  An electron-positron atmosphere is inevitably created around a black hole
accretion disc, the spectrum of which extends to MeV energies. Pairs created in
photon-photon collisions outside the disc are blown away by soft radiation
(which dominates the bolometric luminosity of the disc) and form a
semirelativistic 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 gamma-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 K-alpha 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.

Astrophysics