A paradigm revisited: the accretion disc in AGNs and quasars

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Abstract
A statistical comparison of model spectra of a thin bare accretion disc plus power-law and observations of over 400 active galactic nuclei (AGNs) and quasars are presented. This comparison is not limited to the optical/ultraviolet region of the observed spectra, as has often been the case in the past, but is extended over the entire ∼ 4 eV to ∼ 2 keV energy range. This is achieved by identifying five observable (and model-independent) parameters which cover this energy range and comparing these with values of the parameters obtained from models. These comparisons demonstrate that although the shape and the luminosity of the ultraviolet continuum of AGNs and quasars are similar to the model disc spectra, the continuum energy distribution differs significantly from the model spectra in the ∼ 11 eV to ∼ 0.2 keV energy region. In this energy range the continuum of AGNs and in particular that of quasars is enhanced relative to the disc spectrum. This enhancement is very similar to the ‘ultrasoft excess’ detected in some AGNs, and it is very likely that this ultrasoft excess is common to all AGNs and quasars. A very strong correlation is found between the ultraviolet luminosity and the luminosity of soft X-rays integrated from 0.2 to 2 keV. This correlation is very similar to the correlation between the ultraviolet luminosity and the X-ray luminosity at 2 keV, and this suggests that the shape of the ‘big blue bump’ (from ∼ 4 eV to ∼ 2 keV) is independent of redshift and luminosity. It is very likely that the ‘prime mover’ in the short term variability of the continuum is the hard X-ray emission and the ‘big blue bump’ rides on this power-law. This provides a natural explanation for the near simultaneous variability in the continuum bands in the optical/ultraviolet and X-ray regions.

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