Boundary-condition modification of the Novikov-Thorne accretion-disc models and velocity matching across the inner edge

The Novikov-Thorne relativistic models of accretion discs around black holes are divergent in either radial drift velocity or density (depending on whether they are matter dominated or radiation dominated) at their 'inner edges'. For the modified radiation-dominated models, we show that the radial velocity increases and the density decreases with an increase in the accretion rate M0 thus presenting consistency problems at a certain critical value of M0. However, from other considerations, it is concluded that neither an increase in the accretion rate nor a Lightman-Eardley secular instability can cause density thinning to proceed far enough to trigger a Pringle-Rees (bremsstrahlung inefficiency) thermal instability in the disc itself. (orig./WL)