The power of a relativistic jet depends on the number of leptons and protons
carried by the jet itself. We have reasons to believe that powerful gamma-ray
flat spectrum radio sources emit most of their radiation where radiative
cooling is severe. This helps to find the minimum number of emitting leptons
needed to explain the radiation we see. The number of protons is more
uncertain. If there is one proton per electron, they dominate the jet power,
but they could be unimportant if the emission is due to electron-positron
pairs. In this case the total jet power could be much smaller. However, if the
gamma-ray flux is due to inverse Compton scattering with seed photons produced
outside the jet, the radiation is anisotropic also in the comoving frame,
making the jet to recoil. This Compton rocket effect is strong for light,
electron-positron jets, and negligible for heavy, proton dominated jets. No
significant deceleration, required by fast superluminal motion, requires a
minimum number of protons per lepton, and thus a minimum jet power. We apply
these ideas to the blazar 3C 454.3, to find a robust lower limit to its total
jet power: if the viewing angle theta_v ~ 1/Gamma the jet power is larger than
the accretion luminosity L_d for any bulk Lorentz factor Gamma. For theta_v =0,
instead, the minimum jet power can be smaller than L_d for Gamma<25. No more
than ~10 pairs per proton are allowed.Comment: 5 pages, 2 figures, accepted for publication as a letter to MNRA
