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Continuum Emission
Just like visible light, with its range of energies from red to blue,
X-rays have a continuum, or a range of energies associated with it.
X-rays usually range in energy from around 0.5 keV up to around 1000 keV.
Like line emission, continuum X-ray emission involves charged particles.
Continuum emission is a result of the acceleration of a population of
charged particles.
All X-ray sources contain such particles. These particles must be
at least partially ionized - their electrons need to be unbound from their
nuclei to be free to zip around when they are heated to
extreme temperatures. For an electron
to radiate X-rays, the gas containing the electron
must have extreme conditions, such as temperatures of millions of degrees,
super-strong magnetic fields, or the electrons themselves must be moving
at nearly the speed of light. Extreme conditions
can be found in disks of matter orbiting black holes or in supernova remnants.
Strong magnetic fields, like those created in the wake of a supernova
explosion, can also accelerate fast moving ions in spirals around the
field lines to the point of X-ray emission. Electrons can be accelerated
to nearly the speed of light in the shockwave created by a supernova explosion.
There are three mechanisms that will produce continuum X-ray emission.
They are Synchrotron Radiation, Bremsstrahlung, and Compton
Scattering. Because the populations of electrons have a continuous
range of energies, and they can be accelerated through a range of energies,
the radiation produced is continuous, and not at the discreet energies of
line emission.
courtesy of University of Hertfordshire |
Sychrotron radiation is emitted when a fast electron
interacts with a magnetic field. A magnetic field in an area an
electron is traveling in will cause the electron to change direction by
exerting a force on it perpendicular to the direction the electron is moving.
As a result, the electron will be accelerated, causing it to radiate
electromagnetic energy. This is called magnetic
bremsstrahlung or synchrotron radiation (after radiation
observed from particle accelerators by that name). If the electrons and the
magnetic field are energetic enough, the emitted radiation can be in the form
of X-rays.
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Bremsstrahlung occurs when an electron passes close to a
positive ion, and the strong electric forces cause its trajectory to
change. The acceleration of the electron in this way causes it to
radiate electromagnetic energy - this radiation is called
bremsstrahlung, (literally, from the German meaning 'braking
radiation'). Thermal bremsstrahlung occurs in a hot gas, where many
electrons are stripped from their nuclei, leaving a population of
electrons and positive ions. If the gas is hot enough (millions of
degrees Kelvin), this kind of radiation will primarily take the form
of X-rays. |
courtesy of University of
Hertfordshire |
courtesy of University of Hertfordshire |
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Comptonization is when a photon collides with an electron - the photon
will either give up energy to or gain energy from the electron,
changing the electron's velocity as a result.
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What are some Examples of This in Action?
Gas that is at about 10 million to 100 million degrees, such as the gas
heated by a supernova explosion, produces most of its emission in X-rays
from thermal Bremsstrahlung. Gas can be heated to these temperatures by
the outward moving shock of a supernova explosion, or in an accretion
disk around a black hole or neutron star. Synchrotron
radiation can produce X-rays around supernova remnants (SNR), where the
magnetic fields are strong and ions have been accelerated by the shock
wave to high energies. X-rays produced by SNR require electrons with
energies of about 104 GeV each
(you would have to heat
an electron to a temperature of about ten trillion degrees for it to have
this much energy)! Synchrotron radiation and Compton scattered radiation
are major components of the diffuse X-ray background and emission from
active galaxies.
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