5 Summary

We have presented the data from a long-term flux monitoring project carried out with the Effelsberg radio telescope, on a complete sample of 13 flat spectrum sources from the S5 Survey and 27 other sources. Our observational findings can be summarized as follows:

• Nearly all but the two steep spectrum sources 0835+580 and 0951+699 (excluding the six primary calibrator sources), are variable at most of the observing wavelengths in this 5.5 years monitoring period;

• Among the 32 variable sources, 15 of the 19 IDV sources (79%) show a relatively strong degree of variability (of above $\sim$ 7% at 6 cm) and pronounced outbursts or significant flux variations. The rest 4 of the 19, 0153+744, 0917+624, 1652+398 and 1928+738, do not show strong long-term variability. On the other hand, most of the non-IDV sources (6 of the 9 sources, i.e. 67%) show a relatively weak variability (of below $\sim$ 7% at 6 cm), but 2 sources 0212+735 and 1226+023 show different variability behavior at different wavelengths. This implies that IDV sources vary more strongly. Therefore we suggest that, one might identify an IDV source by its long-term variability characteristics (degree of variability and outbursts);

• As seen from Fig. 28, there is no dependence of the degree of variability on either source galactic latitude, or redshift, or superluminal motions; but both the degree of variability and the rate of occurrence of variability are higher when source spectra are flatter. It appears that there is a weak correlation between the degree of variability and the 6 to 2.8 cm spectral index;

• There is a trend showing that the degree of variability decreases with increasing wavelength, as demonstrated in Fig. 29, which is contrary to the expectations of ISS models, but consistent with shock models;

• The variability in BL Lac objects is stronger than that in quasars, as demonstrated in Fig. 30. Statistically, the mean variability at each wavelength in BL Lacs is about 2 times larger than that in QSOs. This is consistent with the unified scheme of the AGNs.

From this large although not complete sample, we conclude that, the long-term variability behavior does not follow the expectations of the ISS theory: the wavelength dependence and independence of the degree of variability on galactic latitude. Although it is generally accepted that the long term variations discussed here are intrinsic to the source, this result is a useful confirmation.

Acknowledgements

We thank I. Pauliny-Toth for critically reading the manuscript, the referee Dr. M.F. Aller for her valuable comments, R. Nan and Y. Han for helpful discussions, and A.P. Lobanov, E. Ros, J. Klare for helping with the observations. B. Peng thanks the NSF of China for supporting this research. This research has made use of the NASA's Astrophysics Data System Abstract Service and of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration.