1996A&A...306...86B

The contribution of the pair annihilation process in relativistic electron-positron jets to the γ-ray emission of blazars is calculated. Under the same assumptions as for the calculation of the yield of the inverse Compton scattered photons by relativistic particles in the jet (Dermer and Schlickeiser 1993) we calculate the emerging pair annihilation radiation taking into account all spectral broadening effects due to the energy spectra of the annihilating particles and the bulk motion of the jet. It is shown that in spectral appearance the time-integrated pair annihilation spectrum appears almost like the well-known γ-ray spectrum from decaying πdeg-mesons at rest, since it yields a broad bumpy feature located between 50 and 100MeV. We also demonstrate that for pair densities greater 10¹⁰cm^–3 in the jet the annihilation radiation will dominate the inverse Compton radiation, and due to its bump-like spectrum indeed may explain reported spectral bumps at MeV energies. The refined treatment of the inverse Compton radiation including Klein-Nishina cross section corrections, the low-energy cutoff in the radiating relativistic particle distribution function and a realistic greybody accretion disk target photon spectrum leads to spectral breaks of the inverse Compton emission in the MeV energy range with a change in spectral index {DELTA}α larger than 0.5 as detected in PKS 0528+134 and 3C273.