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A radio phoenix is a region of a galaxy cluster emitting radio that is presumed to be the revival of previous radio emission, i.e., some time in the past, an event such as a galaxy merger created the conditions to produce radio (moving plasma gaining density and a magnetic field) and after the radio died down a more recent (nearby) such event raised the same region back to the point that it is currently emitting radio. The presumption is that the plasma was still returning to its "normal" state, and a lesser effect (from a more distant event) was enough to revive the radio emission to the point that we can detect it.