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

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 $μ$Jy beam$^{-1}$ (1$σ$) 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$σ$ level. One highly variable radio source was found with $ΔS > 50$ 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 $μ$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~$μ$Jy.

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