Continuum radiation from active galactic nuclei

Abstract

Active galactic nuclei (AGN) can be divided into two broad classes, where the emitted continuum power is dominated either by thermal emission (radio-quiet AGN), or by nonthermal emission (blazars). Emission in the 0.01–1 μm range is the primary contributor to the bolometric luminosity and is probably produced through thermal emission from an accretion disk, modified by electron scattering and general relativistic effects. The 1–1000 μm continuum, the second most important contributor to the power, is generally dominated by thermal emission from dust with a range of temperatures from ∼ 40 K to ∼ 1000–2000 K. The dust is probably reemitting 0.01–0.3 μm continuum emission, previously absorbed in an obscuring cone (or torus) or an extended disk. The 1–10 keV X-ray emission is rapidly variable and originates in a small region. This emission may be produced through Compton scattering by hot thermal electrons surrounding an accretion disk, although the observations are far from being definitive. The weak radio emission, which is due to the nonthermal synchrotron process, is usually elongated in the shape of jets and lobes (a core may be present too), and is morphologically distinct from the radio emission of starburst galaxies.