Where does the soft X-ray excess come from?
Papers commonly model this as blackbody emission with energies of a few hundred eV, although acknowledging that the accretion disk could not produce such high temperature emission. It’s been hypothesized that this emission could be the result of Comptonization of lower-energy photons by a higher energy `corona’ above the accretion disk. It’s also been suggested that the shape of the soft X-ray excess may be significantly affected by absorption (which may account for its variability?; astro-ph/0407045).
