Introduction

The intense activity on the conception and realization of large high energy neutrino detectors is motivated by the observation of cosmic ray flux extending up to few #tex2html_wrap_inline496#eV. The origin of these cosmic rays and the nature of mechanisms capable to explain the observed energies and luminosities are presently unknown. In order to progress in the understanding of this phenomenon, it is necessary to extend our knowledge by observing high energy cosmic rays of different natures. Therefore, it is important to study both the energy spectra of #tex2html_wrap_inline498#-rays and of neutrinos. Because these particles have no electric charge they are not deflected by the galactic or by the extra-galactic magnetic field. Several examples of detection of cosmic neutrinos exist: solar neutrinos, below 10MeV, and neutrinos from the supernova SN1987a, above 20Mev. It is of major interest to try to detect signals at higher energies. Several attempts have been made with underground detectors, originally devoted to the detection of proton decay. The modest dimensions of these detectors allowed to determine only upper limits on the neutrino luminosities of several celestial bodies. In order to improve the sensitivity to high energy neutrino fluxes the construction of a detector of a km#tex2html_wrap_inline500# scale is now foreseen. Among the possible sources in the Galaxy X-ray binary systems could emit high energy neutrinos. They consist in a compact body, like a neutron star or a black hole, which accretes the matter of its non compact companion. The presence of a very high magnetic field rotating with high frequency and the presence of plasma flow could yield to particle acceleration. The interaction of the accelerated particles with matter could produce mesons which decay in neutrinos. Another source of high energy neutrinos could be the remnants of young supernovæ. Protons could be accelerated by the explosion wind and, again, the neutrino emission could be due to the interactions of the accelerated protons with matter. The typical time duration of this emission would be of the order of 1-10years. Very high energy neutrinos could be emitted by active galactic nuclei (AGN). These potential sources are supposed to be galaxies containing a black hole in the central region. The acceleration power is supplied by the accretion onto the black hole. The emission of gamma-rays up to #tex2html_wrap_inline502# 10GeV from Active Galactic Nuclei and from galactic sources has been well established a large number of them are reported in the Second EGRET Catalog. Nevertheless, only two AGNs (Mrk 421 and Mrk 501) and two pulsars (PSR1706-44 and the Crab) have been also observed as emitters of high energy gammas (above 300GeV). A possible explanation is that high energy gamma-rays are absorbed by interacting with infra-red and cosmic background radiations. It is worth noting that high energy neutrinos are immune to this phenomenon.