Abstract
This lecture describes radio jets, formed by relativistic charged particles ejected by an AGN in the weak regime. Radio-powerful AGNs are only a small fraction of all AGNs (around 10%). A distinction must be made between the highly collimated jets emitted by the black hole itself and the ionized winds emitted by accretion disks, which have a much wider aperture angle. Radio jets form spectacular structures, reaching up to 100 kiloparsecs outside the galaxy. They begin as very fine, asymmetrical jets, as the jet coming towards the observer is boosted by the relativistic Doppler effect, and end in thicker, slower lobes. Several ejection cycles are sometimes observed, suggesting that the typical duration of AGN activity is between 10 and 100 million years. The jets are sometimes distorted by the intergalactic wind, as AGN moves through a cluster at a speed of the order of a few hundred km/s. When the ionized gas is ejected in puffs, their progress can be tracked over time using Very Long Baseline Interferometry (VLBI), and most jets are observed to be superluminal. They appear to be moving at up to 10 times the speed of light, but this is an illusion, due to the fact that the fragments advancing towards the observer have less distance to cover. In other words, relative travel and propagation times are comparable. How are radio jets ejected? It can be shown that energy can be extracted from the black hole if it is rotating: this is the Blandford-Znajek process. The jets are then confined and collimated by magnetic fields, as shown by numerical simulations. Observations of microquasars, whose variations are on a human scale, provide a better understanding of quasar processes, and of the weak-accretion (ADAF, hard X-ray spectrum), strong-accretion (thin disk and soft X-ray spectrum) cycle.
