X-ray astronomy

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X-ray astronomy,

study of celestial objects by means of the X rays they emit, in the wavelength range from 0.01 to 10 nanometers. X-ray astronomy dates to 1949 with the discovery that the sun emits X rays. Since X rays could not be observed from ground-based telescopes, V-2 rockets launched from White Sands, N.Mex., occasionally carried telescopes to study solar X-ray emissions. In 1962 a group led by R. Giacconi launched a small rocket from White Sands to search for celestial sources of X rays with instruments similar to Geiger counters. During the 5-min flight the experiment discovered an X-ray source now called Scorpius X-1, a close binary starbinary star
or binary system,
pair of stars that are held together by their mutual gravitational attraction and revolve about their common center of mass. In 1650 Riccioli made the first binary system discovery, that of the middle star in the Big Dipper's handle, Zeta
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 in which one star expels gas onto a very dense neighbor, which may be a white dwarfwhite dwarf,
in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be easily
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, a neutron starneutron star,
extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933.
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, or a black holeblack hole,
in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping.
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. This mission also found that the earth is bathed in diffuse X rays coming from all directions. Soon afterward X-ray emissions were found coming from the Crab NebulaCrab Nebula,
diffuse gaseous nebula in the constellation Taurus; cataloged as NGC 1952 and M1, the first object recorded in Charles Messier's catalog of nonstellar objects (see Messier catalog).
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 and the radio galaxies (galaxies whose radio emissions constitute an extraordinarily large amount of their total energy output) Centaurus A and Virgo A. Other types of galaxies, particularly Seyfert galaxies (galaxies with extremely bright cores that are strong emitters of radio waves, X rays, and gamma rays), also emit X rays. The center of our galaxy is a strong X-ray source, which is an indicator of the violent activity taking place there.

In 1970 the Uhuru satellite, one of NASA's small astronomy satellites, began to look specifically for X-ray sources. Uhuru used detectors filled with argon, in which incoming X radiation gives off electrons in amounts proportional to its strength. Uhuru mapped more than 400 sources and discovered a series of X-ray binary stars in which ordinary stars orbit neutron stars that emit X rays. One of these sources, Cygnus X-1, is an object with ten times the mass of the sun. Too massive to be a neutron starneutron star,
extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933.
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, it is possibly a black holeblack hole,
in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping.
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Much of the data in X-ray astronomy is now gathered by orbiting satellites. In addition to the United States, Germany and Japan are among the countries having X-ray satellites. In the 1970s the Skylab space station and Orbiting Solar Observatory satellites continued the study, as did the Solar Maximum Mission the following decade. A series of High Energy Astrophysical Observatories (HEAO) were launched during the late 1970s to study X rays, cosmic rays, and gamma rays. HEAO-1, launched in 1977, increased the number of known X-ray sources from 350 to 1,500. HEAO-2—also known as the Einstein Observatory—carried the largest X-ray telescope ever built. It detected several thousand new X-ray sources in our galaxy and beyond, discovered that cataclysmic variable stars in our own galaxy emit X rays when they are in outburst, achieved the first unambiguous detection of X rays from ordinary stars other than the sun, and obtained the first X-ray images of supernovasupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
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 remnants, pulsars, and star clusters. As a result, supernova remnants mapped in X-ray wavelengths can be compared with visible light and radio images. In an example of cooperation between amateur and professional astronomers, the Einstein Observatory was turned toward SS Cygni (see variable starvariable star,
star that varies, either periodically or irregularly, in the intensity of the light it emits. Other physical changes are usually correlated with the fluctuations in brightness, such as pulsations in size, ejection of matter, and changes in spectral type, color, or
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) whenever amateur astronomers with backyard telescopes reported it in outburst. The few days' duration of these outbursts allowed enough time to change the satellite's observing schedule so that it could examine the star, and it discovered the source of the star's X-ray emissions.

During the 1980s the European, Russian, and Japanese space agencies continued to launch successful X-ray astronomy missions, such as the European X-ray Observatory Satellite (EXOSAT), Granat, the Kvant module (of the Mir space stationspace station
or space platform,
artificial earth satellite, usually manned, that is placed in a fixed orbit and can serve as a base for astronomical observations; zero-gravity materials processing; satellite assembly, refueling, and repair; or, possibly, as weapons
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), Tenma, and Ginga. These missions were more modest in scale than the HEAO program in the 1970s and were directed toward in-depth studies of known phenomena.

In 1990, ROSAT [Roentgen Satellite], a joint project of Germany, the United States, and Great Britain, was launched. Operational until 1999, it was instrumental in the discovery of X-ray emissions from cometscomet
[Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months.
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 and conducted an all-sky survey in the X-ray region of the spectrum. Five other satellites launched in the 1990s are still operational. ALEXIS [Array of Low Energy X-ray Imaging Sensors] was launched in 1993; a minisatellite containing six coffee-can-sized wide-angle, ultrasoft-X-ray telescopes, it provided the data for a unique sky map for studying celestial flashes of soft X rays. Also launched in 1993, the Advanced Satellite for Cosmology and Astrophysics is a joint Japanese-American project; containing four X-ray telescopes, its primary purpose is the X-ray spectroscopy of such astrophysical entities as quasars and cosmic background X radiation. In 1995, NASA orbited the Rossi X-ray Timing Explorer (RXTE) to study the variations in the emission of such X-ray sources as black-hole candidates, active galactic nuclei, white dwarf stars, neutron stars, and other high-energy sources. The RXTE played a key role in the discovery in 1996 of a "pulsing burster" located near the center of the Milky WayMilky Way,
the galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon; if not blocked by the horizon, it would be seen as a circle around the entire sky.
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. Unlike other X-ray sources, this one burst, oscillated, and flickered simultaneously, with bursts lasting from 6 to 100 seconds. Before it burned out, the unexplained object was the brightest source of X rays and gamma rays in the sky, radiating more energy in 10 seconds than the sun does in 24 hours. BeppoSAX, a joint Italian-Dutch satellite, was launched in 1996. When on Dec. 14, 1997, for 1 or 2 seconds the most energetic burst of gamma radiation ever detected was recorded by the Compton Gamma Ray Observatory, BeppoSAX recorded the X-ray afterglow of the burst, thereby providing a relatively accurate location for the source. The Chandra X-ray Observatory was deployed from a shuttle and boosted into a high earth orbit in 1999; it focuses on such objects as black holes, quasars, and high-temperature gases throughout the X-ray portion of the electromagnetic spectrum. Also launched in 1999 was X-ray Multimirror Mission, an ESA satellite that carries an optical-ultraviolet telescope together with three parallel mounted X-ray telescopes, allowing it to simultaneously observe phenomena in two regions of the spectrum.

X-ray astronomy

The study of objects lying beyond the Solar System in the photon energy band 100 to 100 000 electronvolts (corresponding to wavelengths from 12 nanometers (nm) to 0.012 nm). X-ray observations are now an integral part of astronomy, relating particularly to galactic and extragalactic systems where violent or energetic phenomena give rise to copious high-energy particles or super-hot gas.

Opacity of the terrestrial atmosphere throughout the X-ray band requires that observations be made above about 150 km and hence X-ray astronomy could begin only after high-altitude rockets became available. The first detection of a cosmic X-ray source was made by Riccardo Giacconi and colleagues in July 1962, during an exploratory rocket launch equipped with an X-ray detector to search for lunar fluorescence. Confirmation of this source (Scorpius X-1) and the discovery of a second source (Taurus X-1) in 1963 began an active period of rocket and balloon observations that, by 1970, had yielded 25–30 sources spread throughout the Galaxy; there was also one likely extragalactic source, apparently associated with the powerful radio galaxy Virgo A (M87). The first optical identification was of Taurus X-1, which was found to coincide with the Crab nebula supernova remnant in a classical lunar occultation observation by Herbert Friedman and his colleagues in the USA.

The launch of the first X-ray astronomy satellite, Uhuru, in Dec. 1970 accelerated the development of the subject, yielding many new sources including a large number at high galactic latitude. In particular, Uhuru discovered X-ray binaries and showed these to be the most common form of galactic X-ray source. A second major discovery was of powerful X-ray emission from clusters of galaxies, with evidence that this emission arose in an extended region comparable in size (about 0.5 megaparsecs) to the cluster.

The launch of other X-ray astronomy satellite experiments continued the rapid expansion of the subject. The Ariel V sky survey extended the Uhuru catalog and led to the establishment of a second major class of extragalactic source: the X-ray Seyfert galaxies. Observations by Copernicus and Ariel V, followed by SAS-3, found the slow X-ray pulsators, periodically variable X-ray sources with periods of a few minutes, substantially longer than those of Centaurus X-3, Hercules X-1, etc. Ariel V, followed by the satellite OSO-8, discovered an emission line near 7 keV in the spectra of several supernova remnants and many of the rich clusters of galaxies, showing the X-rays to be of thermal origin, arising from hot gas at temperatures of 106 to 108 kelvin and containing an abundance of iron similar to that found in the Solar System. X-ray transients and X-ray burst sources were detected by several of the satellites. Optical identifications of a few of these have shown them to be most probably X-ray binaries in which the mass transfer or accretion rate is highly variable. Further major advances were made by the Einstein Observatory, whose greater sensitivity revealed substantial X-ray emission from a wide range of normal stars. This emission comes from the stars' hot coronae.

More recent X-ray satellites, particularly EXOSAT, Ginga, ROSAT, and ASCA, have continued to extend the scope of X-ray observations to the point where it is now the major observational branch of high-energy astrophysics. Thus, for example, stellar activity, the form and evolution of young supernova remnants, the gravitational mass distribution and evolution of clusters of galaxies, the dynamics and energy processes in the centers of active galactic nuclei, as well as the study of accretion in the wide variety of compact-object binary-star systems, are most directly observed by their X-ray emission. This situation seems certain to continue with the diagnostic power of higher-resolution X-ray spectroscopy on future missions, such as AXAF, XMM, ASTRO-E, and Spectrum-X.

X-Ray Astronomy


the branch of observational and theoretical astrophysics that investigates the sources of cosmic X-radiation in the region of wavelengths λ from 100 angstroms (A) to 0.3 A. On the scale of photon energies, this range corresponds to 0.1–30 kiloelectron volts; however, both boundaries are defined rather arbitrarily. In order to conduct astronomical observations in this wavelength region, equipment is lifted above the earth’s atmosphere by rockets or artificial earth satellites, since X-rays are strongly absorbed in the atmosphere. Hard X-radiation can be observed at altitudes of approximately 40 km from high-altitude balloons.

In space, X-radiation can be generated by a hot plasma with a temperature exceeding 106 °K in an optically thin or dense medium, by relativistic electrons in magnetic fields (synchrotron radiation), and by electrons in cosmic rays upon interaction with low-energy photons, for example, optical photons. The last mechanism is called the inverse Compton effect.

The X-radiation of the sun was first detected from a rocket on Aug. 5, 1948, in the USA, although the existence of such radiation had been predicted previously on the basis of geophysical studies of the ionosphere. By the mid-1970’s, solar X-radiation had been investigated in detail throughout the entire spectrum. In the absence of chromospheric flares it extends all the way to 10–20 A. The presence of active regions on the solar disk leads to the appearance of hard X-radiation and gamma radiation (Figure 1). The continuous spectrum is mainly thermal in character, with a temperature ranging from 106 °K to 2 × 107 °K; however, a nonthermal component is also observed at the beginning of the development of a flare. X-radiation is generated within the solar corona and also in the chromosphere and the transition region of the solar atmosphere, which has an extremely narrow altitude range. The gamma radiation of flares, including line radiation, has also been observed. Lines of multiply ionized elements, such as Fe, Ni, Mn, Ar, and Co, are present in the X-ray spectrum. Basically, the spectra of hydrogenlike atoms that have only one remaining electron are observed. Photographs of the solar disk in the soft X-ray region of the spectrum have been obtained by means of grazing-incidence optics. Polarization of X-radiation during flares has been observed.

Figure 1. Spectrum of the sun in the region 1–8 Å

Discrete sources of cosmic X-radiation were discovered by accident in 1962 during a search for lunar fluorescent X-radiation caused by cosmic rays. By 1975, more than 150 sources had been registered. Most of them are concentrated toward the galactic plane, which indicates that they are few in number (according to various estimates, there are only 103–104 such sources in the Milky Way Galaxy) and that the majority are located in the galactic disk (Figure 2). The flux from the brightest source—Sco X-l, in the constellation Scorpio—is equal to 20 quanta/(cm2.sec) in the spectral region 2–8 Å. The weakest sources recorded by 1975 have a flux of 10-3quanta/cm2-sec) in the same region of the spectrum. Only a small number (approximately ten) of the galactic sources have been identified with objects that have been investigated optically; these include the remnants of supernovas. In this case, two types of radiation are observed: synchrotron radiation from an extended nebulosity and thermal radiation from an expanding gaseous shell and from the interstellar gas heated to a temperature of 106 °K. The radiation of a supernova remnant, which is most likely a neutron star, is sometimes observed. The X-radiation of the Crab Nebula (Tau X-1, the second brightest source) with a flux of 2 quanta/(cm2.sec) has a pulsating component with a period of 0.033 sec, which coincides with the period of the optical and radio-frequency radiation of a pulsar.

Figure 2. Distribution of known X-ray sources in the sky, plotted in galactic coordinates. The center of the Milky Way Galaxy is in the middle of the diagram.

X-ray sources belonging to binary stellar systems, such as Her X-1, Cyg X-1, Cyg X-3, Cir X-1, and Cen X-3, have made it possible to investigate in detail the systems’ physical parameters. One such source, Cyg X-l, is probably an object that arose as a result of gravitational collapse (a black hole). The mechanism of the X-ray emission of such sources is the flow of gas from the surface of an ordinary giant star to a neutron star or black hole—a process called disk accretion. Most of the X-ray sources have not been identified with objects observed in the optical spectrum. About 30 sources have been identified with extragalactic objects. These, in particular, are nearby galaxies (the Magellanic Clouds and the galaxy M31 in Andromeda), clusters of galaxies, the radio galaxies Virgo A (M87) and Cen-taurus A (NGC 5128), the quasar 3C 273, and Seyfert galaxies.

In addition to discrete X-ray sources, an isotropic X-ray background is observed. Its spectrum in the range 1–1,000 kiloelectron volts is given as a first approximation by a power law. The isotropic background apparently has an extragalactic origin; however, the mechanism of its emission is not yet clearly understood. Some probable hypotheses point to the inverse Compton effect of intergalactic electrons with infrared photons of active galaxies and submillimeter quanta of the radio background radiation, the superposition of radiation of many unre-solvable, distant extragalactic sources, the thermal radiation of hot intergalactic gas, and various combinations of these mechanisms.

Special photographic materials (for solar investigations), Geiger counters, gas-filled proportional counters, and scintillation counters are used as radiation detectors in the X-ray region. All types of detectors provide a spectral resolution of 1–20, depending on the energy of the registered radiation. The area of proportional counters used to obtain the basic results reaches 1,000 cm2. Collimation (restriction of the field of view) is achieved using honeycomb or slit collimators, which are assembled from thin, perforated steel baffles and having a maximum angular resolution of approximately a few minutes of arc, modulation collimators, which are two or more rows of metal filaments stretched in parallel and have a maximum resolution of approximately 20″, and grazing-incidence hyperbolic and parabolic mirrors, with an angle of incidence greater than 88°, that is, nearly at the tangent to the plane of the mirror. Such mirrors are suitable for obtaining an image formed by soft X rays (λ > 10 Å) with a resolution up to 5″. Bragg crystal spectrometers are used for spectral studies (thus far, only for solar studies).

X-ray astronomy is one of the rapidly developing branches of extra-atmospheric astronomy. It shows great promise in connection with planned launches of rockets and artificial earth satellites carrying large counters and mirror telescopes with an area of 104-105 m2.


Ozernoi, L. M., O. F. Prilutskii, and I. L. Rozental’. Astrofizika vysokikh energii. Moscow, 1973.
Weekes, T. Astrofizika vysokikh energii. Moscow, 1972. (Translated from English.)
Ginzburg, V. L. O fizike i astrofizike: Kakie problemy predstavliaiutsia seichas osobenno vazhnymi i interesny mi?, 2nd ed. Moscow, 1974.
Ul’trafioletovoe izluchenie Solntsa i mezhplanetnaia sreda Moscow, 1962. (A collection of articles translated from English.)


x-ray astronomy

[′eks ‚rā ə′strän·ə·mē]
The study of x-rays mainly from sources outside the solar system; it includes the study of novae and supernovae in the Milky Way Galaxy, together with extragalactic radio sources.
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