Energetic and radiative constraints on highly relativistic jets

We examine constraints on the energetics and radiative efficiencies of highly relativistic, synchrotron-emitting jets. If the observed intraday radio variability of compact radio sources is intrinsic and results from incoherent synchrotron radiation, then the associated jets must have bulk Lorentz factors in the range Gamma varies approximately 30 to 100, several times larger than the largest values inferred from superluminal expansion, and larger even than the values required to avoid the synchrotron self-Compton catastrophe. We show that such highly relativistic jets produce synchrotron radiation with extremely low radiative efficiency. As a result they must carry enormous kinetic energy fluxes, L(sub j) approximately greater than 10(exp 47)(Delta Omega/0.1 sr), where Delta Omega is the solid angle subtended by the jet, in order to produce 'apparent' synchrotron brightness temperatures approximately greater than 10(exp 16) K. Energy losses by such jets should be strongly dominated by Compton scattering of diffuse ambient radiation, and they should produce large X-ray and gamma-ray fluxes.