  The beaming pattern of thermal annihilation radiation is broader than the
beaming pattern produced by isotropic nonthermal electrons and positrons in the
jets of radio-emitting active galactic nuclei which Compton scatter photons
from an external isotropic radiation field. Thus blueshifted thermal
annihilation radiation can provide the dominant contribution to the high-energy
radiation spectrum at observing angles theta > 1/Gamma, where Gamma is the bulk
Lorentz factor of the outflowing plasma. This effect may account for the
spectral features of MeV blazars discovered with the Compton Telescope on the
Compton Gamma Ray Observatory. Coordinated gamma-ray observations of
annihilation line radiation to infer Doppler factors and VLBI radio
observations to measure transverse angular speeds of outflowing plasma blobs
can be used to determine the Hubble constant.
