Signatures of topological defects

We argue that due to various restrictions cosmic strings and monopole-string networks are not likely to produce the observed flux of ultrahigh energy cosmic rays (UHECR). Among the topological defects studied so far, the most promising UHECR sources are necklaces and monopolonia. Other viable sources which are similar to topological defects are relic superheavy particles. All these sources have an excess of pions (and thus photons) over nucleons at production. We demonstrate that in the case of necklaces the diffuse proton flux can be larger than the photon flux, due to absorption of the latter on the radio background, while monopolonia and relic particles are concentrated in the galactic halo, and the photon flux dominates. Another signature of the latter sources is anisotropy imposed by the asymmetric position of the Sun in the galactic halo. In all cases considered so far, including necklaces, photons must be present in ultrahigh energy radiation observed from topological defects, and experimental discrimination between photon-induced and proton-induced extensive air showers can give a clue to the origin of ultrahigh energy cosmic rays.