Relativistic ions with power-law spectra explain radio phoenixes

Radio phoenixes are extended filamentary sources in the intracluster medium (ICM) of galaxy clusters, associated with fossil radio lobes. Their soft, concave-curved spectra, commonly attributed to aged relativistic electrons recently accelerated/compressed, are shown to approach a power-law at high frequencies. Moreover, secondary e± produced by a pure power-law spectrum of relativistic ions explain all phoenix observations with only three free parameters. Hadronic phoenixes require strong magnetization and, for high-curvature, also e± heating, by a factor ≳15 if at ICM pressure.