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For the song(s) by Rush, see Cygnus X-1 duology.
Cygnus X-1 (often abbreviated to Cyg X-1) is an X-ray source in the constellation Cygnus, and is considered to be a black hole. It was one of the first black hole candidates to be discovered and is still one of the star systems most reliably identified as containing a black hole. It is a high-mass X-ray binary system that includes a variable star designated HDE 226868.
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The mapping of stellar x-ray sources was undertaken in 1964 using two Aerobee suborbital rocket launches from the White Sands Missile Range in New Mexico. These surveys discovered eight new sources of cosmic x-rays, including Cyg XR-1 in the constellation cygnus. The estimated coordinates for this source was right ascension 19h53m and declination 34.6°. It was not associated with any prominent radio or optical source at its position.Bowyer, S.; Byram, E. T.; Chubb, T. A.; Friedman, H. (1965). "Cosmic X-ray Sources". Science 147 (3656): 394-398. Retrieved on 2008-03-10.
Subsequent measurements showed that the Cygnus X-1 source varied in intensity. A radio source was discovered in the same region that varied over a comparable period, so it was thought to originate from the same position.Bowyer, S.; Byram, E. T.; Chubb, T. A.; Friedman, H. (1971). "On the Optical Identification of Cygnus X-1". Astrophysical Journal 168: L91–L93. Retrieved on 2008-03-10. More refined measurements of the radio source location then suggested that the star BD 34°3815 was the optical counterpart.Gursky, Herbert (1972). "The Association of X-Ray Sources with Bright Stars". Astrophysical Journal 175: L141–L144. Retrieved on 2008-03-10.
The optical source was found to be a supergiant star that, by itself, is incapable of emitting the observed quantities of x-rays. Hence the star must have a companion that could heat gas to the several million K needed to produce the radiation source for Cygnus X-1.
In 1971, Louise Webster and Paul Murdin, at the Royal Greenwich Observatory,Webster, B. Louise; Murdin, Paul (1972). "Cygnus X-1—a Spectroscopic Binary with a Heavy Companion?". Nature 235 (2): 37-38. doi:10.1038/235037a0. Retrieved on 2008-03-10. and Tom Bolton working independently at the University of Toronto,Bolton, C. T. (1972). "Identification of Cygnus X-1 with HDE 226868". Nature 235 (2): 271-273. doi:10.1038/235271b0. Retrieved on 2008-03-10. announced that the star had a massive hidden companion. Measurements of the Doppler shift of the star\'s spectrum demonstrated the presence of this companion and allowed its mass to be estimated from the orbital parameters. They surmised from the high predicted mass of the object that it may be a black hole, as the largest possible neutron star cannot exceed three solar masses.Bombaci, I. (1996). "The maximum mass of a neutron star". Astronomy and Astrophysics 305: 871–877. Retrieved on 2008-03-11. By the end of 1973, further observations had strengthened the evidence and it became generally conceded by the astronomical community that Cygnus X-1 was most likely a black hole.Rolston, Bruce (November 10, 1997). The First Black Hole. University of Toronto. Retrieved on 2008-03-11. Shipman, H. L. (1975). "The implausible history of triple star models for Cygnus X-1 Evidence for a black hole". Astrophysical Letters 16 (1): 9-12. Retrieved on 2008-03-11.
Results from the Uhuru satellite showed fluctuations in the intensity from Cygnus X-1 that occur several times a second.Oda, M.; Gorenstein, P.; Gursky, H.; Kellogg, E.; Schreier, E.; Tananbaum, H.; Giacconi, R. (1999). "X-Ray Pulsations from Cygnus X-1 Observed from UHURU". Astrophysical Journal 166: L1–L7. Retrieved on 2008-03-11. This rapid variation meant that the energy generation must take place over a relatively small region—on the order of 105 km,This is the distance light can travel in a third of a second. as the speed of light restricts communication between more distance regions. (For comparison, the diameter of the Sun is about 1.4×106 km.) Subsequent measurements 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 disc. Bursts lasting for about a third of a second match the expected time frame of matter falling toward the black hole.Rothschild, R. E.; Boldt, E. A.; Holt, S. S.; Serlemitsos, P. J. (1974). "Millisecond Temporal Structure in Cygnus X-1". Astrophysical Journal 189: 77-115. Retrieved on 2008-03-11.
Cygnus X-1 is a binary star that contains a blue supergiant star and a compact object. These orbit about a common center of mass with a period of 5.60 daysWalker, E. N. (1972). "B and V photometry of Cygnus X-i". Monthly Notices of the Royal Astronomical Society 160: 77-115. Retrieved on 2008-03-10. at a distance of only 3×107km,Luminet, Jean-Pierre (1992). Black Holes. Cambridge University Press. ISBN 0521409063. which is smaller than the orbit of Mercury. The orbit probably has a tilt of about 27–30 degrees; thus more face-on than edge-on. The orbital eccentricity is estimated as 0.06±0.01.Bolton, C. T. (1975). "Optical observations and model for Cygnus X-1". Astrophysical Journal 200: 269–277. Retrieved on 2008-03-12. From the perspective of the Earth, the compact object never goes behind the surface of the other star; in other words, the system does not eclipse. The distance to this system is about 2000 parsecs (6000 ly), as measured by the Hipparcos satellite.
The compact object has a mass of 8.7±0.8 solar masses and is most likely a black hole.Strohmayer, Tod; Shaposhnikov, Nikolai; Schartel, Norbert (May 16, 2007). New technique for ‘weighing’ black holes. ESA. Retrieved on 2008-03-10. This is a region of space with a gravitational field that is strong enough to prevent the escape of electromagnetic energy from the interior. The boundary of this region is called the event horizon and the surface lies at a distance from the center called the Schwarzschild radius. Any matter that passes through this boundary is unable to escape.
The compact object is believed to be orbited by a flat disk of accreted matter. This disk is divided into an inner region with a relatively high level of ionization, and a less ionized outer region that extends out to an estimated 500 times the Schwarzschild radius.Young, A. J.; Fabian, A. C.; Ross, R. R.; Tanaka, Y. (2001). "A Complete Relativistic Ionized Accretion Disc in Cygnus X-1". Monthly Notices of the Royal Astronomical Society 325: 1045–1052. Retrieved on 2008-03-13. The X-rays are produced in the inner accretion disk and modified by Compton scattering in the disk corona and reflection off the surface of the disk. Alternately, the X-rays may be Compton scattered by the base of a jet instead of a disk corona. Cygnus X-1 is the brightest persistent source of hard X-rays (E > 20 keV) in the sky.
X-rays flicker about certain frequencies in a poorly-understood phenomenon called Quasi-periodic oscillations. Exactly repeating pulses have never been seen from Cygnus X-1. Such pulsations are typical of neutron stars, and if seen would rule out a black hole. The pulsations from neutron stars are caused by the neutron star\'s magnetic field and the no hair theorem guarantees that black holes do not have magnetic poles.
The radio emission is thought to arise in a jet of gas emitted perpendicular to the accretion disk.
Cygnus X-1 unpredictably changes between two X-ray states, although the X-rays may vary continuously between those states as well. In the most common state, the X-rays are "hard" (more of the X-rays have high energy). In the less common state, the X-rays are "soft" (more of the X-rays have lower energy) and the X-rays show greater variability.
Measurements of the x-ray output flux from Cygnus X-1 shows that it varies periodically every 5.6 days; occurring during superior conjunction when the orbiting objects are most closely aligned with the Earth. This indicates that the emissions are being partially blocked by circumstellar matter, which is possibly the stellar wind from the star HDE 226868. There is also a roughly 300 day periodicity in the emission that could be caused by the precession of the accretion disk.Kitamoto, S.; E. Wataru, E.; Miyamoto, S.; Tsunemi, H.; Ling, J. C.; Wheaton, W. A.; Paul, B. (2000). "GINGA All-Sky Monitor Observations of Cygnus X-1". Astrophysical Journal 531: 546–552. doi:10.1086/308423.
The space-based Chandra X-ray Observatory was used to measure the spectral signature of iron atoms orbiting near the black hole. A rotating black hole drags the nearby space around with it, which would allow atoms to orbit closer to the event horizon. In the case of Cygnus X-1, none of the atoms were found orbiting closer than 160 km, indicating that the black hole is not rotating to any significant degree.Miller, J. M.; Fabian, A. C.; Nowak, M. A.; Lewin, W. H. G. (July 20-26, 2003). "Relativistic Iron Lines in Galactic Black Holes: Recent Results and Lines in the ASCA Archive". Proceedings of the 10th Annual Marcel Grossmann Meeting on General Relativity. Retrieved on 2008-03-11. Roy, Steve; Watzke, Megan. ""Iron-Clad" Evidence For Spinning Black Hole", Chandra press Room, September 17, 2003. Retrieved on 2008-03-11.
HDE 226868 is a spectral class O9.7 Iab supergiant with a surface temperature of 31,000 kelvin and mass approximately 20–40 times the mass of the Sun. In common with other stars of its spectral type, HDE 226868 is thought to be shedding mass in a stellar wind at a rate of about 10-6 solar masses per year. The ultraviolet lines show P Cygni profiles that demonstrate that the gas is accelerated to speeds of about 1500 kilometers per second.Conti, P.S. 1978, Astronomy & Astrophysics
The star\'s surface is thought to be near its Roche lobe so that the stellar wind is focussed by the gravity of the black hole. The optical brightness of the star varies with the 5.6 day binary orbit. This "ellipsoidal" variation results from the limb darkening and gravity darkening of the surface of the star, which has been distorted to a tear-drop shape by the black hole\'s gravity and the rotation of the system.
X-rays from the black hole heat and ionize the stellar wind. As the black hole moves through different regions of the stellar wind throughout the 5.6 day orbit, the UV lines, the radio emission, and the X-rays themselves all vary.
Cygnus X-1 was the subject of the bet between physicists Stephen Hawking and Kip Thorne, in which Hawking bet against the existence of black holes in the region. Hawking later described this as an "insurance policy" of sorts. To quote from his book, A Brief History of Time, "This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist. But in that case, I would have the consolation of winning my bet, which would win me four years of the magazine Private Eye. If black holes do exist, Kip will get one year of Penthouse. When we made the bet in 1975, we were 80% certain that Cygnus was a black hole. By now, I would say that we are about 95% certain, but the bet has yet to be settled." (1988) According to the updated 10th anniversary edition of A Brief History of Time, Hawking has conceded the bet ("to the outrage of Kip\'s liberated wife") due to subsequent observational data in favour of black holes. In his own book, Black Holes and Time Warps, Thorne reports that Hawking conceded the bet by breaking into Thorne\'s office while he was in Russia, finding the framed bet, and signing it.
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