  Decaying topological defects, in particular cosmic strings, can produce a
significant flux of high energy neutrinos, photons and cosmic rays. According
to the prevailing understanding of cosmic string dynamics in an expanding
Universe, the network of long strings loses its energy first into string loops,
which in turn give off most of their energy as gravitational radiation.
However, it has been suggested by Vincent et al. (VHS) that particle emission
may be the dominant energy loss channel for the long string network. In this
case, the predicted flux of high energy particles would be much larger. Here we
calculate the predicted flux of high energy gamma rays, neutrinos and cosmic
ray antiprotons and protons as a function of the scale of symmetry breaking at
which the strings are produced and as a function of the initial energy m_J of
the particle jets which result from the string decay. Assuming the validity of
the VHS scenario, we find that due to the interactions with the cosmic
radiation backgrounds all fluxes but the neutrino flux are suppressed at the
highest energies. This indicates that the observed events above the GZK cutoff
can only be accounted for in this scenario if the primary particle is a
neutrino and the scale of the symmetry breaking is somewhat less than the GUT
scale. The domain of parameter space corresponding to GUT-scale symmetry
breaking is excluded also by the current observations below the GZK cutoff. A
new aspect of this work is the calculation of the spectrum of the tau neutrinos
directly produced in the decay of the X-particles. This significantly increases
the tau neutrino signal at high energies in all cosmic string scenarios.
