The effects of thermal accretion disk spectra on the emission lines from active galactic nuclei

Abstract

The effects of thermal accretion disk spectra on emission-line production in active galactic nuclei are examined with a view to using these lines as diagnostics of the continuum shape in the extreme ultraviolet. Although changes in ionization parameter and pressure can mimic changes in continuum shape, there are ways to resolve the ambiguity. Two generic methods are discussed, one using global "goodness-of-fit" parameters which measure how well a given photoionization model predicts the entire list of observed line fluxes, the other focusing on combinations of small numbers of lines particularly sensitive to continuum shape. We find that the first method is potentially useful, but the second is very difficult to employ for a variety of technical reasons. We apply the "goodness-of-fit" technique to a compilation of line ratios representing a "typical" quasar spectrum. It weakly favors a spectrum composed of a broken power law added to an accretion disk spectrum of equal power and characteristic temperature 10 eV over two comparison spectra, one a pure broken power law, the other a broken power law added to a hot (80 eV) disk spectrum.