  We investigate the properties of charmonia in a thermal medium, showing that
with increasing temperature the decay widths of these mesons behave in a
non-trivial way. Our analysis is based on a potential model with interaction
potential extracted from thermal lattice QCD calculations of the free-energy of
a static quark-antiquark pair. We find that in the crossover region some decay
widths are extremely enhanced, which might be interpreted as a signature of the
transition from hadronic matter to the deconfined phase. In particular, at
temperatures $T\sim T_c$ the decay widths of the $J/\Psi$ that depend on the
value of the wave function at the origin are enhanced with respect to the
values in vacuum by about a factor 2. In the same temperature range the decay
width of the process $\chi_{c J} \to J/\Psi + \gamma$ is enhanced by
approximately a factor 6 with respect to the value in vacuum. At higher
temperatures the charmonia states dissociate and the widths of both decay
processes become vanishing small.
