  Context: Cygnus X-1 is a black hole X-ray binary system in which the black
hole captures and accretes gas from the strong stellar wind emitted by its
supergiant O9.7 companion star. The irradiation of the supergiant star
essentially determines the flow properties of the stellar wind and the X-ray
luminosity from the system. The results of three-dimensional hydrodynamical
simulations of wind-fed X-ray binary systems reported in recent work reveal
that the ionizing feedback of the X-ray irradiation leads to the existence of
two stable states with either a soft or a hard spectrum. Aims: We discuss the
observed radiation of Cygnus X-1 in the soft and hard state in the context of
mass flow in the corona and disk, as predicted by the recent application of a
condensation model. Methods: The rates of gas condensation from the corona to
the disk for Cygnus X-1 are determined, and the spectra of the hard and soft
radiation are computed. The theoretical results are compared with the MAXI
observations of Cygnus X-1 from 2009 to 2018. In particular, we evaluate the
hardness-intensity diagrams (HIDs) for its ten episodes of soft and hard states
which show that Cygnus X-1 is distinct in its spectral changes as compared to
those found in the HIDs of low-mass X-ray binaries. Results: The theoretically
derived values of photon counts and hardness are in approximate agreement with
the observed data in the HID. However, the scatter in the diagram is not
reproduced. Improved agreement could result from variations in the viscosity
associated with clumping in the stellar wind and corresponding changes of the
magnetic fields in the disk. The observed dipping events in the hard state may
also contribute to the scatter and to a harder spectrum than predicted by the
model.
