Radio and gamma-ray connection in relativistic jets

TL;DR

This paper reviews recent multiwavelength observations of blazars, highlighting the complex connection between radio and gamma-ray emissions, and discusses the challenges in understanding the physics of relativistic jets in active galactic nuclei.

Contribution

It provides new insights from recent campaigns into the radio and gamma-ray connection in blazars, emphasizing the complexity of high-energy emission mechanisms.

Findings

High-energy emission regions are located downstream at parsec scales.

Not all high-energy flares have consistent characteristics across energy bands.

Velocity gradients in structured jets may explain TeV emission behaviors.

Abstract

Relativistic jets are one of the most powerful manifestations of the release of energy related to the supermassive black holes 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 the mechanisms responsible for the high-energy emission and the connection of the variability at different wavelengths are among the greatest challenges in the study of AGN. Recent high resolution radio observations of flaring objects locate the high-energy emitting region downstream the jet at parsec scale distance from the central engine, posing questions on the nature of the seed photons upscattered to gamma-rays. Furthermore, monitoring campaigns of the most active blazars indicate that not all the high energy flares have the same characteristics in the various energy bands, even from the same source, making the interpretation of the mechanism responsible for the high-energy emission not trivial. Although the variability of the most luminous blazars is well explained by the "shock-in-jet" scenario, the sub-class of TeV emitting objects suggests a more complex emission model with velocity gradients in a structured jet. This contribution presents results obtained by recent multiwavelength campaigns of blazars aimed at studying the radio and gamma-ray connection and the physical mechanisms at the basis of the emission in these low and high energy bands.