A 22-year Southern Sky Survey for Transient and Variable Radio Sources using the Molonglo Observatory Synthesis Telescope

We describe a 22-year survey for variable and transient radio sources, performed with archival images taken with the Molonglo Observatory Synthesis Telescope (MOST). This survey covers $2775 \unit{deg^2}$ of the sky south of $\delta < -30\degree$ at an observing frequency of 843 MHz, an angular resolution of $45 \times 45 \csc | \delta| \unit{arcsec^2}$ and a sensitivity of $5 \sigma \geq 14 \unit{mJy beam^{-1}}$. We describe a technique to compensate for image gain error, along with statistical techniques to check and classify variability in a population of light curves, with applicability to any image-based radio variability survey. Among radio light curves for almost 30000 sources, we present 53 highly variable sources and 15 transient sources. Only 3 of the transient sources, and none of the variable sources have been previously identified as transient or variable. Many of our variable sources are suspected scintillating Active Galactic Nuclei. We have identified three variable sources and one transient source that are likely to be associated with star forming galaxies at $z \simeq 0.05$, but whose implied luminosity is higher than the most luminous known radio supernova (SN1979C) by an order of magnitude. We also find a class of variable and transient source with no optical counterparts.