Radio monitoring of a sample of X{ and {ray loud blazars ?

In this paper we present the results of a 4-year (1996{1999) radio flux density monitoring program for as ample of X{ and{ray loud blazars. Our program started in January 1996 and was carried out on a monthly basis at the frequencies of 5 GHz and 8.4 GHz with the 32-m antennas located at Medicina (Bologna, Italy) and Noto (Siracusa, Italy). 22 GHz 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 10 characterised by emission in the X{ and {ray regimes, and target sources for the BeppoSAX X{ray mission. All sources in the sample, except J1653+397 (MKN 501), are variable during the four years of our monitoring program. We classied the type of variability in each source by means of a structure-function analysis. We also computed the spectral index 8:4 5 for all epochs with nearly simultaneous observations at these two frequencies, and found that 8:4 5 starts flattening at the very beginning of a radio flare, or flux density increase.