On the significance of relativistically hot pairs in the jets of FR II radio galaxies

The energetic composition of radio lobes in the FR II galaxies $-$ estimated by comparing their radio luminosities with the powers required to inflate cavities in the external medium $-$ seems to exclude the possibility of their energetic domination by protons. Furthermore, if the jets were dominated by the kinetic energy of cold protons, it would be difficult to efficiently accelerate leptons in the jets' terminal shocks. This suggests that the jets powering the lobes are dominated by pairs not only in number but also in enthalpy. Such a possibility is confronted against the constraints imposed by the jet formation scenarios, and by the properties of jets on parsec and kiloparsec scales. We find a lower limit on the pair content from the energetic dominance of leptons over protons in the radio lobes, which exceeds the estimate of $n_e/n_p \sim 20$ indicated by models of the blazar zones and of the radio cores. This allows us to establish that the average energy of protons in the jet co-moving frame is $<2m_pc^2$, and to estimate the number fluxes of protons and $e^{+}e^{-}$ pairs. The required proton flux is achievable, if the jet carries away about $1\%$ of the accreting matter. Loading the jets by pairs can be achieved by pair creation using soft gamma-ray photons from the accretion disk corona, provided that about $2-3\%$ of the disk radiation extends beyond $1\;{\rm MeV}$. Finally, we discuss possible dissipative mechanisms which may keep the jets dominated by relativistically hot pairs.