Common origin for radio relics and halos: galaxy cluster-wide,
  homogeneous cosmic-ray distribution, and evolving magnetic fields
  Some galaxy clusters show diffuse radio emission in the form of peripheral
relics (so far attributed to primary, shock-(re)accelerated electrons) or
central halos. Analysing radio and X-ray data from the literature, we find new
connections between halos and relics, such as a universal linear relation
between their peak radio brightness and the gas column density. Our results
indicate that halos, relics, and halo-relic bridges in a cluster, all arise
from the same, homogeneous cosmic ray (CR) ion (CRI) distribution. We
analytically derive the signature of synchrotron emission from secondary
electrons and positrons (CREs) produced in hadronic CRI collisions, for an
arbitrary magnetic field evolution. In our model, flat spectrum halos (both
giant and minihalos) arise from steady-state magnetic fields, whereas relics
and steep halos reflect recent or irregular magnetic growth. This naturally
explains the properties of halos, relics, and the connections between them,
without invoking particle (re)acceleration in weak shocks or turbulence. We
find CRI energy densities in the range u_p~10^-[12.4,13.3]erg cm^-3, with a
spectral index s_p=-2.20+-0.05, and identify a ~0.1 magnetic energy fraction in
some halos and behind relics, as far as 2Mpc from the cluster's centre. The CRI
homogeneity suggests strong CR diffusion, D(100GeV)>~10^32cm^2s^-1. The strong
magnetisation imposes strict upper limits on >10GeV CRE (re)acceleration in
weak shocks (efficiency <10^-4) and turbulence; indeed, each weak shock
slightly lowers the energy fraction of flat CRs.
