Ultra-high energy cosmic rays from transient extragalactic sources

Attention is devoted to the origin of those ultra-high energy cosmic rays from those extragalactic sources which have ceased to be visible in the radio, optical and gamma ray regions in the time taken by the particles in diffusing to the earth. Predictions are made of the energy spectra at earth as a function of the various parameters involved: particle charge, temporal and spatial characteristics of the sources, etc. The predictions should be useful when more precise experimental data are available. Application to the presently available data shows that a 'single source' in, say, the VIRGO galaxy cluster, currently 'off', could have provided the particles seen. The apparent clusters of arrival directions are hard to understand in terms of heavy nuclei (such nuclei being preferred for reasons of easier acceleration), unless we have overestimated the strength of the magnetic field in the relevant regions of the intergalactic medium