The structure function of variable 1.4 GHz radio sources based on NVSS
  and FIRST observations
  We augment the two widest/deepest 1.4 GHz radio surveys: the NRAO VLA Sky
Survey (NVSS) and the Faint Images of the Radio Sky at Twenty-Centimeters
(FIRST), with the mean epoch in which each source was observed. We use these
catalogs to search for unresolved sources which vary between the FIRST and NVSS
epochs. We find 43 variable sources (0.1% of the sources) which vary by more
than 4 sigma, and we construct the mean structure function of these objects.
This enables us to explore radio variability on time scales between several
months and about five years. We find that on these time scales, the mean
structure function of the variable sources is consistent with a flat structure
function. A plausible explanation to these observations is that a large
fraction of the variability at 1.4 GHz is induced by scintillations in the
interstellar medium, rather than by intrinsic variability. Finally, for a sub
sample of the variables for which the redshift is available, we do not find
strong evidence for a correlation between the variability amplitude and the
source redshift.
