At the high-energy end of the electromagnetic spectrum, x rays provide a unique window on some of the hottest and most violent objects in the universe. X rays are electromagnetic waves with wavelengths covering a broad range from about 3× 10-8 ft (10-8 m) to 3× 10-11 ft (10-11 m); or about 10 to 0.01 nano-meters (where one nanometer equals one-billionth of a meter). Since the discovery of extra-solar x-ray sources in 1962, scientists have investigated a large number of phenomena that emit x rays. With each new space mission, more sources and more details of the structure of the x-ray universe have been gleaned.
although they are among the most energetic of the electromagnetic spectrum, and thus provide a window on some of the most violent processes in the universe, x rays are not able to penetrate Earth’s atmosphere; they are absorbed at about 62 mi (100 km) above the surface. Thus, only with the advent of rocket and satellite astronomy have astronomers been able to study the wide-ranging phenomena that produce x rays. The highest energy x rays have also been studied by balloons high in Earth’s atmosphere, but there are far fewer photons at these energies than at the lower energies that can be observed above the atmosphere.
X rays are also difficult to bring to a focus, since their energies are so high. Therefore, an important breakthrough in x-ray astronomy was the advent of imaging telescopes, replacing instruments that could only crudely tell in which direction an x-ray source was located. The telescopes with which astronomers are most familiar, consisting of lenses or mirrors that capture light arriving at normal incidence (perpendicular to the surface) will not work in the x-ray region of the spectrum, since the x rays pass through unchanged or are absorbed by the optics. Instead, x-ray astronomers use grazing incidence telescopes, in which the light from the source strikes mirrors at angles of only a few degrees, skipping like stones over the surface of water. By combining two mirrors, the energy can be focused onto a detector in order to provide a sharp image of the source.
although the temperature of the sun’s surface is about 6,000 K (10,341° F; 5,727° C), by the 1930s there was evidence that the outer regions of the solar atmosphere were much hotter, meaning that they could be a source of x rays. At that time, there was no way to verify this prediction, however. After World War II (1939–1945), when captured German V-2 rockets allowed American scientists to place instruments outside the protective atmosphere for the first time, a number of experiments were able to show that the sun did indeed produce x rays.
The strongest early evidence came in 1948, when x-ray detectors registered x rays were coming from the direction of the sun. Further investigations showed that the total x-ray output of the sun was only a tiny fraction of the total energy generated. Because the total x-ray output was so small, despite the fact that the sun is so close in terms of interstellar distances, many believed that no other sources would be found.
In 1962, a rocket was sent up to look for x rays from the moon, which was theorized to generate x rays due to solar wind bombardment. No emission was detected from the moon, but in a surprising discovery, the detector registered an x-ray source in the direction of the constellation Scorpio, along with a diffuse background coming from all directions; the source was called Scorpius X-1.
Since that time, a large number of rocket and Earth-orbiting satellites have discovered tens of thousands of x-ray sources in the sky, many of which are many orders of magnitude brighter than the sun. The Crab Nebula, for instance, produces approximately 2,000 times more energy in the x-ray region of the spectrum than the sun does over all wavelengths. Thus, astronomers now know that the sun is relatively quiet as far as x-ray sources go.
A wide variety of x-ray sources have been seen since the first extrasolar identification in 1962. A few of the most interesting types of sources are:
The sun. A number of x-ray satellites have monitored the sun. Solar flares produce enhancements in its x-ray output.
Stars. Many stars, particularly those with coronae or rapid stellar winds, emit x rays from their outer layers.
Comets. Astronomers have detected x-ray emission from various different comets since the phenomenon was first discovered in 1996 with comet Hyakutake. Scientists believe that x rays are generated by some sort of interaction between the solar wind and the comet’s atmosphere, ionosphere, or atoms within the nucleus.
Groups of galaxies in hot clouds. Bright x-radiation is seen emanating from clusters of galaxies, which, due to their enormous gravitational pull, trap gas in the region. This gas is very hot, and there is a large amount of it. It thus glows in the x-ray region.
X-ray background. The sky is not dark in the x-ray region of the sky like it is in the visible. The diffuse background that was detected in the rocket flight described above is still not understood, although some believe it may be the result of many individual, unresolved sources.
X-ray binaries. These are close binary stars in which gas from one star falls onto its companion, heats up, and gives off x rays. This is especially bright when the companion is a compact stellar remnant such as a neutron star or black hole, because the enormous gravitational field compresses and heats the incoming gas, causing it to glow at x-ray wavelengths.
Supernova remnants. Explosions of stars, or supernovae, show traces of the heavy elements that are formed there when their x-ray spectra are examined.
Quasars and active galactic nuclei. These are among the most energetic objects in the universe, and they emit enormous quantities of radiation at x-ray wavelengths. It is thought that the ultimate source of this energy is a supermassive black hole, surrounded by an accretion disk of in-falling gas that is heated to many millions of degrees.
Among the largest and most productive x-ray missions were Uhuru (1970–1973, also known as X-ray Explorer Satellite), which catalogued 339 x-ray sources; Einstein Observatory (also known as HEAO-2, 1978–1981, was the first fully imaging x-ray telescope in space); and EXOSAT (1983–1986, made over 1,700 observations in the x-ray band of astronomical objects). In addition, there have been many smaller-scale observations.
More recent missions, such as the German ROSAT (Rüntgensatellit), launched in 1990, contain very sophisticated instrumentation, including detectors and grazing incidence telescopes, which can pinpoint the location of an x-ray source to very high accuracy, and take x-ray pictures to show the shape and distribution of the source. This is an important improvement over early missions, which often were not able to determine the exact location of the x-ray sources, making it difficult to correlate the source with an object that could be detected in another wavelength region. It operated until 1999.
Recent missions have also been able to measure the x-ray spectrum, or strength of the radiation in different energy bands. This allows the identification of particular elements in the source. ROSAT identified more than 50,000 x-ray sources during its survey phase, when it scanned the sky for six months. It also finally succeeded in detecting x rays from the moon, nearly 30 years after the first attempt to do so.
NASA launched the Advanced X ray Astrophysics Facility (AXAF), later named the Chandra X-ray Observatory, in 1999. Designed with a resolution 25 times better than any preceding x-ray telescope, CXO passes around the Earth in an elliptical orbit, studying black holes, supernovas, and dark matter and in an attempt to increase scientific understanding of the origin and evolution of the universe. As of November 2006, it is continuing its mission of exploration.
BeppoSAX was an Italian-Dutch satellite launched in 1996 to study x-ray sources and to identify gamma-ray bursts with extra-galactic objects. It ended its mission in 2003. The name Beppo was to honor Italian astronomer Giuseepe (Beppo) Occhialine (1907–1993), and SAX stands for satellite for x-ray astronomy. Similarly, the Swift Gamma-Ray Burst Mission was launched on 2004 by NASA. Its mission
Grazing incidence telescope —A telescope design in which the incoming radiation strikes the mirrors at very small angles.
Spectrum —A display of the intensity of radiation versus wavelength.
was also to study gamma-ray bursts and to observe their afterglows in the x-ray, gamma-ray, ultraviolet, and optical wavelengths. It mission continues as of November 2006.
MacCarone, T.J., R.P. Fender, and L.C. Ho, eds. From X-ray Binaries to Quasars: Black Holes on All Mass Scales. Dordrecht, Germany, and Norwell, MA: Springer, 2005.
Van Grieken, Rene E., and Andrzej A. Markowicz, eds. Handbook of X-ray Spectrometry. New York: Marcel Dekker, 2002.