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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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How astronomers observe 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. 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^{[dead link – history]}). 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 Detectors

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. This works well for astronomical uses, because there just aren't a lot of x-ray photons coming our way - even from the strongest sources like 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), 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, which is occulted by the dark side of the Moon.

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.