The diffuse relic neutrinos with light mass are transparent to Ultrahigh energy (UHE) neutrinos at thousands EeV, born by photoproduction of pions by UHE protons on relic 2.73 K BBR radiation and originated in AGNs at cosmic distances. However these UHE νs may interact with those (mainly heaviest ν µr , ν τr and respective antineutrinos) clustered into HDM galactic halos. UHE photons or protons, secondaries of νν r scattering, might be the final observed signature of such high-energy chain reactions and may be responsible of the highest extragalactic cosmic-ray (CR) events. The chain-reactions conversion efficiency, ramifications and energetics are considered for the October 1991 CR event at 320 EeV observed by the Fly's Eye detector in Utah. These quantities seem compatible with the distance, direction and power (observed at MeV gamma energies) of the Seyfert galaxy MCG 8-11-11. The νν r interaction probability is favoured by at least three order of magnitude with respect to a direct ν scattering onto the Earth atmosphere. Therefore, it may better explain the extragalactic origin of the puzzling 320 EeV event, while offering indirect evidence of a hot dark galactic halo of light (i .e., m ν ∼ tens eV) neutrinos, probably of tau flavour.

Most of the known literature on Inverse Compton Scattering (ICS) is based on earliest theoretical attempts and later approximations led by F.C.Jones and J.B.Blumenthal. We found an independent and more general analytical procedure which provide both relativistic and ultrarelativistic limits for ICS. These new analytical expressions can be derived in a straightforward way and they contain the previously reminded Jones' results. Our detailed solutions may be probed by already existing as well future ICS experiments.

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