Cygnus X-1

Cygnus 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 detectable from Earth, producing a peak X-ray flux density of . 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 "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 a high-mass X-ray binary system located about 7,000 light-years away, that includes a blue supergiant variable star. The supergiant and black hole are separated by about 0.2 AU, or 20% of the distance from 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 relativistic 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 5 million years old and formed from a progenitor star that had more than. 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—hoping to lose. Hawking conceded the bet in 1990 after observational data had strengthened the case that there was indeed a black hole in the system.

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 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– 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 in the constellation Cygnus. The celestial coordinates of this source were estimated as right ascension 19^h53^m 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 X-ray generation must occur over a compact region no larger than ~, as the speed of light restricts communication between more distant regions.
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 1972. 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.

Binary system

The black hole 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 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 Earth remains uncertain, with predictions ranging from 27° to 65°. A 2007 study estimated the inclination as, which would mean that the semi-major axis is about, or 20% of the distance from Earth to the Sun. The orbital eccentricity is thought to be only, meaning a nearly circular orbit. The system is expected to merge into a single black hole in five billion years, possibly generating gravitational waves during the process.
File:V1357CygLightCurve.png|thumb|right|A blue-band light curve for Cygnus X-1, adapted from Kemp et al.
The HDE 226868/Cygnus X-1 system shares a common motion through space with an association of massive stars named Cygnus OB3, which is roughly 7,000 light-years distant. 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. The motion of HDE 226868 with respect to Cygnus OB3 is, a typical value for random motion within a stellar association. HDE 226868 is about from the center of the association and could have reached that separation in about —which roughly agrees with estimated age of the association. The distance to Cyg X-1 is calculated by trigonometric parallax as, and by radio astrometry as.
With a galactic latitude of 4° and galactic longitude 71°, 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.

Compact object

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, while other techniques resulted in 10 solar masses. Measuring periodicities in the X-ray emission near the object yielded a more precise value of. 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 for Cygnus X-1. Anything that passes through this boundary is unable to escape. Measurements published in 2021 yielded an estimated mass of. A 2025 study estimate a mass of 17.5 solar masses, which may be just an upper limit, with another method yielding.
Evidence of just such an event horizon may have been detected in 1992 using ultraviolet observations with the High Speed Photometer on the Hubble Space Telescope. As self-luminous clumps of matter spiral into a black hole, their radiation is 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 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 that it is rotating extremely rapidly, approximately 790 times per second.