The X-ray spectrum of a disc illuminated by ions

The X-ray spectrum from a cool disc embedded in an ion-supported torus is c omputed. The interaction of the hot ions with the disc increases the hard X -ray luminosity of the system. A surface layer of the disc is heated by the protons from the torus. The Comptonized spectrum produced by this layer ha s a shape that depends only weakly on the incident energy flux and the dist ance from the accreting compact object. It consists of a 'blue bump' of un- Comptonized soft photons and a flat high-energy tail, reminiscent of the ob served spectra. The hard tail becomes flatter as the thermalization depth i n the cool disc is increased. Further evidence for ion illumination is the Li abundance in the secondaries of low-mass X-ray binaries and the 450-keV lines sometimes seen in black hole transient spectra.