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

TL;DR

This study analyzes radio source variability at 1.4 GHz using NVSS and FIRST surveys, revealing that most variability is likely caused by interstellar scintillation rather than intrinsic changes, with little correlation to redshift.

Contribution

It introduces a method to identify variable radio sources by combining NVSS and FIRST data and constructs the first mean structure function for these sources over several years.

Findings

43 variable sources identified, constituting 0.1% of the sample.

The structure function is flat, indicating scintillation as the primary variability cause.

No strong correlation between variability amplitude and redshift found.

Abstract

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.