[16] 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.[17] Cygnus X-1 belongs to a high-mass X-ray binary system, located about 2.22 kiloparsecs from the Sun,[5] that includes a blue supergiant variable star designated HDE 226868,[18] which it orbits at about 0.2 AU, or 20% of the distance from Earth to the Sun.

[20][21] A pair of relativistic jets, arranged perpendicularly to the disk, are carrying part of the energy of the infalling material away into interstellar space.

[23] 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.

[25] Observation of X-ray emissions allows astronomers to study celestial phenomena involving gas with temperatures in the millions of degrees.

[26][27] 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.

The rockets carried Geiger counters to measure X-ray emission in wavelength range 1–15 Å across an 8.4° section of the sky.

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.

[39] 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.

X-ray bursts that last for about a third of a second match the expected time frame of matter falling toward a black hole.

[2] 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.

[43] 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.

[44] The compact object and blue supergiant star form a binary system in which they orbit around their center of mass every 5.599829 days.

[5] 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 2000 parsecs from the Sun.

In all cases, the object is most likely a black hole[46][52]—a region of space with a gravitational field that is strong enough to prevent the escape of electromagnetic radiation from the interior.

[5][6] 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 is emitted in a series of pulses that are subject to gravitational redshift as the material approaches the horizon.

Matter hitting a solid, compact object would emit a final burst of energy, whereas material passing through an event horizon would not.

Past analysis of data from the space-based Chandra X-ray Observatory suggested that Cygnus X-1 was not rotating to any significant degree.

[23] 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).

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 kiloelectronvolts—in the sky.

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.

[67] The spectral transition of Cygnus X-1 can be explained using a two-component advective flow solution, as proposed by Chakrabarti and Titarchuk.

[69] The X-ray flux from Cygnus X-1 varies periodically every 5.6 days, especially during superior conjunction when the orbiting objects are most closely aligned with Earth and the compact source is the more distant.

[67] 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.

[74] 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.

[79] When the spectrum of HDE 226868 is compared to the similar star Alnilam, the former shows an overabundance of helium and an underabundance of carbon in its atmosphere.

As the object moves through different regions of the stellar wind during its 5.6-day orbit, the UV lines,[84] the radio emission,[85] and the X-rays themselves all vary.

[86] The Roche lobe of HDE 226868 defines the region of space around the star where orbiting material remains gravitationally bound.

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.