  IceCube have observed neutrinos which are presumably of extra-galactic
origin. Since specific sources have not yet been identified, we discuss what
could be learned from the conceptual point of view. We use a simple model for
neutrino production from the interactions between nuclei and matter, and we
focus on the description of the spectral shape and flavor composition observed
by IceCube. Our main parameters are spectral index, maximal energy, magnetic
field, and composition of the accelerated nuclei. We show that a cutoff at PeV
energies can be achieved by soft enough spectra, a cutoff of the primary
energy, or strong enough magnetic fields. These options, however, are difficult
to reconcile with the hypothesis that these neutrinos originate from the same
sources as the ultra-high energy cosmic rays. We demonstrate that heavier
nuclei accelerated in the sources may be a possible way out if the maximal
energy scales appropriately with the mass number of the nuclei. In this
scenario, neutrino observations can actually be used to test the UHECR
acceleration mechanism. We also emphasize the need for a volume upgrade of the
IceCube detector for future precision physics, for which the flavor information
becomes a statistical meaningful model discriminator as qualitatively new
ingredient.
