  The galactic black hole candidate Cygnus X-1 is observed to be in one of two
X-ray spectral states; either the low/hard (low soft X-ray flux and a flat
power law tail) or high/soft (high blackbody-like soft X-ray flux and a steep
power law tail) state. The physical origin of these two states is unclear. We
present here a model of an ionized accretion disc, the spectrum of which is
blurred by relativistic effects, and fit it to the ASCA, Ginga and EXOSAT data
of Cygnus X-1 in both spectral states. We confirm that relativistic blurring
provides a much better fit to the low/hard state data and, contrary to some
previous results, find the data of both states to be consistent with an ionized
thin accretion disc with a reflected fraction of unity extending to the
innermost stable circular orbit around the black hole. Our model is an
alternative to those which, in the low/hard state, require the accretion disc
to be truncated at a few tens of Schwarzschild radii, within which there is a
Thomson-thin, hot accretion flow. We suggest a mechanism that may cause the
changes in spectral state.
