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Cygnus X-1 (abbreviated Cyg X-1)〔 is a well-known galactic X-ray source, thought to be a black hole,〔 in the constellation Cygnus. 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 ().〔〔 Cygnus X-1 was the first X-ray source widely accepted to be a black hole and it remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 14.8 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 is about .〔 Cygnus X-1 belongs to a high-mass X-ray binary system about from the Sun that includes a blue supergiant variable star designated HDE 226868〔[http://arxiv.org/abs/1401.1035 [1401.1035] Masses of the components of the HDE 226868/Cyg X-1 binary system]〕 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 perpendicular 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 . 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 has not been confirmed due to a lack of direct observation 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 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 astronomy detectorHjellming 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cygnus X-1」の詳細全文を読む スポンサード リンク
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