Astronomical objects that change rapidly give us insight into extreme environments, allowing us to identify new phenomena, test fundamental physics, and probe the Universe on all scales. Transient and variable radio sources range from the cosmological, such as gamma-ray bursts, to much more local events, such as massive flares from stars in our Galactic neighbourhood. The capability to observe the sky repeatedly, over many frequencies and timescales, has allowed us to explore and understand dynamic phenomena in a way that has not been previously possible. In the past decade, there have been great strides forward as we prepared for the revolution in time domain radio astronomy that is being enabled by SKAO pathfinders and precursors such as ASKAP. Murphy and Kaplan review the current status of the field, focussing on image domain (or “slow”) transients, on timescales of seconds to years, and summarise the developments that have happened to get to our current point.

The image above is an updated version of the transient phase space showing radio luminosity versus the product of timescale and observing frequency for different transient source classes. The diagonal lines show contours of brightness temperature, with coherent emitters having values > 10¹² K. Notable is the range of sources that are filling out the centre of this space, straddling the coherent/incoherent divide: long-period radio pulsars are upward-pointing triangles, Galactic Centre Radio Transients (GCRTs) are diamonds, pulsing white dwarf binaries are right-pointing triangles, and long period transients (LPTs) are squares.