7.5 Emission from spiralling electrons: synchrotron radiation
In the very first reading (Activity 1) we encountered the term ‘non-thermal’ describing the spectrum of light emitted from AGN. In this subsection we will learn more about the most important type of non-thermal radiation: synchrotron emission.
When a charged particle moves in the presence of a magnetic field it experiences a Lorentz force, which produces an acceleration whose direction is perpendicular to both the magnetic field line and the velocity of the particle, 
. Consequently the path followed by the electron is a helix, as shown in Figure 12. The radius of the circle described in the plane perpendicular to the magnetic field line is known as the cyclotron radius and is also sometimes referred to as the Larmor radius, rL or the gyroradius. It is given by
where |q| is the magnitude of the charge of the particle, m is its mass, B is the magnetic field strength, and 
is the component of the particle's velocity which is perpendicular to the field.
Any accelerated charged particle produces electromagnetic radiation, and the radiation produced in the case of electrons accelerated by a magnetic field is known as cyclotron radiation. When the electrons in question are high-energy relativistic electrons (i.e. when the electrons have speeds approaching c), the cyclotron radiation is known as synchrotron radiation. In most cases of astrophysical importance, synchrotron radiation is the most plentiful type of non-thermal radiation.
SAQ 4
Question: What two ingredients are required for synchrotron emission to occur?
Answer
Magnetic fields and relativistically moving electrons.