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We augment the two widest/deepest 1.4 GHz radio surveys: the NRAO VLA SkySurvey (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 thesecatalogs to search for unresolved sources which vary between the FIRST and NVSSepochs. We find 43 variable sources (0.1% of the sources) which vary by morethan 4 sigma, and we construct the mean structure function of these objects.This enables us to explore radio variability on time scales between severalmonths and about five years. We find that on these time scales, the meanstructure function of the variable sources is consistent with a flat structurefunction. A plausible explanation to these observations is that a largefraction of the variability at 1.4 GHz is induced by scintillations in theinterstellar medium, rather than by intrinsic variability. Finally, for a subsample of the variables for which the redshift is available, we do not findstrong evidence for a correlation between the variability amplitude and thesource redshift.

The structure function of variable 1.4 GHz radio sources based on NVSS and FIRST observations