Effelsberg Monitoring of a Sample of RadioAstron Blazars: Analysis of
  Intra-Day Variability
  We present the first results of an ongoing intra-day variability (IDV) flux
density monitoring program of 107 blazars, which were selected from a sample of
RadioAstron space very long baseline interferometry (VLBI) targets. The~IDV
observations were performed with the Effelsberg 100-m radio telescope at
4.8\,GHz, focusing on the statistical properties of IDV in a relatively large
sample of compact active galactic nuclei (AGN). We investigated the dependence
of rapid ($<$3 day) variability on various source properties through a
likelihood approach. We found that the IDV amplitude depends on flux density
and that fainter sources vary by about a factor of 3 more than their brighter
counterparts. We also found a significant difference in the variability
amplitude between inverted- and flat-spectrum radio sources, with the former
exhibiting stronger variations. $\gamma$-ray loud sources were found to vary by
up to a factor 4 more than $\gamma$-ray quiet ones, with 4$\sigma$
significance. However a galactic latitude dependence was barely observed, which
suggests that it is predominantly the intrinsic properties (e.g., angular size,
core-dominance) of the blazars that determine how they scintillate, rather than
the directional dependence in the interstellar medium (ISM). We showed that the
uncertainty in the VLBI brightness temperatures obtained from the space VLBI
data of the RadioAstron satellite can be as high as $\sim$70\% due to the
presence of the rapid flux density variations. Our statistical results support
the view that IDV at centimeter wavelengths is predominantly caused by
interstellar scintillation (ISS) of the emission from the most compact,
core-dominant region in an AGN.
