SIMBAD references

We present a study of variability time scales in a large sample of Active Galactic Nuclei at several frequencies between 4.8 and 230GHz. We investigate the differences of various AGN types and frequencies and correlate the measured time scales with physical parameters such as the luminosity and the Lorentz factor. Our sample consists of both high and low polarization quasars, BL Lacertae objects and radio galaxies. The basis of this work is the 22GHz, 37GHz and 87GHz monitoring data from the Metsaehovi Radio Observatory spanning over 25 years. In addition, we used higher 90GHz and 230GHz frequency data obtained with the SEST-telescope between 1987 and 2003. Further lower frequency data at 4.8GHz, 8GHz and 14.5GHz from the University of Michigan monitoring programme have been used. We have applied three different statistical methods to study the time scales: the structure function, the discrete correlation function and the Lomb-Scargle periodogram. We discuss also the differences and relative merits of these three methods. Our study reveals that smaller flux density variations occur in these sources on short time scales of 1-2 years, but larger outbursts happen quite rarely, on the average only once in every 6 years. We do not find any significant differences in the time scales between the source classes. The time scales are also only weakly related to the luminosity suggesting that the shock formation is caused by jet instabilities rather than the central black hole.