  We present an extension of the King et al. (2004) model for the flickering of
black hole accretion discs by taking proper account for the thermal properties
of the disc.
  First we develop a one-dimensional, vertically averaged, one-zone model for
an optically thick accretion disc and study the temporal evolution. This limits
the current model to the so-called high/soft state, where the X-Ray spectrum is
dominated by a thermal black-body component.
  Then we couple this disc model to the flickering process as described in King
et al. (2004). Thus we consider the evolution of a poloidal magnetic field
subject to a magnetic dynamo. By comparing to observations of X-Ray binaries in
the high-soft state, we can constrain the strength of the energy density of the
poloidal magnetic field to a few percent of the energy density of the intrinsic
disc magnetic field.
