  We discuss the origin of the dramatically different X-ray spectral shapes
observed in the Low Hard State (LHS: dominated by thermal comptonisation) and
the High Soft State (HSS: dominated by the accretion disc thermal emission and
non-thermal comptonisation in the corona). We present numerical simulations
using a new code accounting for the so-called synchrotron boiler effect. These
numerical simulations when compared to the data allow us to constrain the
magnetic field and temperature of the hot protons in the corona. For the hard
state of Cygnus X-1 we find a magnetic field below equipartition with
radiation, suggesting that the corona is not powered through magnetic field
dissipation (as assumed in most accretion disc corona models). On the other
hand, our results also point toward proton temperatures that are substantially
lower than typical temperatures of the ADAF models. Finally, we show that in
both spectral states Comptonising plasma could be powered essentially through
power-law acceleration of non-thermal electrons, which are then partly
thermalised by the synchrotron and Coulomb boiler. This suggests that, contrary
to current beliefs, the corona of the HSS and that of the LHS could be of very
similar nature. The differences between the LHS and HSS coronal spectra would
then be predominantly caused by the strong disc soft cooling emission which is
present in the HSS and absent in the LHS.
