Equipartition plasmas near accreting black holes

Abstract

The hard X‐ray/soft gamma‐ray spectra of Seyfert active galactic nuclei and galactic black hole candidates are well fit by thermal Comptonization models. If one assumes equipartition between the accretion energy density and the magnetic field and particle energy densities, then constraints are imposed on the allowed parameters of accretion plasmas near black holes. The additional assumptions of virial equilibrium for the protons and global energy balance for the electrons imply Thomson optical depths τT∼1, electron temperatures kTe∼50 keV, energy spectral indices α∼1, and physical sizes of the equipartition plasma ≊2−20×rI, where rI is the radius of the innermost stable orbit. These values are in accord with physical parameters inferred from observations of accreting black‐hole sources.