ar X iv : a st ro - p h / 05 05 56 3 v 2 2 Se p 20 05 DESY 05 - 053 Z - bursts from the Virgo cluster

Resonant annihilation of ultra-high energy cosmic neutrinos (UHECν) on the cosmic neutrino background (CνB) into Z bosons—the Z-burst mechanism—and its associated absorption and emission phenomenology provide a unique, albeit indirect, probe of the CνB in its present state. In this paper, we examine the implications of gravitational clustering of the CνB in nearby galaxy clusters for the Z-burst phenomenology. In particular, we study the emission features of the Z-decay products originating from the Virgo cluster, and the potential of future cosmic ray experiments to observe clustering-enhanced Z-burst rates. We find that a detector with an exposure equivalent to three years of observations at the Extreme Universe Space Observatory (EUSO) will very likely measure these enhanced rates together with the associated UHECν flux, provided that the latter saturates current observational limits and the neutrino masses are quasi-degenerate, mνi > ∼ 0.1 eV. In the case of UHECν fluxes below the electromagnetic cascade limit, or a hierarchical neutrino mass spectrum, an experimental sensitivity exceeding that of EUSO by at least two orders of magnitude is required to detect the clustering enhancements with any certainty.