2001A&A...379..755V

In this paper we present the results of a 4-year (1996-1999) radio flux density monitoring program for a sample of X- and γ-ray loud blazars. Our program started in January 1996 and was carried out on a monthly basis at the frequencies of 5GHz and 8.4GHz with the 32-m antennas located at Medicina (Bologna, Italy) and Noto (Siracusa, Italy). 22GHz data collected at Medicina from January 1996 to June 1997 will also be presented. The sample of selected sources comprises most radio-loud blazars with δ≥-10° characterised by emission in the X- and γ-ray regimes, and target sources for the BeppoSAX X-ray mission. All sources in the sample, except J1653+397 (MKN501), are variable during the four years of our monitoring program. We classified the type of variability in each source by means of a structure-function analysis. We also computed the spectral index α₅^8.4 for all epochs with nearly simultaneous observations at these two frequencies, and found that α₅^8.4 starts flattening at the very beginning of a radio flare, or flux density increase.