Electron-positron outflows from gamma-ray emitting accretion discs

An e atmosphere is inevitably created around a black hole accretion disc whose spectrum extends to MeV energies. Pairs created in γ − γ collisions outside the disc are blown away by soft radiation (which dominates the bolometric luminosity of the disc) and form a semi-relativistic outflow. We simulate numerically the conversion of the MeV radiation into a vertical e outflow above a disc-like source. The outflowing e ± plasma becomes optically thick to Thomson scattering if the compactness of the γ-ray source exceeds ∼ 30. The scattering by e then collimates the bulk of soft radiation along the disc axis, and the apparent bolometric luminosity of the disc depends strongly on its inclination to the line of sight. The anisotropic central emission may account for the lack of Fe Kα lines in the X-ray spectra of bright radio-quiet quasars. The scattering in e outflows may also explain the orientation of optical polarization in non-blazar active galactic nuclei.