Intra-day variability observations and the VLBI structure analysis of
  quasar S4 0917+624
  The IDV observations of S4 0917+624 were carried out monthly, from August
2005 to January 2010, with the Urumqi 25m radio telescope at 4.8 GHz. The
quasar S4 0917+624 exhibits only very weak or no IDV during our 4.5 year
observing interval. Prior to the year 2000, the source S4 0917+624 was one of
the most prominent IDV sources. Our new data indicate that the previous strong
IDV has ceased. We analyzed the long-term VLBI structural variability using
Gaussian model-fitting. From this we obtained the flux densities and the
deconvolved sizes of core and inner-jet components of the source. We studied
the properties such as core fraction, angular size, spectral index, and
brightness temperature of VLBI core for S4 0917+624, as well as the time delay
between 5 and 15 GHz variations, and compared them with the IDV properties of
S4 0917+624. The source shows ejection of several jet components that are
suspected to have partially reduced the IDV amplitude of S4 0917+624. However,
during 2005-2006, the VLBI core size was comparable to the size before the year
2000, but no strong IDV was detected in the period, suggesting that the
quenching effect due to source size changes may not be responsible for the lack
of strong IDV after the year 2000. The refractive scattering properties for the
strong IDV phase of S4 0917+624 before the year 2000 are discussed. The
disappearance of strong IDV in S4 0917+624 after the year 2000 is a mystery and
cannot be explained via the quenching effect by changes in the observable VLBI
structure. However, it may be caused by changes in the interstellar medium,
i.e. by interstellar weather, which induces changes in the scintillation
pattern on timescales of several years. Further coordinated multi-frequency
observations will be required to distinguish between the effect of
source-intrinsic variability and changing properties of the interstellar
medium.
