X-Ray Astronomy (redirected from Supersoft x-ray source)

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 star in which one star expels gas onto a very dense neighbor, which may be a white dwarf, a neutron star, or a black hole. 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 Nebula 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 star, it is possibly a black hole.

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 supernova 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 star) 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 station), 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 comets 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 Way. 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.

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X-ray astronomy

Study of astronomical objects and phenomena that emit radiation at X-ray wavelengths. Because Earth's atmosphere absorbs most X-rays, X-ray telescopes and detectors are taken to high altitudes or into space by balloons and spacecraft. In 1949 detectors aboard sounding rockets showed that the Sun gives off X-rays, but it is a weak source; it took 30 more years to clearly detect X-rays from other ordinary stars. Beginning with the Uhuru X-ray satellite (launched 1970), a succession of space observatories carried increasingly sophisticated instruments into Earth orbit. Astronomers discovered that most types of stars emit X-rays but usually as a tiny fraction of their energy output. Supernova remnants are more powerful X-ray sources; the strongest sources known in the Milky Way Galaxy are certain binary stars in which one star is probably a black hole. In addition to myriad point sources, astronomers have found a diffuse background of X-ray radiation emanating from all directions; unlike cosmic background radiation, it appears to have many distant individual sources. The Chandra X-Ray Observatory and XMM-Newton X-ray satellite (both launched 1999) have made numerous discoveries relating to the nature and quantity of black holes in the universe, the evolution of stars and galaxies, and the composition and activity of supernova remnants.

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x-ray astronomy [′eks ‚rā ə′strän·ə·mē]

(astronomy)

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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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 10^3–10⁴ 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.

REFERENCES

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.)

V. G. KURT