  The origin of dramatically different electron distributions responsible for
Comptonization in black hole X-ray binaries (BHBs) in their various states is
discussed. We solve the coupled kinetic equations for photons and electrons
without approximations on the relevant cross-sections accounting for Compton
scattering, synchrotron radiation, and Coulomb collisions. In the absence of
external soft photons, the electrons are efficiently thermalized by synchrotron
self-absorption and Coulomb scattering even for pure nonthermal electron
injection. The resulting quasi-thermal synchrotron self-Compton spectra have
very stable slopes and electron temperatures similar to the hard states of
BHBs. The hard spectral slopes observed in the X-rays, the cutoff at 100 keV,
and the MeV tail together require low magnetic fields ruling out the magnetic
dissipation mechanism. The motion of the accretion disk toward the black hole
results in larger Compton cooling and lower equilibrium electron temperature.
Our self-consistent simulations show that in this case both electron and photon
distributions attain a power-law-dominated shape similar to what is observed in
the soft state. The electron distribution in the Cyg X-1 soft state might
require a strong magnetic field, being consistent with the magnetically
dominated corona.
