Cyclo-synchrotron emission from magnetically dominated active regions above accretion discs

We discuss the role of thermal cyclo-synchrotron emission in a magnetically dominated corona above an accretion disc in an active galaxy or a Galactic black hole candidate. The dissipation occurs in localized active regions around the central black hole. Cyclo-synchrotron radiation is found to be an important process. In the case of Galactic black hole candidates, its emission can dominate the inverse Compton scattering of the soft photon field produced in the disc by thermal electrons. We discuss observational predictions and the detectability of cyclo-synchrotron radiation both for these sources and for radio-quiet active galactic nuclei.