On pair content and variability of subparsec jets in quasars

X-ray observations of blazars associated with optically violently variable (OVV) quasars put strong constraints on the e(+)e(-) pair content of radio-loud quasar jets. From those observations, we infer that jets in quasars contain many more e+e- pairs than protons, but dynamically are still dominated by protons. In particular, we show that pure e(+)e(-) jet models can be excluded, since they overpredict soft X-radiation; likewise, pure proton-electron jets can be excluded, since they predict too weak nonthermal X-radiation. An intermediate case is viable. We demonstrate that jets that are initially proton-electron ("protojets") can be pair-loaded via interaction with 100-300 keV photons produced in hot accretion disc coronae, likely to exist in active galactic nuclei in general. If the coronal radiation is powered by magnetic flares, the pair loading is expected to be nonuniform and nonaxisymmetric. Together with radiation drag, this leads to velocity and density perturbations in a jet and the formation of shocks, where the pairs are accelerated. Such a scenario can explain rapid (timescale of similar to a day) variability observed in OVV quasars.