During (1) nearly past 50 years, the empirical search for neutrinos has spanned roughly six orders of magnitude in energy, from approximately 10-3 GeV up to approximately 103 GeV. The lower energy edge corresponds to solar neutrinos, whereas the upper energy edge corresponds to atmospheric neutrinos. The intermediate energy range include the terrestrial and supernova neutrinos. This search has already given us remarkable insight into neutrino interaction properties as well as its intrinsic properties such as mixing and mass. Here, I briefly review the possibility of having neutrinos with energy greater than 103 GeV. The upper energy edge for high energy astrophysical neutrinos is limited only by the concerned experiments. A main motivation to search for such high energy astrophysical neutrinos is to get more accurate information about the origin of observed high energy photons (and ultra high energy cosmic rays) that is presently not possible through conventional gamma ray astronomy. For instance, the observation of high energy gamma ray flux alone from active centers of nearby galaxies (AGNs) such as M 87 and distant sites of gamma ray bursts (GRBs) does not allow us to identify its origin in purely electromagnetic or purely hadronic interactions unambiguously. Sizable high energy astrophysical neutrino flux is expected if latter interactions are to play a dominant role. Search for high energy astrophysical neutrinos will thus provide us a complementary and yet unexplored view about some of the highest energy phenomenons occurring in the known universe. For a general introduction of the subject of high energy astrophysical neutrinos, see Bahcall & Halzen (1996), Protheroe (1999), Bahcall (2001). See, also Battiston (2002).
1 Talk given at IAU 8th Asian Pacific Regional Meeting, 2-5 July, 2002, Tokyo, Japan. Back.