Synchrotron radiation is named after the particle accelerators developed in the 1930's and 1940's to produce high-energy electrons. In 1950 Hannes Alfven, Nicolai Herlofson, and Karl Keipenheuer brought this form of plasma radiation to astronomers' attention. Alfven, who later won a Nobel prize in physics for his solar studies, proposed that streams of electrons move at nearly the speed of light along magnetic-field lines not only in Earth's magnetosphere and above the Sun, but also throughout the cosmos.
Electromagnetic waves propagated in cosmic space derive from a variety of mechanisms. The major contribution in the optical region of the spectrum is from radiation resulting from bound–bound electron transitions between discrete atomic or molecular states, free–bound transitions during recombination, and free–free transitions in the continuum. In the latter case, when for transitions between levels, radiation classified as bremsstrahlung results from the acceleration of electrons traveling in the vicinity of the atom or ion.
In addition, there are other mechanisms of considerable importance operating in the radio region. In particular, there are noncoherent and coherent mechanisms connected with the existence of sufficiently dense plasmas which are responsible for radiation derived from plasma oscillations, such as the sporadic solar radio emissions. This radiation cannot be attributed to the motion of individual electrons in a vacuum but is due to the collective motion of electrons at the plasma frequency. This often occurs in cosmic plasma when electron beams propagate through a neutralizing plasma background.
When a plasma is subjected to a magnetic field there is yet another mechanism which plays an extremely important role in radio astronomy. The frequency and angular distribution of the radiation from free electrons moving in the presence of a magnetic field undergoes dramatic changes as the electron energy is increased from nonrelativistic to extreme relativistic energies. Essentially three types of spectra are found. Names such as cyclotron emission and magnetobremsstrahlung are used to describe the emission from nonrelativistic and mildly relativistic electron energies, whereas the name synchrotron radiation is traditionally reserved for highly relativistic electrons because it was first observed in 1948 in electron synchrotrons.
Synchrotron radiation is characterized by a generation of frequencies appreciably higher than the cyclotron frequency of electrons (or positrons) in a magnetic field, a continuous spectra whose intensity decreases with frequency beyond a certain critical frequency, highly directed beam energies, and polarized electromagnetic wave vectors.
In astrophysics, nonthermal (nonequilibrium) cosmic radio emission is, in a majority of cases, synchrotron radiation. This is true for general galactic radio emission, radio emission from the envelopes of supernovae, and radio emission from double radio galaxies and quasars (continuum spectra). Synchrotron radiation also appears at times as sporadic radio emission from the sun, as well as from Jupiter. In addition, optical synchrotron radiation is observed in some instances (Crab nebula, the radiogalaxy and “jet” in M87–NGC 4486, M82, and others). This apparently is also related to the continuous optical spectrum sometimes observed in solar flares. Synchrotron radiation in the X ray region can also be expected in several cases, particularly from the Crab nebula.
When cosmic radio or optical emission has the characteristics of synchrotron radiation, a determination of the spectrum makes possible a calculation of the concentration and energy spectrum of the relativistic electrons in the emission sources. Therefore, the question of cosmic synchrotron radiation is closely connected with the physics and origin of cosmic rays and with gamma- and X ray astronomy.
Synchrotron radiation was first brought to the attention of astronomers by H. Alfvén; and N. Herlofson in 1950, a remarkable suggestion at a time when plasma and magnetic fields were thought to have little, if anything, to do in a cosmos filled with “island” universes (galaxies). The recognition that this mechanism of radiation is important in astronomical sources has been one of the most fruitful developments in astrophysics. For example, it has made possible the inference that high-energy particles exist in many types of astronomical objects, it has given additional evidence for the existence of extensive magnetic fields, and it has indicated that enormous amounts of energy may indeed be converted, stored, and released in cosmic plasma.