Radio observations of the Galactic Centre and the Coma cluster as a
  probe of light dark matter self-annihilations and decay
  We update our earlier calculations of gamma ray and radio observational
constraints on annihilations of dark matter particles lighter than 10 GeV. We
predict the synchrotron spectrum as well as the morphology of the radio
emission associated with light decaying and annihilating dark matter candidates
in both the Coma cluster and the Galactic Centre. Our new results basically
confirm our previous findings: synchrotron emission in the very inner part of
the Milky Way constrains or even excludes dark matter candidates if the
magnetic field is larger than 50 micro Gauss. In fact, our results suggest that
light annihilating candidates must have a S-wave suppressed pair annihilation
cross section into electrons (or the branching ratio into electron positron
must be small). If dark matter is decaying, it must have a life time that is
larger than t = 3. 10^{25} s. Therefore, radio emission should always be
considered when one proposes a "light" dark matter candidate.
