X-ray and Gamma-ray Astronomy

Type of physical science: Astronomy; Astrophysics

Field of study: Observational techniques

X-ray and γ-ray (gamma-ray) astronomy involve the observation of events in the universe that occur at energies far greater than what is normally shown by visible light or other forms of astronomy. The state of X-ray and gamma-ray astronomy is less advanced than that of other forms, partly because the fields are younger, but they promise great advances in understanding the formation and fate of matter in the universe.

Overview

X-ray and gamma-ray astronomy involve the observation of events in the universe that occur at energies far greater than what is normally shown by visible light or other forms of astronomy. Both x-rays and gamma rays are electromagnetic radiation, like light, but at much shorter wavelengths and correspondingly higher frequencies. Because of this extreme difference, it is much more difficult to collect large quantities of x-rays and gamma rays and to detect their source. In general, the two fields are part of what is called high-energy astrophysics, a subject that also studies ultraviolet radiation and cosmic rays.

Because of the extremely short wavelengths involved, x-rays and gamma rays are both measured in terms of their energy, from thousands to billions of electronvolts. Although there is no firm demarcation, x-rays span the range of one thousand to one hundred thousand electronvolts, and gamma rays are generally those above one hundred thousand electronvolts. For context, visible light is around one to two electronvolts. X-rays are emitted by the innermost electrons of an atom releasing energy. Gamma rays, by definition, are emitted by reactions in the nuclei of atoms or by other subatomic reactions. In truth, there is some overlap between the two spectral bands, and it may be impossible to tell if an emission was from an energetic electron or a nuclear reaction.

Since x-rays and gamma rays both pass through solid matter, it is difficult to focus them on a detector and difficult to make them interact with the matter in the detector itself. A common method of focusing high-energy radiation is to use collimators to exclude all radiation except that from a particular direction. The basic technique can be imitated by looking through a cluster of straws held at arm's length. The view is narrow and restricted; essentially, all light that is nearly on axis (that is, parallel with the center line of the straws) passes through, while light that is off axis strikes the sides of the straw. The longer the straws, the narrower the field of view. A variety of complex collimation techniques have been developed to allow only radiation from a desired source to fall directly on a detector. While simple, this method generally yields images of the sky that lack the finer resolution of optical telescopes and thus leave much uncertainty about the location of energy sources.

In the case of x-rays, this problem has been addressed by the use of grazing-incidence mirrors to focus "soft" (low-energy) x-rays. Grazing incidence occurs when radiation strikes a surface at an extremely shallow angle and is reflected rather than being absorbed or scattered. The effect is readily seen when visible light strikes a windshield or pond at less than the critical angle and is reflected to cause glare. Because of the energy of x-rays, however, the angle of incidence must be extremely shallow, typically less than two degrees, and the surface must be exceptionally smooth to allow an image to form. X-ray telescopes generally use two reflectors, a parabolic primary and a hyperbolic secondary, to focus the radiation into an image without aberration. To provide the shallow angle of incidence, the mirrors resemble tubes; the segment of the paraboloid or hyperboloid surface is more distant from the focus than with conventional telescopes. This also requires that the secondary mirror be mounted directly behind and precisely aligned with the primary. Such an arrangement is known as a Wolter type I telescope. A number of variations are available. Because only a small region of the radiation is intercepted by the mirrors, modern x-ray telescopes typically will use a nesting scheme in which up to six complete telescopes are built within each other.

Another type of x-ray telescope is the Kirkpatrick-Baez telescope, which uses curved plates of glass in banks one behind the other. This arrangement focuses the light in first one axis and then the other. While lacking the fine resolution of Wolter telescopes, the Kirkpatrick-Baez arrangement is well suited to all-sky surveys.

Grazing-incidence telescopes were developed in the 1960s and 1970s. The 1980s saw the development of a radically new approach: normal-incidence x-ray mirrors. In Bragg crystals, the internal structure of a crystal can refract x-rays almost as effectively as glass refracts light. Normal-incidence mirrors are built up in layers of microscopic crystals that intercept the x-rays and reverse their direction. When these mirrors are laid on parabolic or hyperbolic surfaces, one can build an x-ray telescope that resembles a conventional reflector telescope and provide comparable resolution for low- to medium-energy x-rays.

At higher energies, the efficiency of reflectors decreases until virtually no image is being focused, so constructing a telescope capable of focusing high-energy x-rays was a significant challenge. In 2005, the High-Energy Focusing Telescope (HEFT), which consisted of several coaligned grazing-incidence telescopes and multilayered coatings, was launched in a balloon from Fort Sumner, New Mexico, for a twenty-five-hour flight. It was able to focus x-rays between twenty and seventy thousand electronvolts. Subsequently, in 2012, the National Aeronautics and Space Administration (NASA) launched the Nuclear Spectroscopic Telescope Array (NuSTAR), the first orbiting telescope capable of focusing high-energy x-rays—up to eighty thousand electronvolts. NuSTAR successfully captured the first ever focused image of hard x-rays in the universe.

For a long time, scientists thought there was simply no way to focus gamma rays. However, in May 2012, physicists at the Ludwig Maximilian University of Munich and the Institut Laue-Langevin in Grenoble, France, announced that they had successfully used a silicon prism to bend gamma rays of over seven hundred thousand electronvolts at an angle of approximately one millionth of a degree. While such a minute angle may not seem significant, the experiment demonstrated that it is, in fact, possible to bend gamma rays and raised the possibility of a telescope capable of focusing gamma-rays in the future.

X-rays and gamma rays, because of the energies involved, are detected less directly than light; specifically, the energy yielded by interaction with some intermediate object is what is detected. The first and simplest x-ray detector was photographic film. Exposure to x-rays will cause the film to darken, producing a negative image. Originally, film was used in x-ray telescopes that could be recovered, such as manned satellites and sounding rockets; various electronic detectors were used in unmanned satellites. The two were complementary in use, with film providing the highest spatial resolution and finer details but over a broad energy range, while electronic detectors could measure energy levels with a fair degree of accuracy but at the expense of spatial resolution. More recently, as the technology improved, x-ray telescopes began to only use electronic detectors. Typically, these detectors transmit information as raw data, and that data is then translated into images by astronomers on Earth.

Various different types of electronic detectors are available for imaging x-rays. The most common type is the charge-coupled device (CCD), which is an electronic analogue of the retina and comprises thousands of small photosensors that are read individually by computer. CCDs are a main component of digital imaging and are used in most modern x-ray telescopes. Unlike the visible-light CCDs found in cameras, in which a single photon charges one pixel with a single electron, the CCDs in x-ray telescopes work by using a single x-ray photon to produce hundreds or even thousands of electrons, allowing the energy of each individual x-ray to be measured and recorded.

Another type of detector is the proportional counter, which is similar to a Geiger-Müller counter. A high-energy photon enters a gas-filled chamber, intercepts a gas atom or molecule, and generates an ion and an electron. These are attracted to the anode in the tube, causing an increase in the electrical current that is measured by the instrument's electronics. Proportional counters can also be used for spectral analysis of x-rays.

Scintillation detectors work by trapping the incoming radiation in a crystal, causing a flash of light that is measured by a photomultiplier tube. The most sensitive of these instruments require the crystal to be supercooled by liquefied or solid gas so that the body temperature of the crystals does not cause false readings. A variation of this effect is Compton scattering, in which a high-energy x-ray strikes a free charged particle and is scattered at lower energy, having transferred part of its energy to the charged particle, usually an electron. At higher energies, however, the efficiency of this effect declines. Gamma-ray interactions can also produce electron-positron (antielectron) pairs that can be detected in spark chambers, which cause sparks between electrified plates or wires. These are then registered by photomultiplier tubes.

To increase the chances of detecting an x-ray, detectors sometimes are electrically charged so that a single x-ray will produce a shower of electrons that can be readily detected. The electrically charged device is a microchannel plate, which is like a microscopic collimator with its channels at a slant. This ensures that an x-ray will strike the wall of the channel, which has an electric charge not quite high enough to cause an electric arc. The incoming x-ray provides the needed extra jolt and releases a cascade of electrons that exits the back of the plate to be measured by the wires of a grid behind the detector.

Because of the immense, possibly insurmountable difficulty of focusing gamma rays, different techniques are used to determine the shape of a gamma-ray source. The instrument can be tilted back and forth and the flux change measured to determine its origin. Alternatively, the instrument can be built as a pair, one behind the other, and the two sets of signals can be collated to determine the origin of each gamma ray.

X-ray and gamma-ray detectors generally have anticoincidence detectors that are designed to detect cosmic rays and thus allow the signals they generate to be subtracted from the instrument signal, somewhat like filtering the noise in a radio.

Applications

X-ray and gamma-ray astronomy must be conducted above the atmosphere of the earth because the atmosphere absorbs all x-rays; even the thin layer above thirty-six thousand meters absorbs soft x-rays. While both fields are best conducted from satellites, both started on (and still use) suborbital platforms: x-ray astronomy on suborbital rockets and gamma-ray astronomy on balloons because the lower fluxes required larger, heavier detectors. Suborbital rockets, also called sounding rockets, can expose a payload to the space environment for several minutes, depending on the weight of the instrument and the power of the rocket. Suborbital rockets were a major tool during much of early x-ray astronomy and continue to serve the same purpose as new telescopes are developed and tested. They also have filled the gap between flights of major x-ray satellites. Gamma-ray instruments have to be larger and spend more time at altitude to intercept a sufficient flux from gamma-ray sources, so unmanned balloons have been their preferred suborbital platform. In either case, though, satellites are the best means of operation, since they provide essentially indefinite observing time for medium to heavy instruments.

Several x-ray astronomy satellites have been launched, some notable early ones being the High Energy Astrophysical Observatory (HEAO) series, the European X-Ray Observatory Satellite (EXOSAT), and the Röntgensatellit (ROSAT). Wolter type I x-ray telescopes were carried by three of these satellites (HEAO-2, EXOSAT, ROSAT) to produce images and spectra of the heavens.

X-ray satellites launched in the twenty-first century include NuSTAR, discussed above, as well as the European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL), launched in 2002, and NASA's spacecraft Swift, launched in 2004. INTEGRAL carries four instruments for detecting x-rays and gamma rays, between them capable of observing energy ranging from three thousand to ten million electronvolts. Swift, the object of NASA's Swift Gamma-Ray Burst Mission, carries three instruments, one of which is the Swift X-Ray Telescope (XRT), a CCD imaging spectrometer intended to capture the aftermath of gamma-ray bursts as they fade into the x-ray spectrum. Another notable x-ray satellite is the Chandra X-Ray Observatory, launched by NASA in 1999. Originally called the Advanced X-Ray Astrophysics Facility (AXAF), Chandra was renamed in honor of Nobel Prize–winning astrophysicist Subrahmanyan Chandrasekhar, who died in 1995. The observatory was conceived as an x-ray complement to the optical Hubble Space Telescope and has a set of four nested Wolter type I mirror pairs, each pair being equivalent to a single telescope. This high-resolution mirror assembly (HRMA) is capable of producing x-ray images with resolutions of 0.5 arcsecond or better. Chandra also carries the Advanced CCD Imaging Spectrometer (ACIS), which is capable of accurately measuring the energy of a single photon, and the High Resolution Camera (HRC); both of these instruments can detect x-rays of up to ten thousand electronvolts.

Fewer gamma-ray satellites have been launched, with notable early ones including Cos-B, launched by the European Space Research Organization in 1975, and HEAO-3, which was launched by NASA in 1979 and included a gamma-ray spectrometer. The Compton Gamma Ray Observatory (CGRO), which orbited from 1991 to 2000, carried four different but complementary gamma-ray instruments that between them covered the widest range of the electromagnetic spectrum to date: twenty thousand to thirty billion electronvolts (or thirty gigaelectronvolts). The Burst and Transient Source Experiment (BATSE) consisted of eight detectors that surveyed the entire sky for gamma-ray bursts. The Oriented Scintillation Spectrometer Experiment (OSSE) contained four scintillation units that measured the spectra of gamma rays; the units typically worked in pairs, with one observing the gamma-ray source while the other observed background radiation levels near the source. The Imaging Compton Telescope (COMPTEL) was actually two scintillators that made use of the Compton effect. Light was scattered and recorded as the gamma ray encountered the first detector, and the gamma ray was reemitted at a lower energy to be detected in the same manner at the next detector. The total energy of the photon revealed the energy of the original gamma ray, and the locations of the two "hits" pointed back toward the source. Finally, the Energetic Gamma Ray Experiment Telescope (EGRET) was designed to detect even low fluxes of the highest-energy gamma rays. A gamma ray entering the telescope would strike interleaved tantalum sheets, creating an electron-positron pair that then traveled through two spark chambers and finally into a crystal scintillator. A complex anticoincidence system discounted sparks caused by cosmic rays. The energy of the gamma ray was recorded, and its direction was revealed by the paths of the two particles it created.

In June 2008, NASA, in conjunction with the US Department of Energy and the governments of France, Germany, Italy, Japan, and Sweden, launched the Fermi Gamma-Ray Space Telescope (FGST), previously the Gamma-Ray Large Area Space Telescope (GLAST). The observatory carried two instruments, the Large Area Telescope (LAT) and the Gamma-Ray Burst Monitor (GBM). Like EGRET, the LAT operates by detecting the conversion of gamma rays into electron-positron pairs; however, instead of spark chambers, it uses silicon microstrip detectors. The LAT has a better angular resolution than EGRET—less than 3.5 degrees at one hundred million electronvolts, compared to EGRET's 5.8 degrees—and can observe energies as high as three hundred gigaelectronvolts. The GBM features twelve sodium iodide scintillators, which can detect gamma rays up to approximately one million electronvolts, and two bismuth germanate scintillators, which can detect rays as high as thirty million electronvolts. Together, these two sets of detectors provide information on the source position and spectra of gamma-ray bursts. The GBM is sensitive to a greater energy range than BATSE—ten thousand to twenty-five million electronvolts, versus twenty-five thousand to ten million—and can observe the entire sky at once, apart from the region hidden behind Earth.

Context

X-ray and gamma-ray astrophysics has become one of the most revealing disciplines in astrophysics since its development following World War II. Extraterrestrial cosmic rays and x-rays were detected in the 1800s by balloon crews who carried electrostatic instruments and cloud chambers aloft. As laboratory physics developed an understanding of nuclear fusion and how matter decays, it became obvious that x-rays and gamma rays were generated by the stars. Nevertheless, it was doubted that the flux, or total energy flow, would be great enough to be measured, at least for any star other than Earth's sun.

With the availability of captured V-2 rockets to carry instruments aloft in tests after World War II, however, some rudimentary instruments were flown, followed by larger instruments aboard unmanned balloons starting in the 1950s. It was discovered that as sensitivity increased, what could be seen became richer and more detailed. In the early 1960s, the sky appeared to be suffused with a strong background glow of x-rays. The Uhuru (meaning "freedom" in Swahili) satellite carried an all-sky survey detector that had sufficient resolution to detect more than three hundred discrete sources among the background glow. This led to the development and launch of a series of three High Energy Astrophysical Observatories. HEAO-1 and 3 carried detectors that mapped the entire sky in x-rays and gamma rays, and HEAO-3 also carried cosmic-ray detectors. HEAO-2 carried the first stellar x-ray telescope and discovered that the x-ray background was composed largely of point sources that could not be distinguished at lower resolutions. This led to the discovery of the x-ray components of known visible objects and of previously unknown objects. In many cases, what was seen in x-rays matched very nicely with the visible and radio components. In other cases, it seemed as though two different objects were being viewed.

Comparable work was done in solar physics. The Orbiting Solar Observatories (OSO) carried a number of x-ray and gamma-ray instruments in the 1960s and 1970s, and the Skylab space station in 1973–74 included x-ray spectrometers and imaging telescopes in its array of eight solar telescopes. The Solar Maximum Mission (SMM) satellite (1980–88) carried x-ray-burst detectors and gamma-ray spectrometers to measure the output of the sun.

In general, astronomers have found that the universe is far more energetic than previously believed. The Crab Nebula, the remnant of a star that exploded in 1054, was found to pulse in x-rays at the same rate as its visible pulsar, thus suggesting a very compact object. X-rays have been found emanating from the cores of quasars and most "normal" galaxies.

Gamma-ray astronomy provides a different view of the universe. Specifically, it reveals the creation and destruction of matter (properly, its conversion from energy to matter and back) in supernovas, neutron stars, pulsars, black holes, quasars, active galaxies, and other objects. Gamma-ray astronomy has confirmed that the heavier elements are created in the blast furnace of a supernova when a star self-destructs. The famous Supernova 1987A produced gamma-ray lines indicating that cobalt 56 was decaying into iron. Cobalt 56 is unstable and had to have been created shortly before the observation—that is, when the star exploded. In addition, observation of gamma rays has raised the possibility that the heart of the Milky Way might be the site of dark-matter annihilation; in 2012, instruments tentatively measured gamma rays being emitted from the galactic core at an energy of 130 gigaelectronvolts, which corresponds to the predicted interaction of two weakly interacting massive particles (WIMPs), hypothetical particles that may constitute dark matter.

The most important result of high-energy astronomy, as well as radio and infrared astronomy, is the increasing awareness that objects must be studied in terms of their total output rather than as emitters in different spectral bands. What puzzles scientists in one band may be solved in another, or at least illuminate a new line of investigation.

Principal terms

ELECTROMAGNETIC RADIATION: energy encompassing an electric element and a magnetic element that travels in the form of "rays," as distinct from radiation composed of high-speed atomic nuclei or particles; light and radio waves are the best known forms

GRAZING INCIDENCE: an event in which incoming radiation strikes a surface at an extremely shallow angle, typically less than 1 degree

SCINTILLATION: the light emitted when high-energy radiation is absorbed by matter, then reemitted at lower energies

SPECTROSCOPY: the measurement of the intensity of light at specific wavelengths (energy levels) in the spectrum

Bibliography

Cartwright, Jon. "Gamma-Ray Bending Opens New Door for Optics." Science. AAAS, 8 May 2012. Web. 18 Dec. 2013.

Hirsch, Richard F. Glimpsing the Emerging Universe: The Emergence of X-Ray Astronomy. Cambridge: Cambridge UP, 1983. Print. A history of x-ray astronomy through the High Energy Astrophysical Observatories. Written for the well-informed reader.

Kaufmann, William J., III. Universe. New York: Freeman, 1985. Print. A college-level introductory text covering the field of astronomy. Contains descriptions of astrophysical questions and their relationships.

McLean, Ian S. Electronic and Computer-Aided Astronomy: From Eyes to Electronic Sensors. Chichester: Horwood, 1989. Print. A survey of the history and applications of electronics and astronomy. Mostly a technical survey, with several chapters that provide a general history and explanation of charge-coupled devices.

Moskowitz, Clara. "Center of Attention: Space Telescope May Hone in on Heart of the Milky Way in Hunt for Dark Matter." Scientific American. Scientific Amer., 1 Oct. 2013. Web. 19 Dec. 2013.

Tucker, Wallace H. The Cosmic Inquirers: Modern Telescopes and Their Makers. Cambridge: Harvard UP, 1986. Print. Contains literate, well-written descriptions of modern observatories and the scientific questions that led to their construction.

Tucker, Wallace H. The Star Splitters: The High Energy Astronomy Observatories. Washington: NASA, 1984. Print. A history of the HEAO satellite program and an overview of its results. Includes descriptions of x-ray optics and detectors.

Weniger, Christoph. "A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope." Journal of Cosmology and Astroparticle Physics 2012.8 (2012): n. pag. Web. 19 Dec. 2013.

X-ray telescope

Close binary system

Hubble Space Telescope