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

Understand

(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.