Editing Radio galaxy (section)

== Emission processes ==

The [[radio waves|radio emission]] from radio-loud active galaxies is [[synchrotron radiation|synchrotron emission]], as inferred from its very smooth, broad-band nature and strong [[Polarization (waves)|polarization]]. This implies that the radio-emitting [[Plasma (physics)|plasma]] contains, at least, [[electron]]s with [[special relativity|relativistic]] speeds ([[Lorentz factor]]s of ~10<sup>4</sup>) and [[magnetic field]]s. Since the plasma must be neutral, it must also contain either [[proton]]s or [[positron]]s. 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.<ref>{{cite journal |last=Burbidge  |first=G |date=1956 |title=On synchrotron radiation from Messier 87 |journal=Astrophysical Journal |volume=124 |pages=416 |doi=10.1086/146237 |bibcode=1956ApJ...124..416B|doi-access=free }}</ref>

A sister process to the synchrotron radiation is the [[Compton scattering|inverse-Compton]] process, in which the relativistic electrons interact with ambient photons and [[Thomson scattering|Thomson scatter]] them to high energies. Inverse-Compton emission from radio-loud sources turns out to be particularly important in X-rays,<ref>{{cite journal |display-authors=4 |author=Croston JH |author2=Hardcastle MJ |author3=Harris DE|author4=Belsole E |author5=Birkinshaw M|author6=Worrall DM |date=2005 |title=An X-ray study of magnetic field strengths and particle content in FRII radio sources |journal=Astrophysical Journal |volume=626 |issue=2 |pages=733–47 |arxiv=astro-ph/0503203 |doi=10.1086/430170|bibcode = 2005ApJ...626..733C |s2cid=10241874 }}</ref> 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 that are detected in the radio wavelengths may also be seen in the [[infrared]], [[visible spectrum|optical]], [[ultraviolet]] or even [[X-ray]]. In the latter case the responsible electrons must have energies in excess of 1 [[electronvolt|TeV]] in typical magnetic field strengths. Again, polarization and continuum spectrum are used to distinguish the synchrotron radiation from other emission processes. [[astrophysical jets|Jet]]s and hotspots are the usual sources of high-frequency synchrotron emission. It is hard to distinguish observationally between the synchrotron and inverse-Compton radiation, making them a subject of ongoing research.

Processes, collectively known as particle acceleration, produce populations of relativistic and non-thermal particles that give rise to synchrotron and inverse-Compton radiation. [[Fermi acceleration]] is one plausible particle acceleration process in radio-loud active galaxies.