  There have been many speculations about the presence of cosmic ray protons
(CRps) in galaxy clusters over the past two decades. However, no direct
evidence such as the characteristic gamma-ray signature of decaying pions has
been found so far. These pions would be a direct tracer of hadronic CRp
interactions with the ambient thermal gas also yielding observable synchrotron
and inverse Compton emission by additionally produced secondary electrons. The
obvious question concerns the type of galaxy clusters most likely to yield a
signal: Particularly suited sites should be cluster cooling cores due to their
high gas and magnetic energy densities. We studied a nearby sample of clusters
evincing cooling cores in order to place stringent limits on the cluster CRp
population by using non-detections of EGRET. In this context, we examined the
possibility of a hadronic origin of Coma-sized radio halos as well as radio
mini-halos. Especially for mini-halos, strong clues are provided by the very
plausible small amount of required CRp energy density and a matching radio
profile. Introducing the hadronic minimum energy criterion, we show that the
energetically favored CRp energy density is constrained to 2% +/- 1% of the
thermal energy density in Perseus. We also studied the CRp population within
the cooling core region of Virgo using the TeV gamma-ray detection of M 87 by
HEGRA. Both the expected radial gamma-ray profile and the required amount of
CRp support this hadronic scenario.
