A Census of Variable Radio Sources at $3\,$GHz

Abstract

A wide range of phenomena, from explosive transients to active galactic nuclei, exhibit variability at radio wavelengths on timescales of a few years. Characterizing the rate and scale of variability in the radio sky can provide keen insights into dynamic processes in the Universe, such as accretion mechanics, jet propagation, and stellar evolution. We use data from the first two epochs of the Very Large Array Sky Survey (VLASS) to conduct a census of the variable radio sky. Approximately $3,600$ compact sources are found to significantly vary in brightness during the $\sim2.5\,$ years between observations. In this work we focus on sources that are detected in both VLASS epochs, but estimate there may be $>10,000$ additional variable radio sources in VLASS that are only detected in either the first or second epoch. For objects detected in both epochs whose mean flux density across the two epochs, $\mu_{S}$, is brighter than $20\,$mJy, $5\,$% show brightness variations $>30\,$%, rising to $9\,$% at $\mu_{S}>300\,$mJy. We analyze the redshift distributions, infrared colors, and $\gamma$-ray properties of the variable radio sources, finding that most have multiwavelength characteristics that are consistent with blazars and quasars. Blazars in particular are found to be overrepresented among the variable radio sources, and the largest absolute changes in flux density are produced by blazars. The largest fractional changes in brightness are exhibited by galactic sources. We discuss our results, including some of the more interesting and extreme examples of variable radio sources identified, as well as future research directions.