Twenty years of blazar monitoring with the INAF radio telescopes

¹ INAF Istituto di Radioastronomia, Via Gobetti 101, 40127 Bologna, Italy
² INAF Istituto di Radioastronomia, Stazione di Medicina, Via Fiorentina 3513, 40059, Villafontana (BO), Italy
³ INAF, Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ INAF Istituto di Radioastronomia, Stazione di Noto, Contrada Renna Bassa, 96017 Noto, Italy
⁷ Osservatorio Astrofisico di Catania, INAF, Via S. Sofia 78, 95123 Catania, Italy
⁸ Physics Department, University of Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
⁹ INFN – Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹⁰ Scuola Normale Superiore di Pisa, P.zza dei Cavalieri 7, 56126 Pisa, Italy
¹¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (Rome), Italy

^★ Corresponding author: nicola.marchili@inaf.it

Received: 28 July 2025
Accepted: 2 September 2025

Abstract

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.