Intrinsic disc emission and the Soft X-ray Excess in AGN

TL;DR

This paper presents a self-consistent model explaining the soft X-ray excess in AGN as intrinsic disc emission modified by Compton scattering, supported by spectral fits to various Seyfert galaxies and implemented in XSPEC.

Contribution

It introduces a novel energetically self-consistent model linking disc emission, Comptonisation, and corona effects, available in XSPEC for broader application.

Findings

The soft X-ray excess can originate from intrinsic disc emission with Compton upscattering.

The model successfully fits spectra of different AGN types across Eddington ratios.

Spectral sequences correlate with Eddington ratio, supporting the model's physical basis.

Abstract

(Abridged) Narrow Line Seyfert 1 (NLS1) galaxies have low mass black holes and mass accretion rates close to (or exceeding) Eddington, so a standard blackbody accretion disc should peak in the EUV. However, the lack of true absorption opacity in the disc means that the emission is better approximated by a colour temperature corrected blackbody, and this colour temperature correction is large enough ($\sim 2.4$) that the bare disc emission from a zero spin black hole can extend into the soft X-ray bandpass. Part of the soft X-ray excess seen in these objects must be intrinsic emission from the disc unless the vertical structure is very different to that predicted. However, the soft excess is much broader than predicted by a bare disc spectrum, indicating some Compton upscattering by cool, optically thick material. We associate this with the disc itself, so it must ultimately be powered by mass accretion. We build an energetically self consistent model assuming that the emission thermalises at large radii, but that at smaller radii the gravitational energy is split between powering optically thick Comptonised disc emission (forming the soft X-ray excess) and an optically thin corona above the disc (forming the tail to higher energies). We show examples of this model fit to the extreme NLS1 REJ1034+396, and to the much lower Eddington fraction Broad Line Seyfert 1 PG1048+231. We use these to guide our fits and interpretations of three template spectra made from co-adding multiple sources to track out a sequence of AGN spectra as a function of $L/L_{Edd}$. The new model is publically available within the {\sc xspec} spectral fitting package.