Synchrotron radiation (or magnetobremsstrahlung, magneto-bremsstrahlung, magneto-brems, MBS) is radiation emitted by a charged particle traveling at near the speed of light when it is accelerated. It was discovered in a synchrotron (particle accelerator). It can be generated in magnetic fields in space. Characteristics:
Radiation from slower-moving charged particles is called cyclotron radiation.
Since synchrotron radiation is (at least somewhat) directional, often it has to be inferred by its effects, e.g., clouds that scatter it or are warmed and produce black body radiation. The synchrotron luminosity of directly-observed synchrotron radiation could be worked out by the spectrum, looking for the power law component, but when indirectly observed, more modeling is required to work it out.
Proton synchrotron radiation is that specifically generated by protons to distinguish it from the more frequent electron-generated radiation. It is found in proton accelerators and in astrophysical phenomena that accelerate protons to relativistic speeds. It is relatively rare because a proton's much larger mass makes achieving the relativistic speeds less frequent.
A contributor to the spectrum of synchrotron radiation is synchrotron self-absorption and synchrotron self-scattering. The latter refers to absorption and subsequent emission. This results in a synchrotron-specific radiative transfer, with photons generated, then absorbed/re-emitted before leaving the medium. Of the synchrotron spectrum the lower-frequency (longer-wavelength) photons are more likely to be absorbed, thus the net effect on the spectrum seen externally is a reduction at the low-frequency end.