Twenty years of blazar monitoring with the INAF radio telescopes

Context. The extreme variability of blazars, in both timescale and amplitude, is generally explained as the effect of a relativistic jet closely aligned with the observer’s line of sight. Via causality arguments, variability characteristics translate into spatial information about the emitting region of blazars. Since radiation at different wavelengths is emitted in different parts of the jet, multi-frequency observations provide us with a virtual view of the structure of the jet on different scales. Radio-γ-ray correlations, moreover, are essential to revealing where and how the high-energy radiation is produced.Aims. We present the observations collected within the blazar radio monitoring programme that we are running at the Medicina and Noto telescopes. Its aim is to investigate how the variability characteristics and spectral energy distribution of blazars evolve in time.Methods. Beginning in 2004, observation were performed at 5, 8, 24, and 43 GHz on 47 targets with a monthly cadence; the monitoring programme is still active at frequencies of 8 and 24 GHz.Results. The database we have built over more than 20 years of activity comprises to date about 21 000 flux density measurements. Some basic analysis tools have been applied to the data to characterise the detected variability and offer a first glance at the wealth of information that such a programme can provide about blazars.