Multi-band observations of relativistic jets

Relativistic jets are one of the most powerful manifestations of the release of energy produced around supermassive black holes (SMBH) at the centre of active galactic nuclei (AGN). Their emission is observed across the entire electromagnetic spectrum, from the radio band to gamma rays. Despite decades of efforts, many aspects of the physics of relativistic jets remain elusive. In particular, the location and mechanisms responsible for the high-energy emission and their connection to the jet structure are among the greatest challenges in the study of AGN. There is observational evidence supporting either a location of the gamma-ray emitting region close to the SMBH, or a site downstream along the jet, at parsec scale distance from the central engine. The emission in radio-loud AGN is explained either by single-zone models where radio and gamma-ray emission is produced by the same mechanism, like a shock, or by two-zone models, where radio and gamma-rays originate in different regions, like in a stratified jet. By analysing high-resolution VLBI observations of gamma-ray emitting AGN, it is possible to discriminate between the models, either detecting superluminal components which are the manifestation of a propagating shock, or observing changes along the jet surface brightness, like the transition from edge-brightening to limb-brightening. In this talk, I will present recent results on multiwavelength campaigns of different kind of radio-loud AGN detected by Fermi-LAT and I will focus on the importance of high angular resolution observations.