German Title: Zeitabhängige Akkretionsflüsse auf Schwarze Löcher

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Translation of abstract (English)

Accretion onto compact objects plays a fundamental role in a number of astrophysical systems like active galactic nuclei (AGN) and black hole X-ray binaries (BHXB). The standard thin disk model fails to reproduce the spectra of these systems. They exhibit a blackbody-like component, generally attributed to the cold standard disk, and a hard power law-like component extending to several 100 keV, which cannot be accounted for by the standard disk. This situation can be remedied by assuming that the accretion flow consists not only of the cold standard disk, but also of a hot optically thin plasma in the immediate vicinity. This hot flow cools through advection rather than through radiation. This work addresses the question whether an inner hot optically thin disk can be self-consistently connected to an outer cold standard disk. We come to the conclusion that radial transitions between the two flow types are in principle possible for a wide range of transition radii, which depend on the flow parameters. But while the flow evolves in time, the transition radius drifts radially outward until this reaches its outermost allowed location, which is in agrement with previous steady models. A second major result is, that the transition region is highly unstable. Oscillating modes of the Rayleigh-instability leak into the inner hot flow and make its hydrodynamical and spectral properties highly variable. The characteristic frequency of the oscillations is near to the local keplerian orbital frequency, which makes them a potential candidate for the high-frequency quasi-periodic oscillations present in the X-ray light-curves of BHXB's.