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Keywords:

  • radiation mechanisms: non-thermal;
  • gamma-ray burst: general;
  • galaxies: active;
  • galaxies: jets;
  • X-rays: general

ABSTRACT

The matter content of extragalactic relativistic jets is still an unsolved issue. There are strong arguments against pure electron–positron pair jets, but pairs could outnumber the electrons associated with protons by a factor of 10–20. This impacts on the estimate of the jet kinetic power, by reducing it by the same factor, and on the total energy delivered to leptons by the particle acceleration mechanism. Pairs cannot be created in the same jet zone responsible for the high-energy γ-ray emission we see in blazars, because the reprocessing of the created pairs would overproduce the X-ray flux. Copious pair creation could occur in the inner zone of the still accelerating jet, where the bulk Lorentz factor is small. It is found that the inner zone can produce a sufficient number of pairs to replenish the zone of the jet where most of the luminosity is emitted, but only if the γ-ray luminosity of the inner jet is above 1044 erg s−1 at ∼1 MeV. Since the beaming is modest, this emission can be observed at large viewing angles, and detected in radio galaxies and lobe-dominated quasars at the flux level of 10−12–10−11 erg cm−2 s−1 for a source at a redshift z= 0.1.