Testing the ionized disc reprocessing model for the soft X-ray emission of quasars

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Abstract
One of the current explanations for the soft X-ray emission of active galactic nuclei (AGN) is reprocessing of the hard X-rays by partially ionized, optically thick matter. This idea is very appealing because it would explain the shape of the AGN soft X-ray spectrum in terms of atomic physics. While at present the reflection model correctly describes the soft X-ray spectra of a few low-luminosity Seyfert galaxies, it is not clear whether or not it can be applied to higher luminosity quasars. To investigate this issue quantitatively, we have fitted the high signal-to-noise ratio Position Sensitive Proportional Counter (PSPC) spectra of 11 AGN of different luminosities with a model consisting of a direct hard X-ray component, reflection from the ionized surface of an accretion disc and the direct thermal emission of the disc. We find that the AGN with an acceptable fit are a minority, and all have a low optical (and bolometric) luminosity, flat PSPC energy index αPSPC and the flattest αox of the sample, while those with the worst fit all have high optical (and bolometric) luminosity, steep αPSPC and the steepest αox of the sample. We conclude that either the reprocessing model is not correct and the form of the soft X-ray spectrum of AGN cannot be simply explained in terms of characteristic atomic features (i.e. highly ionized oxygen K-edges), or the origin of the soft X-ray emission of AGN is not ‘universal’.

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