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We have tested the stability of a thin, orbiting accretion disk near a black hole. Under conditions appropriate for a binary X-ray source, with the usual (ad hoc) assumptions about viscosity, the disk is always secularly unstable on time scales of a few seconds or less. Therefore current thin-disk models for such X-ray sources are self-inconsistent. We mention possibilities for alternative models; perhaps the secular instability explains chaotic time variations in Cygnus X-t. Sitbject headings: binaries - black holes - instabilities
Lightman et al. (Tue,) studied this question.