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Optical Properties of faint FIRST Variable Radio Sources

A sample of 123 radio sources that exhibit significant variations at 1.4 GHz on a 7 year baseline has been created using FIRST VLA B-configuration data from 1995 and 2002 on a strip at δ = 0 near the south Galactic cap. This sample spans the range of radio flux densities from ∼2 to 1000 mJy. It presents both in size and radio flux density range a unique starting point for variability studies of galaxies and quasars harboring lower luminosity active galactic nuclei (AGNs). We find, by comparing our variable and nonvariable control samples with the Sloan Digital Sky Survey, the following: (1) The quasar fraction of both the variable and nonvariable samples declines as a function of declining radio flux density levels; (2) our variable sample contains a consistently higher fraction of quasars than the nonvariable control sample, irrespective of radio flux; (3) the variable sources are almost twice as likely to be retrieved from the optical SDSS data than the nonvariable ones; (4) based on relative numbers, we estimate that quasars are about 5 times more likely to harbor a variable radio source than are galaxies; and (5) there does not appear to be any significant optical color offset between the two samples, even though the suggestive trend for sources to be bluer when a variable has been detected before and may be real. This leads us to conclude that both radio variability and radio flux density levels, in combination with accurate optical information, are important discriminators in the study of (radio) variability of galaxies. The latter start to dominate the source counts below ∼20 mJy. In any case, variability appears to be an intrinsic property of radio sources and is not limited to quasars. Radio variability at low flux density levels may offer a unique tool in AGN unification studies.