'Cold' material in non-thermal sources

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Abstract
Accretion-driven compact sources of non-thermal radiation, such as active galactic nuclei and quasars, must contain significant amounts of gas in their central continuum-emitting regions unless they are well below the Eddington limit and very efficient. This gas has to be included in any model for the radiation spectrum of these sources as it is capable of reprocessing a large fraction of the energy. Despite being exposed to an intense flux of hard radiation, it would be dense enough for radiative cooling to maintain it at ≲ 105 K. Even if the underlying mechanism for converting gravitational energy into radiation produces a simple power-law spectrum, the radiation which escapes the source is likely to have a much more complicated form due to reprocessing in the ‘cold’ gas. Photoelectric absorption will affect the spectrum in the soft and medium band, and Comptonization will produce a spectral feature in the 100 keV to 1 MeV region. Also the consequent heating of the gas enhances the far-UV and soft X-ray luminosity.

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