The High Energy Continuum of Agns: Thermal or Non-Thermal?
The bulk of the luminosity of a radio quiet active galactic nucleus is likely to be emitted primarily in X-rays, which are partly reprocessed by cold matter to form the lower frequency emission. After more than a decade of debate about the thermal or non-thermal nature of X-rays, OSSE on the Compton satellite can now settle the issue: a steep spectrum, consistent with thermal radiation, has already been seen in the few OSSE observations of low luminosity Seyfert galaxies. After a brief review of the standard non-thermal e ± pair model and its predictions, I discuss the problems of thermal models, in particular the thermalization timescales of two-body interactions, and the apparent fine tuning needed to obtain the appropriate spectrum. I then discuss the proposed suggestions to overcome these problems, emphasizing that e ± pairs play a key role also in thermal models, since they can establish a typical temperature of order 50–100 keV, very close to what is observed by OSSE. If this temperature is typical of AGNs, then they can form the entire cosmic X-ray background from 3 to 100 keV.
Key wordsX-rays Active Galactic Nuclei X-ray background
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