Relativistic jets in active galactic nuclei

This review discusses the basic characteristics of the jets inactive galactic nuclei (AGN). It concentrates on the highly relativistic variety found in blazars, since these are the closest cousins to the jets seen in some X-ray binaries (microquasars) and the collimated, ultra-fast flows inferred to exist in gamma ray bursts. The fastest AGN jets are gamma-ray bright, with flow Lorentz factors that range up to at least 45, although such high values must be rare unless the radiative efficiency is usually very low. If jets are magnetically launched, observations indicate that the acceleration and collimation occur over an extended region and the Poynting flux is converted into both flow energy and turbulence. Particle domination of the pressure occurs during outbursts, as might be possible in shocks. The cores of AGN jets may have different origins depending on frequency: points where the Doppler factor becomes high, conical recollimation shocks, or the optically thick/thin transition region. Superluminal knots and outbursts are well explained by shocks, but the polarization properties require than in many cases the shock fronts are oblique to the jet axis. In two radio galaxies, appearances of superluminal knots in the jet follow low-hard X-ray states, reminiscent of the behavior of microquasars. Models that relate the accretion state to the geometry of the magnetic field must explain how the transition back to a radiative inner disk releases an ultra-fast moving energetic disturbance into the jet.