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X-Ray Astronomy
I INTRODUCTION
X-Ray Astronomy, detection and study of electromagnetic energy radiating from celestial bodies in the form of X rays. X-ray astronomy provides astrophysicists with a means to study violent and energetic events in the universe. Nearly every class of astronomical object, from nearby stars to distant quasars, emits X rays at some point in its life cycle. See also X ray; Astronomy.
X rays are part of a wide spectrum of energy called electromagnetic radiation. Electromagnetic waves range from high energy, short wavelength gamma rays, to visible light, to low-energy, long wavelength radio waves. X rays have shorter wavelengths and higher energies than visible light and ultraviolet radiation, but longer wavelengths than gamma rays. They are powerful enough and have short enough wavelengths to pass through many materials that reflect or absorb visible light.
Objects and regions in space emit X rays for one of two reasons. Most X rays come from regions in which gas is heated to tens of millions of degrees Celsius or Fahrenheit. This heating may be a result of shock waves from huge stellar explosions, gas plunging into intense gravitational fields (see Gravitation), or other energetic events, and it causes gas to emit X rays. X-ray emission caused by hot gas is called thermal emission. X rays may also be emitted when powerful magnetic fields accelerate electrons to nearly the speed of light. This kind of X-ray emission is called nonthermal emission.
II DETECTING X RAYS
Although X rays pass easily through many solid objects, Earth's atmosphere absorbs most of the X-ray radiation that hits it. Therefore X-ray astronomy requires instruments to be above Earth's atmosphere. Astronomers use rockets, balloons, and satellites to place their instruments above the atmosphere. They must also use special telescopes and detectors because X rays pass right through ordinary telescopes.
X-ray telescopes reflect and focus X rays to produce an image that astronomers can use. Many X-ray telescopes use a metal mirror in the shape of a hyperbola, a parabola, or a combination of the two. The mirror in these X-ray telescopes—called grazing-incidence telescopes—is not shallow like that in optical wavelength telescopes, but is instead almost cylindrical. When X rays reach the mirror, they barely graze the mirror's surface. The angle between the mirror and the incoming X rays is just large enough that the mirror reflects the X rays toward a central focus, but not large enough to allow the mirror to absorb the X rays or to allow the X rays to pass through the mirror. Most X-ray telescopes contain several concentric mirrors; each mirror adds to the radiation-gathering power of the telescope.
Another type of X-ray telescope—called a multilayer telescope—has a shallower mirror, more like that of an optical reflecting telescope. Many of the X rays that hit the mirror of a multilayer telescope pass through it, but very thin layers of special materials intensify the X rays that do get reflected. Multilayer telescopes can gather light from a wider area of the sky than grazing-incidence telescopes can.
The mirrors of grazing-incidence and multilayer X-ray telescopes gather and focus light, but the information that they gather must be recorded to be of use to astronomers. Detectors that perform like electronic cameras detect and record radiation at the focal point of the telescope. A detector can be a charge-coupled device (CCD), a microchannel plate detector, or an ionization chamber (see Particle Detectors: Ionization Chamber). All three types of detectors record as an electronic signal the location of each X-ray photon that hits the detector.
A CCD is an array of photodiodes, electrical circuits that are sensitive to electromagnetic radiation. The CCD records the location of each photodiode that gets hit by an X-ray photon (a packet of X-ray radiation). It also records the energy, or frequency and wavelength, of the photon.
A microchannel plate detector is an array of tiny hollow tubes. The tubes are coated with a substance that releases electrons when struck by an X-ray photon. When a photon enters one of the tubes, the tube sends out an electrical signal. The detector records the location of each photon, but cannot determine the frequency of the radiation.
An ionization chamber is a container containing a gas and a network of wires. When an X-ray photon enters the gas, it produces an electron that ionizes the nearby gas by stripping electrons off gas molecules or adding electrons to them, leaving behind positively or negatively charged molecules called ions. The positive or negative gas ions send an electrical signal along the nearest wire, giving an approximate location of the photon. Ionization chambers are not as good as the other detectors at determining the location of the photon, but they can measure the photon's energy better than microchannel plates can. Astronomers interested in determining the exact location and size of an X-ray-emitting object need good location measurements. Scientists measuring the characteristics of the radiation emitted by an object need good wavelength and energy measurements.
III X-RAY SOURCES
Any celestial object that produces hot gas or strong magnetic fields can emit X rays. These objects include sources within our galaxy, the Milky Way, and sources outside of the Milky Way. Galactic sources include stars, binary stars, pulsars, X-ray bursters, and remnants of supernova stars (see Supernova). Extragalactic X-ray sources include X-ray galaxies, quasars, and the X-ray background radiation. Some objects emit only a tiny portion of their total energy in X rays. Others may be dim in visual wavelengths, but emit strong X rays.
A Galactic Sources
A normal star such as our Sun produces X rays in the hot outer layer, or corona, of the star. Hot solar or stellar flares also emit X rays. The surface of most ordinary stars is too cool to generate X rays.
An X-ray binary is a pair of stars that emit X rays. X-ray binaries consist of a normal star that orbits a compact object such as a white dwarf, a neutron star, or a black hole. The gravitational pull of the compact object pulls off some of the normal star's outer atmosphere. The stream of material from the stellar atmosphere settles into a disk that swirls around the compact star. Friction heats the trapped gas, which emits X rays.
Neutron stars may emit X rays because of the strong magnetic field surrounding the stars. A neutron star is the very dense, rapidly spinning, collapsed core of a star that has exploded in a supernova. Neutron stars have powerful magnetic fields—up to one trillion times as strong as Earth's magnetic field. If a neutron star has a close companion star, the neutron star's intense gravity pulls matter off of the companion star. The magnetic field of some neutron stars is so strong that it forces the trapped material to trickle onto the neutron star's surface through magnetic funnels at the neutron star's poles. Friction heats the incoming material and causes it to emit X rays. The neutron star's magnetic field also influences the released X rays, forcing them out in narrow beams from the star's magnetic poles. The neutron star seems to pulsate X rays because the star rotates many times a second, turning the hot spots at the poles toward Earth like a rotating lighthouse beacon. Neutron stars that emit X rays in this way are called X-ray pulsars.
Some neutron stars have weaker magnetic fields that allow incoming material to settle onto the entire surface of the neutron star. Eventually, so much material builds up that the surface layer becomes dense enough to set off a vast thermonuclear explosion, called an outburst. The explosion heats gas to produce X rays. Such a neutron star—called an X-ray burster—can increase its X-ray production by a million times during an outburst. The X-ray glow fades over time, and the binary system enters a long, quiet period as material from the companion star again begins to accumulate on the neutron star's surface.
Some very massive stars explode into supernovas at the end of their lives. The star begins to use up its nuclear fuel and begins to collapse in on itself. Eventually the star's increasing density triggers one final, huge nuclear explosion. The shock waves that this explosion generates crash into interstellar gas and heat it so much that the gas radiates X-ray light for thousands of years.
B Extragalactic Sources
Most galaxies contain the same types of X-ray-emitting objects that the Milky Way does, but the other galaxies are so distant that these sources usually cannot be detected from Earth. Only extremely energetic objects in other galaxies, such as large supernovas and supermassive black holes, are detectable from Earth. The centers of many galaxies are powerful X-ray emitters. Astronomers believe that this is because almost every galaxy has a supermassive black hole at its center. These black holes are a million to a billion times the mass of the Sun and produce intense gravitational and magnetic fields. Galaxies that are especially active in the X-ray spectrum (X-ray galaxies) often have large amounts of material surrounding the black hole in the galactic nucleus. As matter swirls toward the black hole, friction heats the gas and dust enough to produce X rays. Electrons also get caught and accelerated in the black hole's magnetic field and produce X rays. Astronomers do not know exactly what quasars are, but many believe that quasars may be galaxies that appear at strange angles from Earth.
Clusters of galaxies are among the most luminous X-ray sources in the sky. Huge amounts of very hot hydrogen gas in the clusters produce the X-ray radiation. Astronomers have found that the amount of visible matter in the cluster is not sufficient to explain how so much gas gets squeezed and heated to such high temperatures. They have concluded that a vast amount of dark matter—matter that does not emit electromagnetic radiation and is therefore invisible to observers from Earth—must be present to provide the gravitational energy to hold the gas in the cluster.
The universe also glows in X rays. The background X-ray glow is strong and uniform, or the same in all directions. This uniformity is one of the reasons astronomers believe that nearly every galaxy in the universe contains an X-ray-producing black hole at its center. The distribution of galaxies in the universe is uniform on a large scale, so X rays from galactic black holes would explain the uniformity of the X-ray background radiation.
IV HISTORY OF X-RAY ASTRONOMY
German physicist Wilhelm Roentgen accidentally discovered X rays during an 1895 laboratory experiment. Roentgen dubbed the radiation X rays because x is a common mathematical symbol for the unknown. Roentgen and other scientists quickly discovered the ability of X rays to pass through many substances and applied their research on X rays to medicine and industry.
Astronomical X rays were not discovered until after World War II (1939-1945). Scientists at the United States Naval Research Laboratory equipped a captured German V-2 rocket with an X-ray detector and sent it through Earth's atmosphere. The X-ray detector registered X-ray radiation from the Sun.
In 1962 a group of American scientists—including Riccardo Giacconi, Herbert Gursky, Frank Paolini, and Bruno Rossi—sent a rocket-mounted X-ray detector to study the Sun's effect on the Moon's X-ray emission. Instead, the detector picked up X rays from a bright X-ray source in the constellation Scorpius. This object, called Scorpius X-1 or Sco X-1, was the first black hole discovered.
Encouraged by this unexpected discovery, Giacconi led development of the first Earth-orbiting observatory, named Uhuru (“freedom” in Swahili). Launched in 1970, Uhuru mapped the entire sky and discovered about 300 X-ray sources—far more than anyone had ever imagined.
In the late 1970s the U.S. National Aeronautics and Space Administration (NASA) launched a series of remarkably successful X-ray satellites, called the High Energy Astronomy Observatories (HEAO). HEAO 2—also called the Einstein Observatory—was the first space observatory capable of making X-ray images comparable to the images produced by optical telescopes. The Einstein telescope revealed that many active galaxies and quasars are strong X-ray emitters.
During the 1980s the European, Japanese, and Russian space agencies continued to launch successful X-ray astronomy missions largely directed toward in-depth studies of phenomena such as X-ray bursters and X-ray pulsars. In the 1990s a German X-ray satellite, called the Roentgen Satellite (ROSAT), provided a sharper, more sensitive, and wider view of the X-ray sky. ROSAT's all-sky survey has identified nearly 60,000 X-ray sources across the universe. In addition, an armada of other satellites now in operation are contributing to important new breakthroughs, including the U.S. Advanced Satellite for Cosmology and Astrophysics (ASCA), the U.S. Rossi X-ray Timing Explorer (RXTE), and the Italian Satellite for X-ray Astronomy (Beppo SAX).
In 1999 NASA launched the most sensitive X-ray satellite to date, the Chandra X-Ray Observatory. Named for American astrophysicist Subrahmanyan Chandrasekhar, the telescope aboard Chandra has eight times the resolution of any previous X-ray telescope. Also in 1999, the European Space Agency launched the X-ray Multi-Mirror (XMM) Newton satellite, whose unique configuration of nested mirrors was designed to help scientists discover many more astronomical sources of X rays.
Contributed By:
Ray Villard
X-Ray Telescope
In order to observe celestial X-ray sources, astronomers use a special kind of telescope launched into orbit, because the earth's atmosphere absorbs X rays from space. X rays are so short that lenses do not refract, or bend, them as they do ordinary light. However, X rays can be reflected if they make grazing contact with a metal surface. An X-ray telescope uses sets of nested, slightly tapering cylinders to focus X rays onto a detector.
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