X-ray emission from accreting black holes

In this thesis I use state of the art models of x-ray emission from accreting black holes in order to observationally constrain the accretion geometry. The broad-band RXTE PCA and HEXTE spectrum of Cyg X-1 from 3 – 200keV is fit with reflection models which calculate the vertical ionization structure of an x-ray illuminated disc. Two geometries were considered corresponding to a truncated disc/inner hot flow and magnetic flares above an untruncated disc. Both models are able to fit the PCA 3 - 20keV data, but with very different spectral components. In the magnetic flare models the 3 - 20keV PCA spectrum contains a large amount of highly ionized reflection while in the truncated disc models the amount of reflection is rather small. The Compton downscattering rollover in the reflected emission means that the magnetic flare models predict a break in the spectrum at the high energies covered by the HEXTE bandpass which is not seen. By contrast the weakly illuminated truncated disc models can easily fit the 3 – 200keV spectra, strongly suggesting that this is the preferential geometry of galactic black hole binaries in the hard/low state.