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X-ray source
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What is an X-ray source in astronomy?
In astronomy, an X-ray source is any cosmic object emitting radiation at X-ray wavelengths. Since Earth's atmosphere absorbs X-rays, telescopes and detectors must be placed in space to observe them.
Most stars emit a tiny fraction of their energy as X-rays. Supernova remnants and binary stars with a black hole are more powerful sources within the Milky Way. Other galaxies, such as radio galaxies and quasars, also emit X-rays from the activity at their cores, thought to be caused by hot gases surrounding a central black hole. Galaxy clusters emit X-rays from hot gas between galaxies, heated by supernovas. There is also a diffuse X-ray background from distant active galaxies.
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How do X-ray sources produce X-rays in space?
X-ray sources in space produce X-rays through various mechanisms, including the acceleration of charged particles, atomic transitions between discrete energy levels, and radioactive decay of atomic nuclei.
- Acceleration of charged particles In an X-ray tube, high-energy electrons bombard a solid target, causing them to decelerate abruptly. This deceleration results in the emission of bremsstrahlung, a continuous spectrum of electromagnetic radiation with a peak intensity in the X-ray region. Synchrotron particle accelerators and storage rings can also produce a continuum of X-rays.
- Atomic transitions When fast-moving electrons collide with target atoms, inner-shell electrons may be ejected, and outer-shell electrons fall into the inner shell to fill the vacancy. This process emits a photon with energy corresponding to the X-ray region of the spectrum.
- Binary star systems Some binary star systems emit copious X-rays, with the strongest sources producing over 1,000 times the entire energy output of the Sun in the X-ray region alone.
Supernova remnants are also strong sources of X-rays, sometimes associated with synchrotron radiation or atomic emissions from extremely hot gases.
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What kinds of objects in space can be X-ray sources?
X-ray sources in space include various objects that emit radiation at X-ray wavelengths. These objects are studied using X-ray telescopes and detectors carried by spacecraft.
Examples of cosmic X-ray sources:
* Stars The Sun was the first celestial object found to emit X-rays. Young, massive stars are the most powerful X-ray emitters.
* Supernova remnants Gaseous shells ejected during a star's explosion can emit X-rays from hot gas. The Crab Nebula is one example.
* Binary stars Certain binary stars, called X-ray binaries, are powerful X-ray sources. These systems often involve a compact object like a neutron star or black hole.
* Active galaxies Galaxies with violent activity at their cores, such as radio galaxies and quasars, emit variable X-ray energy.
* Galaxy clusters X-rays from galaxy clusters come from hot gas between the galaxies.5 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do astronomers detect and study X-ray sources?
Astronomers detect and study X-ray sources using specialized X-ray telescopes that must be positioned at high altitudes via rockets, balloons, or spacecraft due to atmospheric absorption of X-rays.
Unlike conventional optical telescopes, X-ray telescopes use grazing incidence to capture X-rays, bouncing them off mirrors at very low angles to focus them into an image. X-ray detectors like Geiger counters, proportional counters, and scintillation counters are utilized, requiring large collecting areas and high efficiency to detect weak X-ray emissions against cosmic-ray-induced background radiation. Spectral analysis of the radiation helps identify emission processes.
The Chandra X-Ray Observatory and XMM-Newton are examples of X-ray satellites that have contributed to discoveries about black holes, stars, galaxies, and supernova remnants.
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Why can't we observe X-ray sources from the ground without special equipment?
X-ray astronomy requires special equipment because Earth's atmosphere absorbs most X-rays. Therefore, X-ray telescopes and detectors must be taken to high altitudes via rockets or balloons, or placed in orbit outside the atmosphere.
X-ray telescopes differ significantly from conventional optical telescopes because X-ray photons possess high energy and would pass through a standard reflector's mirror. To capture X-rays, they must be bounced off a mirror at a very low angle, a technique known as grazing incidence.
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How have discoveries about X-ray sources helped us understand the universe?
Discoveries about X-ray sources have significantly advanced our understanding of the universe. X-ray astronomy involves studying astronomical objects emitting radiation at X-ray wavelengths.
X-ray telescopes, such as the Chandra X-Ray Observatory and XMM-Newton, have enabled numerous discoveries about black holes, the evolution of stars and galaxies, and the nature of supernova remnants. For example, the cosmic X-ray background, once puzzling, was found to originate from discrete sources like distant galaxies with massive black holes. Also, X-ray observations of binary star system Cygnus X-1 offered key evidence supporting the existence of black holes. Pulsars emit X-rays, giving insights into neutron stars and their environments.
These observations contribute to our knowledge of the cosmos.
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X-ray source, in astronomy, any of a class of cosmic objects that emit radiation at X-ray wavelength. Because the Earth’s atmosphere absorbs X rays very efficiently, X-ray telescopes and detectors must be carried high above it by spacecraft to observe objects that produce such electromagnetic radiation.
A brief treatment of astronomical X-ray sources follows. For full treatment, see Cosmos.
Advances in instrumentation and improved observational techniques have led to the discovery of an increasing number of X-ray sources. By the late 20th century, thousands of these objects had been detected throughout the universe.
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The Sun was the first celestial object determined to give off X rays; rocket-borne radiation counters measured X-ray emissions from its corona (outer atmosphere) in 1949. The Sun, however, is an intrinsically weak X-ray source, and it is prominent only because it is so close to the Earth. The unambiguous detection of X rays from other more distant ordinary stars was achieved 30 years later by the orbiting HEAO 2 satellite known as the Einstein Observatory. It detected more than 150 ordinary stars by the X-radiation from their coronas. The stars observed cover almost the entire range of star-types—main sequence, red giants, and white dwarfs. Most stars emit only an extremely small fraction of their energy in the form of X rays. Young, massive stars are the most powerful X-ray emitters. They usually occur in nebulas, and their hot coronal gases can expand to make a nebula itself a detectable X-ray source.
A more powerful type of X-ray source is a supernova remnant, the gaseous shell ejected during the violent explosion of a dying star. The first to be observed was the Crab Nebula, the remnant of a supernova explosion whose radiation reached the Earth in ad 1054. It is, however, a very atypical remnant because its X rays are synchrotron radiation produced by high-speed electrons from a central pulsar. The X-radiation from most other supernova remnants emanates instead from hot gas. The gases ejected by a supernova explosion are relatively cool, but as they sweep outward at a speed of several thousand kilometres per second they accumulate interstellar gas. The strong shock wave heats this gas to a temperature high enough for X-ray emission—namely, about 10,000,000 K.
The most powerful X-ray sources in the Milky Way Galaxy are certain binary stars. These so-called X-ray binaries have an X-ray output 1,000 times as great as the Sun’s output at all wavelengths. X-ray binaries account for most of the sources discovered during the initial years of X-ray astronomy, including Scorpius X-1. A typical X-ray binary source consists of a close double star system in which one member is a very compact object. This object may be a neutron star that contains approximately the mass of two Suns condensed into a sphere only about 20 km (12 mi) across, or alternatively an even more compact black hole, a collapsed star whose gravity is so strong that not even light can escape from it. As gas from the companion star falls toward the compact star, the latter swirls round into an accretion disk. Viscous processes in the disk convert the orbital energy of the gas into heat, and when sufficiently high temperatures are attained large amounts of X rays are emitted.
There are several types of X-ray binaries. In an X-ray pulsar, the gas is channeled to the poles of a neutron star and the radiation is given off as pulses in very regular periods. In objects known as bursters, a neutron star’s magnetic field suspends the gas until the accumulated weight crushes the field temporarily and the falling gas emits a sudden burst of X rays. A transient occurs in stellar pairs in which the orbit is elongated and gas is only transferred occasionally (i.e., when the component stars are closest together). Astronomers generally classify the compact object in an X-ray binary as a neutron star unless its calculated mass exceeds three solar masses. In such cases, they identify the object as a black hole. Two very strong black hole candidates are Cygnus X-1 (nine solar masses) and LMC X-3 (seven solar masses).
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Nearby galaxies (e.g., the Andromeda Galaxy) are detected by the emission from constituent X-ray binaries. They are relatively weak sources compared to active galaxies, which fall into various categories such as radio galaxies, Seyfert galaxies, and quasars. These galactic types are all characterized by violent activity at their cores, usually explained as arising from an accretion disk of hot gases that surrounds a central black hole having a mass of about 1,000,000,000 Suns. The X-ray energy of these galaxies is highly variable. The quasar OX 169, for example, has been observed to vary substantially in X-ray output in less than two hours, implying that the region producing this radiation is less than two “light-hours” across (i.e., smaller than the solar system).
Other powerful extragalactic X-ray sources are galaxy clusters. The X rays from a cluster do not come from its member galaxies but rather from a pool of hot gas between them, which is kept within the cluster by the galaxies’ combined gravitational pull. The gas is typically at a temperature of 100,000,000 K, and it may have originated as hot gas ejected by numerous supernovas.
Finally, there is a diffuse background of X-radiation emanating from great distances and from all directions. Although it was discovered in 1962, its nature was not finally resolved until 2000. The background consists mainly of X rays from numerous active galaxies.