[en] The discovery of high-energy neutrinos of cosmic origin aims at providing an unambiguous signature for acceleration of hadrons and this way contributing to address the question of the origin of cosmic rays. Due to the small cross-section and low expected fluxes this is a difficult task and no indication of cosmic neutrino signals has been collected so far. Here we review the perspectives of opening a new observational window with the establishment of neutrino astrophysics.

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[en] Cosmic rays are particles which originate from space and impinge on the earth's atmosphere with energies spanning over fourteen orders of magnitude. Despite first discovered almost hundred years ago, the origin of cosmic rays is still not completely unveiled. At very high energies (above 10¹⁵ eV) the composition and the spectral behavior are not fully assessed; the sources are not yet unambiguously identified as well and unclear are the underlying acceleration mechanisms. Strong indications are however provided by the evidence of non-thermal emission from several objects, which suggest acceleration of particles in plasma shock waves in the proximity of violent astrophysical environments. With the operation of a new generation of imaging Cherenkov telescopes, enormous progress has been achieved in the last years in the discovery of sources of high energy γ-rays. If originated in the decay of neutral pions, the observed γ-rays can represent the fingerprint of cosmic ray sources. In most cases, however, the observed electromagnetic emission can be attributed to other mechanisms. The discovery of high energy neutrinos of cosmic origin would instead provide an unambiguous signature for acceleration of hadrons, but no indication of cosmic neutrino signals has been collected so far. Here we review the success achieved during the latest years in observing the γ-ray imprint of the universe and the perspectives of opening a new observational window with the establishment of neutrino astrophysics

[en] For the IceCube and MAGIC Collaborations: Fourth generation neutrino telescopes are now being constructed (IceCube) and designed (KM3NET). While no neutrino flux of cosmic origin has been discovered so far, the first signals arc expected to be discerned in the next years. Multi-messenger investigations aim at addressing the problem of extracting these signals from irreducible backgrounds. One application is the search for time correlations of high energy neutrinos and established signals. In this talk we show the first adaptation of a Target of Opportunity strategy to collect simultaneous data of high energy neutrinos and γ-rays. A non-established signal (neutrino) can be used to alert monitoring observations (γ-ray). In some cases the detection of positive coincidences could enhance the discovery chance. More generally the availability of simultaneous observations is increased. In case of positive detection of neutrino signals, this would allow time correlation studies and therefore constraints on the source modeling. A first technical implementation of this scheme involving AMANDA and MAGIC has been realized for few pre-selected sources in a short test run, with the aim of a feasibility study. The principles of the NToO test run and its first outcomes are shown and the physics potential with IceCube discussed. (orig.)

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[en] The IceCube Collaboration has observed a high-energy astrophysical neutrino flux and recently found evidence for neutrino emission from the blazar TXS 0506+056. These results open a new window into the high-energy universe. However, the source or sources of most of the observed flux of astrophysical neutrinos remains uncertain. Here, a search for steady point-like neutrino sources is performed using an unbinned likelihood analysis. The method searches for a spatial accumulation of muon-neutrino events using the very high-statistics sample of about 497,000 neutrinos recorded by IceCube between 2009 and 2017. The median angular resolution is $\sim <!--\; ???\; -->$ 1${}^{\circ <!--\; ???\; -->}$ at 1 TeV and improves to $\sim <!--\; ???\; -->$ 0.3${}^{\circ <!--\; ???\; -->}$ for neutrinos with an energy of 1 PeV. Compared to previous analyses, this search is optimized for point-like neutrino emission with the same flux-characteristics as the observed astrophysical muon-neutrino flux and introduces an improved event-reconstruction and parametrization of the background. The result is an improvement in sensitivity to the muon-neutrino flux compared to the previous analysis of $\sim <!--\; ???\; -->$ 35% assuming an E${}^{-<!--\; ???\; -->2}$ spectrum. The sensitivity on the muon-neutrino flux is at a level of E${}^2$dN/dE = 3 $\cdot <!--\; ???\; -->$ 10${}^{-<!--\; ???\; -->13}$ TeV cm${}^{-<!--\; ???\; -->2}$ s${}^{-<!--\; ???\; -->1}$. No new evidence for neutrino sources is found in a full sky scan and in an a priori candidate source list that is motivated by gamma-ray observations. Furthermore, no significant excesses above background are found from populations of sub-threshold sources. The implications of the non-observation for potential source classes are discussed. (orig.)