A search for variable and transient radio sources in the extended Chandra Deep Field South at 5.5 GHz

TL;DR

This deep 5.5 GHz radio survey of the Chandra Deep Field South identified variable active galactic nuclei but found no transient sources, providing constraints on transient event rates and insights into AGN variability.

Contribution

This is the deepest 5.5 GHz radio survey to date, revealing the nature and prevalence of variable AGN and setting upper limits on transient radio source densities.

Findings

Detected 124 sources above 5.5σ threshold.

Identified 1 highly variable source with >50% flux change.

Placed an upper limit on transient surface density < 7.5 deg$^{-2}$.

Abstract

We present a three epoch survey for transient and variables in the extended Chandra Deep Field South at 5.5 GHz with the Australia Telescope Compact Array. A region covering $\sim$0.3 deg$^{2}$ was observed on timescales of 2.5 months and 2.5 years and typical sensitivities 12.1 $-$ 17.1 $\mu$Jy beam$^{-1}$ (1$\sigma$) were achieved. This survey represents the deepest search for transient and variable radio sources at 5.5 GHz. In total 124 sources were detected above the 5.5$\sigma$ level. One highly variable radio source was found with $\Delta S > 50%$ implying a surface density of $\sim$3 deg$^{-2}$. A further three radio sources were found with lower levels of variability equating to a surface density of $\sim$13 deg$^{-2}$ above a detection threshold of 82.3 $\mu$Jy. All of the variable sources have inverted radio spectra (between 1.4 and 5.5 GHz) and are associated with active galactic nuclei. We conclude that these variables are young gigahertz peaked-spectrum sources with active and self-absorbed radio jets. We explore the variability completeness of this sample and conclude that the fairly low levels of variability would only be detectable in 3$-$25% of all sources within the field. No radio transients were detected in this survey and we place an upper limit on the surface density of transient events $ < 7.5$ deg$^{-2}$ above a detection threshold of 68.8~$\mu$Jy.