ar X iv : a st ro - p h / 01 11 50 3 v 1 2 7 N ov 2 00 1 Annual Modulation in the Variability Properties of the IDV - Source 0917 + 624 ?

There is new evidence which identifies seasonal changes of the variability time scale in IDV-sources with refractive interstellar scintillation effects. Such a RISSmodel takes the annual change of the Earth velocity relative to the scattering medium into account. In September 1998 we found a remarkable prolongation of the variability time scale in the IDV-source 0917+624 with only small variations in flux density during a period of 5 days. This was explained as a seasonal effect, in which the velocity vector of the Earth and the interstellar medium nearly canceled. In order to further investigate the applicability of the model for 0917+624, we performed an Effelsberg 6cm-flux monitoring program over the course of one year. Since September 2000, the source appears to be remarkably inactive and yet (May 2001), no return to its normal, faster and stronger variability pattern is observed. Here, our observational results and a possible explanation for the current quiescence are presented.