The X-ray spectrum of a disk illuminated by ions
  The X-ray spectrum from a cool disk embedded in an ion supported torus is computed. The interaction of the hot ions with the disk increases the hard X-ray luminosity of the system}. A surface layer of the disk is heated by the protons from the torus. The Comptonized spectrum produced by this layer has a shape that depends only weakly on the incident energy flux and the distance from the accreting compact object. It consists of a `blue bump' of unComptonized soft photons and a flat high energy tail, reminiscent of the observed spectra. The hard tail becomes flatter as the thermalization depth in the cool disk is increased. Further evidence for ion illumination are the Li abundance in the secondaries of low mass X-ray binaries and the 450 keV lines sometimes seen in black-hole transient spectra. 
H.C. Spruit and F. Haardt