CHILES VERDES: Radio Variability at an Unprecedented Depth and Cadence in the COSMOS Field

Although it is well established that some extragalactic radio sources are time-variable, the properties of this radio variability, and its connection with host galaxy properties, remain to be explored-particularly for faint sources. Here we present an analysis of radio variable sources from the CHILES Variable and Explosive Radio Dynamic Evolution Survey (CHILES VERDES)-a partner project of the 1.4 GHz COSMOS H I Large Extragalactic Survey. CHILES VERDES provides an unprecedented combination of survey depth, duration, and cadence, with 960 hr of 1-2 GHz continuum VLA data obtained over 209 epochs between 2013 and 2019 in a 0.44 deg² section of the well-studied extragalactic deep field, COSMOS. We identified 18 moderate-variability sources (showing 10%-30% flux density variation) and 40 lower-variability sources (2%-10% flux density variation). They are mainly active galactic nuclei (AGNs) with radio luminosities in the range of 10²²-10²⁷ W Hz^-1, based on cross-matching with COSMOS multiwavelength catalogs. The moderate-variability sources span redshifts z = 0.22-1.56, have mostly flat radio spectra (α > -0.5), and vary on timescales ranging from days to years. The lower-variability sources have similar properties, but generally have higher radio luminosities than the moderate-variability sources, extending to z = 2.8, and have steeper radio spectra (α < -0.5). No star-forming galaxy showed statistically significant variability in our analysis. The observed variability likely originates from scintillation on short (~week) timescales, and Doppler-boosted intrinsic AGN variability on long (month-year) timescales.