n.
The branch of astronomy that deals with the origin and nature of emissions from extraterrestrial sources in the x-ray range of electromagnetic radiation rather than in the visible range.
x-ray astronomy
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n.
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The study of x-ray emission from extrasolar sources, including virtually all types of astronomical objects from stars to galaxies and quasars. The x-ray region of the electromagnetic spectrum extends from wavelengths of about 10 picometers to a few tens of nanometers, with shorter wavelengths corresponding to higher-energy photons (1 nm corresponds to about 1000 eV). X-ray astronomy is traditionally divided into broad bands—soft and hard—depending on the energy of the radiation being studied. Observations in the soft band (below about 10 keV) must be carried out above the atmosphere from rockets or satellites, while hard x-ray observations can be made at high altitudes achievable by balloons. See also Electromagnetic radiation; Rocket astronomy; Satellite astronomy; X-rays.
Most observations have been in the soft band. Sky surveys, particularly the ROSAT all-sky survey, have located tens of thousands of sources at a sensitivity of about 1/100,000 of the strength of the brightest source, Sco X-1. Some of these sources are concentrated along the galactic equator. Such a concentration corresponds to objects in the Milky Way Galaxy, particularly in the disk, which contains most of the galactic stars and the spiral arms. Other x-ray sources are uniformly spread over the sky, associated mainly with extragalactic objects, such as individual galaxies, clusters of galaxies, and quasars. See also Milky Way Galaxy.
Galactic sources
Galactic x-ray sources have been identified with different types of unusual objects. The Crab Nebula, which was the first nonsolar x-ray source to be identified with a specific celestial object, is a supernova remnant left over from the explosive death of a star. It contains a rapidly rotating neutron star at its center as well as a nebula consisting of hot gas and energetic particles. About 10% of the x-ray emission from the Crab is pulsed radiation from the neutron star; the rest is extended emission associated with the nebula. Many other galactic supernova remnants have been detected as x-ray sources, and there have also been detections of supernova remnants in the nearest neighboring galaxies, especially in the Large Magellanic Cloud. See also Crab Nebula; Neutron star; Pulsar; Supernova.
While normal stars radiate some energy in the x-ray band, most of their luminosity is output in the visible region of the spectrum. X-ray surveys have discovered a class of objects where the majority of energy is radiated in the x-ray portion of the electromagnetic spectrum. These are known as x-ray stars. With accurate locations, it has been possible to identify them with binary star systems. The combination of x-ray and optical data leads to the conclusion that these systems usually consist of a relatively normal visible star and one subluminous star in a gravitationally bound orbit about each other. The x-ray star is often a collapsed object, such as a white dwarf, neutron star, or even a black hole. Such compact sources form the majority of galactic emitters.
The theoretical model for x-ray emission in these systems consists of matter being transferred from the normal star to the compact object. This process, known as accretion, usually leads to the creation of a disk of infalling material spiraling down toward the surface of the compact star. During infall the matter reaches very high temperatures as gravitational energy is converted to heat, which is then radiated as x-rays. It is believed that similar processes are involved regardless of whether the compact object is a white dwarf, neutron star, or black hole. However, different types of detailed behavior are expected for each of these objects. Rotating neutron stars lead to x-ray pulsars. For white dwarfs, there appear to be fluctuations in x-ray intensity on time scales of hours to days that might be indicative of changes at the surface of the star where accreted material is collecting, and then flashing in a burst of thermonuclear energy release. In the case of black hole candidates, there are variations in x-ray intensity at extremely short time scales that indicate very small regions of x-ray emission.
Another type of time behavior is represented by the so-called bursters. These sources emit at some constant level, with occasional short bursts or flares of increased brightness. These outbursts may well be instabilities in the x-ray emission processes associated with this type of source. See also Binary star; Black hole; Cataclysmic variable;
Extragalactic sources
Among the more interesting types of objects detected have been apparent normal galaxies, galaxies with active nuclei, radio galaxies, clusters of galaxies, and quasars.
In active nuclei galaxies (those with strong optical emission lines and nonthermal continuum spectra), x-ray emission is usually orders of magnitude in excess of that from normal galaxies. In most cases the emission comes from the galactic nucleus, and must be confined to a relatively small region on the basis of variability and lack of structure at the current observational limits of a few seconds of arc. Many astrophysicists believe that galactic nuclei are the sites of massive black holes, at least 106–109 times the mass of the Sun, and that the radiation from these objects is due to gravitational energy released by infalling material. The broad range of properties, such as x-ray luminosity, may reflect the size of the black hole and availability of infalling matter. Quasars may represent the extreme case of this mechanism. Many of the observable properties depend on the viewing angle of the observer as well as the size of the nuclear black hole. See also Quasar.
Clusters of galaxies are collections of hundreds to thousands of individual galaxies which form a gravitationally bound system. The space between galaxies in such clusters has been found to contain hot (approximately 107–108 K) tenuous gas which glows in x-rays, and whose mass is equal to (or exceeds) the mass of the visible galaxies. See also Astrophysics, high-energy; Galaxy, external.
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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 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.
| Wikipedia: X-ray astronomy |
X-ray astronomy is an observational branch of astronomy, which deals with the study of X-ray emission from celestial objects. X-ray radiation is absorbed by the Earth's atmosphere, so instruments to observe X-rays must be taken to high altitude, in the past with balloons and sounding rockets. Nowadays, X-ray astronomy is part of space research and X-ray detectors are placed in satellites.
X-ray emission is expected in sources which contain an extremely hot gas at temperatures from a million to hundred million kelvins, in general in objects in which the atoms and/or electrons have a very high energy. The discovery of the first cosmic X-ray source in 1962 came as a surprise. This source is called Scorpius X-1, the first X-ray source found in the constellation of Scorpius, located in the direction of the center of the Milky Way. Based on this discovery, Riccardo Giacconi received the Nobel Prize in Physics in 2002. Later it was found that the X-ray emission of this source is 10,000 times greater than its optical emission. In addition, the energy output in X-rays is 100,000 times greater than the total emission of the Sun in all wavelengths. It is now known that such X-ray sources are compact stars, such as neutron stars and black holes. The energy source is gravitational energy, which comes from gas heated by the fall in the strong gravitational field of such objects.
Nowadays, many thousands of X-ray sources are known. In addition, it appears that the space between galaxies in a cluster of galaxies is filled with a very hot, but very dilute gas at a temperature of between 10 and 100 megakelvins. The total amount of hot gas is five to ten times the total mass in the visible galaxies.
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X-rays observation
X-rays span 3 decades in wavelength, frequency and energy. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft x-rays, and from 0.1 nm to 0.01 nm (about 12 to 120 keV) as hard X-rays.
Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ) can penetrate the air at least for distances of a few meters (they would never have been detected and medical X-ray machines would not work if this was not the case) the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air.
To observe X-rays from the sky, the X-ray detectors must be flown above most of the Earth's atmosphere. There are three main methods of doing so: sounding rocket flights, balloons, and satellites. Satellites are the method preferred by scientists now.
Sounding rocket flights
A detector is placed in the nose cone section of a sounding rocket and launched above the atmosphere. This was first done at White Sands Missile Range in New Mexico with a V-2 rocket in 1949. X-rays from the Sun were detected by the Navy's experiment on board. An Aerobee 150 rocket launched in June 1962 detected the first X-rays from other celestial sources (Scorpius X-1). The largest drawback to rocket flights is their very short duration (just a few minutes above the atmosphere before the rocket falls back to Earth) and their limited field of view. A rocket launched from the United States will not be able to see sources in the southern sky; a rocket launched from Australia will not be able to see sources in the northern sky.
Balloons
Balloon flights can carry instruments to altitudes of up to 40 kilometers above sea level, where they are above as much as 99.997% of the Earth's atmosphere. Unlike a rocket where data are collected during a brief few minutes, balloons are able to stay aloft for much longer. However, even at such altitudes, much of the X-ray spectrum is still absorbed. X-rays with energies less than 35 keV (5,600 aJ) cannot reach balloons. One of the recent balloon-borne experiments was called the High Resolution Gamma-ray and Hard X-ray Spectrometer (HIREGS). It was first launched from McMurdo Station, Antarctica in December 1991, when steady winds carried the balloon on a circumpolar flight lasting for about two weeks.
Satellites
A detector is placed on a satellite which is then put into orbit well above the Earth's atmosphere. Unlike balloons, instruments on satellites are able to observe the full range of the X-ray spectrum. Unlike sounding rockets, they can collect data for as long as the instruments continue to operate. In one instance, the Vela 5B satellite, the X-ray detector remained functional for over ten years.
Satellites in use today include the XMM-Newton observatory (low to mid energy X-rays 0.1-15 keV) and the INTEGRAL satellite (high energy X-rays 15-60 keV), and both were launched by the European Space Agency. NASA has launched the Rossi X-ray Timing Explorer (RXTE), and the Swift and Chandra observatories. One of the instruments on Swift is the Swift X-Ray Telescope (XRT). SMART-1 contained an X-ray telescope for mapping lunar X-ray fluorescence. Past observatories included ROSAT, the Einstein Observatory, the ASCA observatory and BeppoSAX.
X-ray telescopes/mirrors
X-ray telescopes have varying directionality or imaging ability based on glancing angle reflection rather than refraction or large deviation reflection [1] [2]. This limits them to much narrow fields of view than visible or UV telescopes.
The mirrors can be made of ceramic or metal foil [3].
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Detectors
Most astronomical X-ray detectors can measure the energy of the photons[citation needed]. They can be characterised by the minimum energy they can detect[citation needed], and their effective number of pixels.
CCDs
Most existing X-ray telescopes use CCD detectors, similar to those in visible-light cameras. In visible-light, a single photon can produce a single electron of charge in a pixel, and an image is built up by accumulating many such charges from many photons during the exposure time. When an X-ray photon hits a CCD, it produces enough charge (hundreds to thousands of electrons, proportional to its energy) that the individual X-rays have their energies measured on read-out.
Microcalorimeters
Microcalorimeters can only detect x-rays one photon at a time (but can measure the energy of each). This works well for ground-based astronomical uses as few x-ray photons reach the earth, even from the strongest sources such as black holes See Microcalorimeters and X-ray microcalorimeter
Transition Edge Sensors
TES devices are the next step in microcalorimetery. In essence they are super-conducting metals kept as close as possible to their transition temperature. This is the temperature at which these metals become super-conductors and their resistance drops to zero. These transition temperatures are usually just a few degrees above absolute zero (usually less than 10 K).
Astronomical sources of X-rays
Several types of astrophysical objects emit X-rays, from galaxy clusters, through black holes in active galactic nuclei (AGN) to galactic objects such as supernova remnants, stars, and binary stars containing a white dwarf (cataclysmic variable stars and super-soft x-ray sources), neutron star or black hole (X-ray binaries). Some solar system bodies emit X-rays, the most notable being the Moon, although most of the X-ray brightness of the Moon arises from reflected solar X-rays. A combination of many unresolved X-ray sources is thought to produce the observed X-ray background.
Black holes give off radiation because matter falling into them loses gravitational energy which may result in the emission of radiation before the matter falls into the event horizon. The infalling matter has angular momentum, which means that the material cannot fall in directly, but spins around the black hole. This material often forms an accretion disk. Similar luminous accretion disks can also form around white dwarfs and neutron stars, but in these the infalling gas releases additional energy as it slams against the high-density surface with high speed. In case of a neutron star, the infall speed can be a sizeable fraction of the speed of light.
In some neutron star or white dwarf systems, the magnetic field of the star is strong enough to prevent the formation of an accretion disc. The material in the disc gets very hot because of friction, and emits X-rays. The material in the disc slowly loses its angular momentum and falls into the compact star. In neutron stars and white dwarfs, additional X-rays are generated when the material hits their surfaces. X-ray emission from black holes is variable, varying in luminosity in very short timescales. The variation in luminosity can provide information about the size of the black hole.
Clusters of galaxies are formed by the merger of smaller units of matter, such as galaxy groups or individual galaxies. The infalling material (which contains galaxies, gas and dark matter) gains kinetic energy as it falls into the cluster's gravitational potential well. The infalling gas collides with gas already in the cluster and is shock heated to between 107 and 108 K depending on the size of the cluster. This very hot gas emits X-rays by thermal bremsstrahlung emission, and line emission from metals (in astronomy, 'metals' often means all elements except hydrogen and helium). The galaxies and dark matter are collisionless and quickly become virialised, orbiting in the cluster potential well.
The X-rays of the solar system bodies are produced by fluorescence. Scattered solar X-rays provide an additional component.
References
- ^ http://swift.sonoma.edu/about_swift/instruments/xrt.html SWIFT X-ray mirrors
- ^ http://chandra.harvard.edu/about/telescope_system.html Chandra X-ray focusing mirrors
- ^ http://astrophysics.gsfc.nasa.gov/xrays/MirrorLab/xoptics.html
See also
- Gamma-ray astronomy
- X-Ray telescopes Alphabetic list
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- Superluminal motion
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