Spectral Tests of Models for Accretion Disks Around Black Holes
  A critical review is presented of the current status of our understanding of the the spectral output of thermally-radiating accretion disks around black holes, with special attention to those around supermassive black holes in active galactic nuclei. Uncertainties in disk physics (e.g. the nature of the inter-ring stresses, the distribution of dissipation with altitude) contribute to uncertainties in the output. So, too, do inadequacies in calculations (e.g. incomplete treatment of non-LTE effects, neglect of transitions in elements heavier than He, difficulty of coupling Comptonization to genuine transfer). Existing models do not yet adequately explain the absence of Lyman edge features, or the observed flux in the EUV. 
J.H. Krolik