957, PMID 12169716, S2CID 118027201
- Wen, Han Chin (March 1998), Ten Microsecond Time Resolution Studies of Cygnus X-1, Stanford University, p. 6, Bibcode:1997PhDT.........6W
- Stella, L.; et al. (1985), "The discovery of 4.4 second X-ray pulsations from the rapidly variable X-ray transient V0332 + 53" (PDF), Astrophysical Journal Letters, 288: L45–L49, Bibcode:1985ApJ...288L..45S, doi:10.1086/184419
- Narayan, Ramesh (2003), "Evidence for the black hole event horizon", Astronomy & Geophysics, 44 (6): 77–115, arXiv:gr-qc/0204080, Bibcode:2003A&G....44f..22N, doi:10.1046/j.1468-4004.2003.44622.x
- Torres, Diego F.; et al. (2005), "Probing the Precession of the Inner Accretion Disk in Cygnus X-1", The Astrophysical Journal, 626 (2): 1015–1019, arXiv:astro-ph/0503186, Bibcode:2005ApJ...626.1015T, doi:10.1086/430125, S2CID 16569507
- S.K. Chakrabarti; L.G. Titarchuk (1995). "Spectral Properties of Accretion Disks around Galactic and Extragalactic Black Holes". Astrophysical Journal. 455: 623–668. arXiv:astro-ph/9510005v2. Bibcode:1995ApJ...455..623C. doi:10.1086/176610. S2CID 18151304.
- S.K. Chakrabarti; S. Mandal (2006). "The Spectral Properties of Shocked Two-Component Accretion Flows in the Presence of Synchrotron Emission". The Astrophysical Journal. 642 (1): L49–L52. Bibcode:2006ApJ...642L..49C. doi:10.1086/504319.
- Kitamoto, S.; et al. (2000), "GINGA All-Sky Monitor Observations of Cygnus X-1", The Astrophysical Journal, 531 (1): 546–552, Bibcode:2000ApJ...531..546K, doi:10.1086/308423
- Begelman, Mitchell C. (2003), "Evidence for Black Holes", Science, 300 (5627): 1898–1903, Bibcode:2003Sci...300.1898B, doi:10.1126/science.1085334, PMID 12817138, S2CID 46107747
- Russell, D. M.; et al. (2007), "The jet-powered optical nebula of Cygnus X-1", Monthly Notices of the Royal Astronomical Society, 376 (3): 1341–1349, arXiv:astro-ph/0701645, Bibcode:2007MNRAS.376.1341R, doi:10.1111/j.1365-2966.2007.11539.x, S2CID 18689655
- Sackmann, I.-Juliana; Boothroyd, Arnold I.; Kraemer, Kathleen E. (1993), "Our Sun. III. Present and Future", The Astrophysical Journal, 418: 457–468, Bibcode:1993ApJ...418..457S, doi:10.1086/173407
- Gallo, E.; et al. (2005), "A dark jet dominates the power output of the stellar black hole Cygnus X-1", Nature, 436 (7052): 819–821, arXiv:astro-ph/0508228, Bibcode:2005Natur.436..819G, doi:10.1038/nature03879, PMID 16094361, S2CID 4404783
- Albert, J.; et al. (2007), "Very High Energy Gamma-ray Radiation from the Stellar-mass Black Hole Cygnus X-1", Astrophysical Journal Letters, 665 (1): L51–L54, arXiv:0706.1505, Bibcode:2007ApJ...665L..51A, doi:10.1086/521145, S2CID 15302221
- Iorio, Lorenzo (2008), "On the orbital and physical parameters of the HDE 226868/Cygnus X-1 binary system", Astrophysics and Space Science, 315 (1–4): 335–340, arXiv:0707.3525, Bibcode:2008Ap&SS.315..335I, doi:10.1007/s10509-008-9839-y, S2CID 7759638
- Miller, J. M.; et al. (2005), "Revealing the Focused Companion Wind in Cygnus X-1 with Chandra", The Astrophysical Journal, 620 (1): 398–404, arXiv:astro-ph/0208463, Bibcode:2005ApJ...620..398M, doi:10.1086/426701, S2CID 51806148
- Caballero, M. D. (16–20 February 2004), "OMC-INTEGRAL: Optical Observations of X-Ray Sources", Proceedings of the 5th INTEGRAL Workshop on the INTEGRAL Universe (ESA SP-552). 16–20 February 2004, Munich, Germany: ESA, 552: 875–878, Bibcode:2004ESASP.552..875C
- Cox, Arthur C. (2001), Allen's Astrophysical Quantities, Springer, p. 407, ISBN 0-387-95189-X
- Canalizo, G.; et al. (1995), "Spectral variations and a classical curve-of-growth analysis of HDE 226868 (Cyg X-1)", Revista Mexicana de Astronomía y Astrofísica, 31 (1): 63–86, Bibcode:1995RMxAA..31...63C
- Conti, P. S. (1978), "Stellar parameters of five early type companions of X-ray sources", Astronomy and Astrophysics, 63: 225, Bibcode:1978A&A....63..225C
- Sowers, J. W.; et al. (1998), "Tomographic Analysis of Hα Profiles in HDE 226868/Cygnus X-1", The Astrophysical Journal, 506 (1): 424–430, Bibcode:1998ApJ...506..424S, doi:10.1086/306246
- Hutchings, J. B. (1976), "Stellar winds from hot supergiants", The Astrophysical Journal, 203: 438–447, Bibcode:1976ApJ...203..438H, doi:10.1086/154095
- Vrtilek, Saeqa D.; Hunacek, A.; Boroson, B. S. (2006), "X-Ray Ionization Effects on the Stellar Wind of Cygnus X-1", Bulletin of the American Astronomical Society, 38: 334, Bibcode:2006HEAD....9.0131V
- Pooley, G. G.; Fender, R. P.; Brocksopp, C. (1999), "Orbital modulation and longer-term variability in the radio emission from Cygnus X-1", Monthly Notices of the Royal Astronomical Society, 302 (1): L1–L5, arXiv:astro-ph/9809305, Bibcode:1999MNRAS.302L...1P, doi:10.1046/j.1365-8711.1999.02225.x, S2CID 2123824
- Gies, D. R.; et al. (2003), "Wind Accretion and State Transitions in Cygnus X-1", The Astrophysical Journal, 583 (1): 424–436, arXiv:astro-ph/0206253, Bibcode:2003ApJ...583..424G, doi:10.1086/345345, S2CID 6241544
- Margon, Bruce; Bowyer, Stuart; Stone, Remington P. S. (1973), "On the Distance to Cygnus X-1", The Astrophysical Journal, 185 (2): L113–L116, Bibcode:1973ApJ...185L.113M, doi:10.1086/181333
- Interstellar Reddening, Swinburne University of Technology, retrieved 2006-08-10
- Kaler, Jim, Cygnus X-1, University of Illinois, retrieved 2008-03-19
- Hawking, Stephen (1988), A Brief History of Time, Bantam Books, ISBN 0-553-05340-X
- Hawking, Stephen (1998), A Brief History of Time (Updated and Expanded Tenth Anniversary ed.), Bantam Doubleday Dell Publishing Group, ISBN 0-553-38016-8
- Thorne, Kip (1994), Black Holes and Time Warps: Einstein's Outrageous Legacy, W. W. Norton & Company, ISBN 0-393-31276-3
- Vaughan, Simon. "Hawking Thorne wager". University of Leicester. Retrieved 4 February 2018.
- "Black Hole Apocalypse". PBS.org. Retrieved 4 February 2018.
|Wikimedia Commons has media related to Cygnus X-1.|
- Staff (June 10, 2005). "Artist's impression of Cygnus X-1". ESA. Retrieved 2008-03-24.
- Staff (April 1, 1996). "Cygnus X-1, the black hole". Astronomical Observatory of the Jagiellonian University. Retrieved 2008-03-24.
- Cyrmon, W.; et al. (December 18, 2002). "Black Hole in Cygnus". ESA. Retrieved 2008-03-29.
- Nemiroff, R.; Bonnell, J., eds. (June 8, 2009). "Possible Jet Blown Shells Near Microquasar Cygnus X-1". Astronomy Picture of the Day. NASA. Retrieved 2009-06-08.
- Cygnus X-1 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
- Cygnus X-1 at Constellation Guide
- NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk Michael Parker, 29 May 2015
- NuSTAR's First View of High-Energy X-ray Universe NASA/JPL-Caltech June 28, 2012
Cyg X-1 is the first black hole discovered
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