Active Galactic Nucleus - Observational Characteristics

Observational Characteristics

There is no single observational signature of an AGN. The list below covers some of the historically important features that have allowed systems to be identified as AGN.

• Nuclear optical continuum emission. This is visible whenever we have a direct view of the accretion disc. Jets can also contribute to this component of the AGN emission. The optical emission has a roughly power-law dependence on wavelength.
• Nuclear infra-red emission. This is visible whenever the accretion disc and its environment are obscured by gas and dust close to the nucleus and then re-emitted ('reprocessing'). As it is thermal emission, it can be distinguished from any jet or disc-related component.
• Broad optical emission lines. These come from cold material close to the central black hole. The lines are broad because the emitting material is revolving around the black hole with high speeds, emitting photons at varying Doppler shifts.
• Narrow optical emission lines. These come from more distant cold material, and so are narrower than the broad lines.
• Radio continuum emission. This is always due to a jet. It shows a spectrum characteristic of synchrotron radiation.
• X-ray continuum emission. This can arise both from a jet and from the hot corona of the accretion disc via scattering processes: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is a `soft excess' in the X-ray emission in addition to the power-law component. The origin of the soft excess is not clear at present.
• X-ray line emission. This is a result of illumination of cold heavy elements by the X-ray continuum. Fluorescence gives rise to various emission lines, the best-known of which is the iron feature around 6.4 keV. This line may be narrow or broad: relativistically broadened iron lines can be used to study the dynamics of the accretion disc very close to the nucleus and therefore the nature of the central black hole.