The Beaming Pattern of Doppler Boosted Thermal Annihilation Radiation: Application to MeV Blazars

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.