This article concerns the physical phenomenon of synchrotron radiation. For details on the production of this radiation and applications in laboratories, see Synchrotron light source. Synchrotron radiation is
electromagnetic radiation, similar to
cyclotron radiation, but generated by the acceleration of
ultrarelativistic (i.e., moving near the
speed of light) charged particles through magnetic fields.
This may be achieved artificially in
synchrotrons or
storage rings, or naturally by fast electrons moving through magnetic fields in space.
The radiation produced may range over the entire electromagnetic spectrum, from
radio waves to
infrared light,
visible light,
ultraviolet light,
X-rays, and
gamma rays.
It is distinguished by its characteristic
polarization and
spectrum.
History
General Electric
synchrotron accelerator built in 1946, the origin of the discovery of synchrotron radiation. The arrow indicates the evidence of
arcing.
The radiation was named after its discovery in a General Electric synchrotron accelerator built in 1946 and announced in May 1947 by Frank Elder, Anatole Gurewitsch, Robert Langmuir, and Herb Pollock in a letter entitled "Radiation from Electrons in a Synchrotron". Pollock recounts:
"On April 24, Langmuir and I were running the machine and as usual were trying to push the electron gun and its associated pulse transformer to the limit. Some intermittent sparking had occurred and we asked the technician to observe with a mirror around the protective concrete wall. He immediately signaled to turn off the synchrotron as "he saw an arc in the tube." The vacuum was still excellent, so Langmuir and I came to the end of the wall and observed. At first we thought it might be due to
Cherenkov radiation, but it soon became clearer that we were seeing
Ivanenko and
Pomeranchuk radiation."
Emission mechanism
When high-energy relativistic electrons are forced to travel in a curved path by a
magnetic field, synchrotron radiation is produced, similar to a radio antenna, but with the difference that the relativistic speed changes the observed frequency due to the Doppler effect by the
Lorentz factor,
.
Relativistic
time contraction then bumps the frequency observed in the lab by another factor of
, thus multiplying the GeV frequency of the resonant cavity that accelerates the electrons into the X-ray range.
Another dramatic effect of
relativity is that the radiation pattern is distorted from the isotropic dipole pattern expected from non-relativistic theory into an extremely forward-pointing cone of radiation.
This makes artificial synchrotron radiation the brightest known source of X-rays.
The planar acceleration geometry makes the radiation linearly polarized when observed in the orbital plane, and circularly polarized when observed at a small angle to that plane.
Synchrotron radiation from accelerators
Synchrotron radiation may occur in accelerators either as a nuisance, causing undesired energy loss in
particle physics contexts, or as a
deliberately produced radiation source for numerous laboratory applications.
Electrons are accelerated to high speeds in several stages to achieve a final energy that is typically in the GeV range.
Synchrotron radiation in astronomy
M87's Energetic Jet.,
HST image.
The blue light from the jet emerging from the bright
AGN core, towards the lower right, is due to synchrotron radiation.
Synchrotron radiation is also generated by astronomical objects, typically where relativistic electrons spiral (and hence change velocity) through magnetic fields.
Two of its characteristics include non-thermal
power-law spectra, and
polarization.
History
It was first detected in a jet emitted by
M87 in 1956 by
Geoffrey R. Burbidge , who saw it as confirmation of a prediction by
Iosif S. Shklovskii in 1953, but it had been predicted several years earlier by
Hannes Alfvén and Nicolai Herlofson in 1950.
T. K. Breus noted that questions of priority on the history of astrophysical synchrotron radiation is quite complicated, writing:
"In particular, the Russian physicist
V.L. Ginsburg broke his relationships with
I.S. Shklovsky and did not speak with him for 18 years. In the West,
Thomas Gold and Sir
Fred Hoyle were in dispute with
H. Alfven and N. Herlofson, while K.O. Kiepenheuer and G. Hutchinson were ignored by them."
s have been suggested for producing synchrotron radiation, by
relativistic beaming of jets produced by gravitationally accelerating ions through magnetic fields.
Pulsar wind nebulae
A class of astronomical sources where synchrotron emission is important is the
pulsar wind nebulas, or
plerions, of which the
Crab nebula and its associated
pulsar are archetypal.
Pulsed emission gamma-ray radiation from the Crab has recently been observed up to ≥25 GeV, probably due to synchrotron emission by electrons trapped in the strong magnetic field around the pulsar.
Polarization in the Crab at energies from 0.1 to 1.0 MeV illustrates a typical synchrotron radiation.
See also
