variable radio source
variable radio sourceA radio source whose measured parameters change with time, especially the flux density. Many extragalactic radio sources have been observed at frequencies above 400 megahertz to vary over periods of weeks or months. The time-scale of the variations imposes an upper limit on the linear size of the varying region since the conditions are unlikely to change more quickly than the time it takes for light to travel across the region.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006