Editing Cygnus X-1 (section)

==Discovery and observation==
Observation of X-ray emissions allows [[astronomer]]s to study celestial phenomena involving gas with temperatures in the millions of degrees. However, because X-ray emissions are blocked by [[Atmosphere of Earth|Earth's atmosphere]], observation of [[X-ray astronomy|celestial X-ray sources]] is not possible without lifting instruments to altitudes where the X-rays can penetrate.<ref name=herbert2002/><ref name=apj611_2_1084/> Cygnus&nbsp;X-1 was discovered using [[X-ray astronomy detector|X-ray instruments]] that were carried aloft by a [[Sub-orbital spaceflight|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–{{val|15|u=[[Ångström|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.<ref name=science3656/>

As a result of these surveys, eight new sources of cosmic X-rays were discovered, including Cyg&nbsp;XR-1 (later Cyg&nbsp;X-1) in the constellation Cygnus. The [[Celestial coordinate system|celestial coordinates]] of this source were estimated as [[right ascension]] 19<sup>h</sup>53<sup>m</sup> and [[declination]] 34.6°. It was not associated with any especially prominent [[radio astronomy|radio]] or [[light|optical]] source at that position.<ref name=science3656/>

Seeing a need for longer-duration studies, in 1963 [[Riccardo Giacconi]] and [[Herbert Gursky|Herb Gursky]] proposed the first orbital satellite to study X-ray sources. [[NASA]] launched their [[Uhuru (satellite)|Uhuru]] satellite in 1970,<ref name=heasarc20030626/> which led to the discovery of 300 new X-ray sources.<ref name=giacconi20021208/> Extended Uhuru observations of Cygnus&nbsp;X-1 showed fluctuations in the X-ray intensity that occurs several times a second.<ref name=apj166_L1/> This rapid variation meant that the X-ray generation must occur over a compact region no larger than ~{{val|e=5|u=km}} (roughly the size of [[Jupiter]]),<ref>This is the distance light can travel in a third of a second.</ref> 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.&nbsp;Hjellming and Campbell Wade at the [[National Radio Astronomy Observatory]],<ref name=apj168_L91/> detected radio emission from Cygnus&nbsp;X-1, and their accurate radio position pinpointed the X-ray source to the star AGK2&nbsp;+35 1910 = HDE&nbsp;226868.<ref name=nature232_5308_246/><ref name=vrsb9_100_173/> On the [[celestial sphere]], this star lies about half a [[Degree (angle)|degree]] from the [[apparent magnitude|4th-magnitude]] star [[Eta Cygni]].<ref name=bernard_stecker1999/> 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&nbsp;X-1.

[[Betty Louise Turtle|Louise Webster]] and [[Paul Murdin]], at the [[Royal Observatory, Greenwich|Royal Greenwich Observatory]],<ref name=nature235_2_37/> and [[Tom Bolton (astronomer)|Charles Thomas Bolton]], working independently at the [[University of Toronto]]'s [[David Dunlap Observatory]],<ref name=nature235_2_271/> announced the discovery of a massive hidden companion to HDE&nbsp;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.<ref name=luminet1992/> 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 [[solar mass|mass of the Sun]].<ref name=aaa305_871/>

With further observations strengthening the evidence, by the end of 1973 the astronomical community generally conceded that Cygnus&nbsp;X-1 was most likely a black hole.<ref name=rolston1997/><ref name=apl16_1_9/> More precise measurements of Cygnus&nbsp;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.<ref name=apj189_77/>

[[File:Cygnus x1 xray.jpg|right|thumb|X-ray image of Cygnus&nbsp;X-1 taken by a balloon-borne telescope, the [[High-Energy Replicated Optics]] (HERO) project]]
Cygnus&nbsp;X-1 has since been studied extensively using observations by orbiting and ground-based instruments.<ref name=SIMBAD/> The similarities between the emissions of X-ray binaries such as HDE&nbsp;226868/Cygnus&nbsp;X-1 and [[active galactic nuclei]] suggests a common mechanism of energy generation involving a black hole, an orbiting accretion disk and associated [[Relativistic jet|jets]].<ref name=mnras372_3_1366/> For this reason, Cygnus&nbsp;X-1 is identified among a class of objects called [[microquasar]]s; an analog of the [[quasar]]s, or quasi-stellar radio sources, now known to be distant active galactic nuclei. Scientific studies of binary systems such as HDE&nbsp;226868/Cygnus&nbsp;X-1 may lead to further insights into the mechanics of [[active galaxy|active galaxies]].<ref name=brainerd20050720/>