Cygnus X-1

Artistic representation of the binary system HDE 226868 Cygnus X-1. (ESA/Hubble Illustration)

Cygnus X-1 (abbreviated as Cyg X-1) is a black hole that creates a very bright source of X-rays located in the constellation Cygnus. It was discovered in 1964 from an X-ray detector aboard an Aerobee suborbital rocket launched from the White Sands Missile Range. Cyg X-1 is highly variable but in hard X-rays (X-rays with energy greater than 30 keV) it is usually be the brightest source in the sky.

It is a classic example of a X-ray binary, systems composed of a compact object, which can be a black hole or a neutron star, and a companion star. In the case of Cygnus X-1, the compact object is a black hole of 14.81±0.98MΔ Δ {displaystyle M_{odot }orbiting the supergigante blue star HDE 226868 of apparent magnitude 8.9. As in all X-ray binaries, it is not the black hole that emits the X-rays, but the matter that is about to fall into it. This matter (gas and plasma) forms a growing disc that orbits around the black hole and reaches temperatures of millions of kelvin.

Cygnus X-1 is the first case in which the presence of a black hole could be proven.

Matter jets are also observed that extend from a few astronomical units to several parsecs, where they collide with the interstellar medium and give rise to an emission arc in the optic. To generate this arc, the jet must have a power of 20,000 times the power of our sun.^{[citation needed]}

The system is about 6,000 light-years from Earth.

Discovery and observation

Observing X-ray emissions allowed astronomers to study celestial phenomena involving gas with temperatures of millions of degrees. However, because X-ray emissions are blocked by the Earth's atmosphere, observation of celestial X-ray sources is not possible unless instruments are raised to heights where the X-rays penetrate. Cygnus X-1 was discovered using X-ray instruments that were carried aboard a suborbital space rocket from the White Sands Missile Range in New Mexico. As part of an effort to survey these sources, a survey was conducted in 1964 using two Aerobee suborbital rockets. The rockets carried Geiger counters to measure X-ray emissions in a wavelength range 1–15 Å in an 8.4° sector of the sky. These instruments swept the sky as the rockets rotated, producing a map of equally spaced reliefs.

As a result of these surveys, eight new cosmic X-ray sources were discovered, including Cyg XR-1 (later Cyg X-1) in the constellation Cygnus. The celestial coordinates of this source were estimated as a right ascension of 19^h53^m and declination 34.6°. No particularly prominent radio or optical source was associated with that position.

Seeing the need for longer-term studies, in 1963 Riccardo Giacconi and Herb Gursky proposed the first orbiting satellite to study X-ray sources. NASA launched its Uhuru satellite in 1970, which made it possible to discover 300 new X-ray sources X. Extended Uhuru observations of Cygnus X-1 showed fluctuations in X-ray intensity occurring several times per second. This rapid variation meant that power generation had to take place in a relatively small region of about 10⁵km, since the speed of light restricts communication between more distant regions. For size comparison, the diameter of the Sun is about 1.4x10⁶km.

In April–May 1971, Luc Braes and George K. Miley of the Leiden Observatory, and independently Robert M. Hjellming and Campbell Wade at the National Radio Astronomy Observatory, detected the radio emission from Cygnus X-1, and its precise radio position pointed the X-ray source to the star AGK2 +35 1910 = HDE 226868. On the celestial sphere, this star lies within half a degree of the 4th magnitude star Eta Cygni. It is a supergiant star that is incapable of emitting the observed amounts of X-rays by itself. star must have a companion that can heat gas to the millions of degrees needed to produce Cygnus X-1's radiation source.

Louise Webster and Paul Murdin, at the Royal Observatory, Greenwich, and Charles Thomas Bolton, working independently at the University of Toronto's David Dunlap Observatory, announced the discovery of a hidden massive companion to HDE 226868 in 1972. Measurements of the Doppler shift of the star's spectrum demonstrated the presence of the companion and allowed to estimate its mass from the orbital parameters. Based on the high predicted mass of the object, they conjectured that it could be a black hole, since the largest possible neutron star cannot exceed three times the mass of the Sun..

With new observations reinforcing the evidence, by late 1973 it was generally agreed by the astronomical community that Cygnus X-1 was most likely a black hole. More precise measurements from Cygnus X-1 demonstrated a variability of up to a single millisecond. This interval is consistent with turbulence in a disk of accretion matter surrounding a black hole—the accretion disk. X-ray bursts lasting about a third of a second match the expected time for matter falling towards a black hole.

Cygnus X-1 X-ray image taken by a balloon-mounted telescope, the High-Energy Replicated Optics (HERO) project.

Since then, Cygnus X-1 has been extensively studied by observations with instruments in orbit and on the ground. Similarities between the emissions from X-ray binaries such as HDE 226868/Cygnus X-1 and active galactic nuclei suggest a common power generation mechanism 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 analogue of 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 a greater understanding of the mechanics of active galaxies.

Binary system

The compact object and the blue supergiant star form a binary system in which they orbit their center of mass every 5.599829 days. From Earth's perspective, the compact object never trails behind the other star; in other words, the system is not eclipsed. However, the inclination of the orbital plane with respect to Earth's line of sight remains uncertain, with predictions ranging from 27° to 65°. A 2007 study estimated the inclination at 48.0±6.8°, which would mean that the semi-major axis is about 0.2 AU, or 20% of the distance from Earth to the Sun. The orbital eccentricity is thought to be only 0.0018 ±0.002, which means a nearly circular orbit. The distance from Earth to this system is approximately 1,860 ± 120 parsecs (6,070 ± 390 light-years).

A blue band light curve for Cygnus X-1, adapted from Kemp et al. (1987)

The HDE 226868/Cygnus X-1 system shares a common motion through space with an association of massive stars called Cygnus OB3, which is located about 2000 parsecs from the Sun. This implies that HDE 226868, Cygnus X-1 and this OB may have formed at the same time and place. If so, the age of the system is about 5±1.5 million years. 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 lies about 60 parsecs from the center of the assemblage and could have reached that separation in about 7±2 million years—roughly matching the estimated age of the assemblage.

With a galactic latitude of 4° and a galactic longitude of 71°, this system lies inland along the same Orion Spur as the Sun within the Milky Way, near where the spur approaches the Arm of Sagittarius. Cygnus X-1 has been described as belonging to the Sagittarius Arm, although the structure of the Milky Way is not well established.