  The high energy continuum in Seyfert galaxies and galactic black hole
candidates is likely to be produced by a thermal plasma. There are difficulties
in understanding what can keep the plasma thermal, especially during fast
variations of the emitted flux. Particle-particle collisions are too
inefficient in hot and rarefied plasmas, and a faster process is called for. We
show that cyclo-synchrotron absorption can be such a process: mildly
relativistic electrons thermalize in a few synchrotron cooling times by
emitting and absorbing cyclo-synchrotron photons. The resulting equilibrium
function is a Maxwellian at low energies, with a high energy tail when Compton
cooling is important. Assuming that electrons emit completely self absorbed
synchrotron radiation and at the same time Compton scatter their own
cyclo-synchrotron radiation and ambient UV photons, we calculate the time
dependent behavior of the electron distribution function, and the final
radiation spectra. In some cases, the 2-10 keV spectra are found to be
dominated by thermal synchrotron self-Compton process rather than by thermal
Comptonization of UV disk radiation.
