Radio Galaxy - Emission Processes

Emission Processes

The radio emission from radio-loud active galaxies is synchrotron emission, as inferred from its very smooth, broad-band nature and strong polarization. This implies that the radio-emitting plasma contains, at least, electrons with relativistic speeds (Lorentz factors of ~104) and magnetic fields. Since the plasma must be neutral, it must also contain either protons or positrons. There is no way of determining the particle content directly from observations of synchrotron radiation. Moreover, there is no way to determine the energy densities in particles and magnetic fields from observation: the same synchrotron emissivity may be a result of a few electrons and a strong field, or a weak field and many electrons, or something in between. It is possible to determine a minimum energy condition which is the minimum energy density that a region with a given emissivity can have, but for many years there was no particular reason to believe that the true energies were anywhere near the minimum energies.

A sister process to synchrotron radiation is the inverse-Compton process, in which the relativistic electrons interact with ambient photons and Thomson scatter them to high energies. Inverse-Compton emission from radio-loud sources turns out to be particularly important in X-rays, and, because it depends only on the density of electrons, a detection of inverse-Compton scattering allows a somewhat model-dependent estimate of the energy densities in the particles and magnetic fields. This has been used to argue that many powerful sources are actually quite near the minimum-energy condition.

Synchrotron radiation is not confined to radio wavelengths: if the radio source can accelerate particles to high enough energies, features which are detected in the radio may also be seen in the infrared, optical, ultraviolet or even X-ray, though in the latter case the electrons responsible must have energies in excess of 1 TeV in typical magnetic field strengths. Again, polarization and continuum spectrum are used to distinguish synchrotron radiation from other emission processes. Jets and hotspots are the usual sources of high-frequency synchrotron emission. It is hard to distinguish observationally between synchrotron and inverse-Compton radiation, and there is ongoing disagreement about what processes we are seeing in some objects, particularly in the X-ray.

The process(es) that produce the population of relativistic, non-thermal particles that give rise to synchrotron and inverse-Compton radiation are collectively known as particle acceleration. Fermi acceleration is one plausible particle acceleration process in radio-loud active galaxies.