Being dominated by non-thermal emission from aligned relativistic jets, blazars allow us to elucidate the physics of extragalactic jets and, ultimately, how the energy is extracted from the central black hole in radio-loud active galactic nuclei. Crucial information is provided by their broad-band spectral energy distributions (SED) in different brightness states, the trends of SEDs with luminosity, and by the cross-correlation analysis of their multi-frequency variability. Since December 2004 we perform a monthly multi-frequency radio monitoring of a sample of 35 blazars at the 32-m antennas in Medicina and Noto.
Some of these sources belong to the list of targets observed by the Whole Earth Blazar Telescope (WEBT;
https://www.oato.inaf.it/blazars/webt), with which we have been collaborating since the beginning. The WEBT can obtain well-sampled optical light curves, allowing us to perform a radio-optical cross-correlation analysis.
Most of our sources are gamma-ray emitters, and are continuously monitored by the Fermi satellite, which means that we can compare their low-energy behaviour with that at high energies.
Moreover, we also monitor blazars that are TeV emitters, and we have a fruitful collaboration with the MAGIC team, observing them in the TeV energy domain.
The radio monitoring program started in 2004 and was led by Uwe Bach (Max-Planck Institute for Radioastronomy of Bonn) until 2018, when the coordination task passed to Nicola Marchili (INAF-IRA).