Spectrum and polarization of x-rays from accretion disks around black holes

The emergent radiation spectrum and polarization of X-rays from accretion disks around black holes are calculated. A linear polarization of a few percent should be observed above approx.1 keV from compact binary X-ray sources containing black holes if a disk exists. The observed, hard X-ray component of the Cyg X-1 spectrum cannot be explained by the standard, instability-free accretion disk model