Synchrotron radiation

The synchrotron radiation of relativistic and ultrarelativistic electrons is the process which dominates high energy astrophysics. It is the radiation emitted by very high energy electrons gyrating in a magnetic field. It was originally observed in early betatron experiments, in which electrons were first accelerated to ultrarelativistic energies. This same mechanism is responsible for the radio emission from the Galaxy, from supernova remnants and extragalactic radio sources. It is also responsible for the non-thermal optical emission observed in the Crab Nebula and possibly for the optical and X-ray continuum emission of quasars. The reasons for these assertions will become apparent in the course of this chapter.

The word non-thermal is used frequently in high energy astrophysics to describe the emission of high energy particles. I find this an unfortunate terminology, since all emission mechanisms are ‘thermal’ in some sense. The word is conventionally taken to mean ‘continuum radiation from particles, the energy spectrum of which is not Maxwellian’. In practice, continuum emission is often referred to as ‘nonthermal’ if it cannot be accounted for by the spectrum of thermal bremsstrahlung or black-body radiation.

It is a very major undertaking to work out properly all the properties of synchrotron radiation, and that is beyond the scope of this book. For details, I refer the enthusiast to the books by Bekefi (1966), Pacholczyk (1970) and Rybicki and Lightman (1979) and the three review articles by Ginzburg and his colleagues (see the References section for this chapter).