With the completion of the first cubic-kilometer class neutrino telescopes, IceCube, the race for the discovery of the first cosmic high-energy neutrino sources enters into a new phase. The usage of neutrinos as cosmic messengers has the potential to significantly enhance and extend our knowledge on Galactic and extragalactic sources of the high-energy universe. This article gives a short review on the status of neutrino telescopes and their sensitivities concentrating on point-like sources. It discusses the current upper limits on neutrino emissions and their implications for models of different source classes.
Up to now, information on objects in our galaxy and beyond has nearly exclusively been obtained using electromagnetic waves as cosmic messengers. In addition to this "electromagnetic" information, we know from measurements of the cosmic-ray spectrum that there exist sources in the universe which accelerate protons or heavier nuclei up to energies of ∼ 10 20 eV, 10 7 times higher than the most energetic man-made accelerator, the LHC at CERN. These highest energies are believed to be reached in extragalactic sources like gamma-ray bursts (GRBs) or active galactic nuclei (AGNs) whereas Galactic sources like supernova remnants (SNRs) or micro-quasars are thought to accelerate particles at least up to energies of 3 × 10 15 eV. However, despite the detailed measurements of the cosmic-ray spectrum and 100 years after their discovery by Victor Hess, we still do not know what the sources of the cosmic rays are as they are deflected in the Galactic and extragalactic magnetic fields and hence have lost all information about their origin when reaching Earth. Only at the highest energies beyond ∼ 10 19. 6 GeV cosmic rays may retain enough directional information to locate their sources.
Alternative messengers for locating the sources of the cosmic rays must have two distinct properties: they have to be electrically neutral and essentially stable. Only two of the known elementary particles meet these requirements: photons and neutrinos. Both particles are inevitably produced when the accelerated protons or nuclei collide with matter or photons inside or near the sources. In these reactions neutral and charged pions are produced which then decay into highenergy photons and neutrinos, respectively. However, only high-energy neutrinos are a smokinggun evidence for the sources of cosmic rays as TeV photons are also produced in the up-scattering of photons in reactions with accelerated electrons (inverse-Compton scattering).
In order to detect the low fluxes of cosmic neutrinos, large volumes of natural transparent media like ice or water have to be instrumented with a three-dimensional array of optical sensors (photomultipliers). Neutrinos are reconstructed by detecting the arrival time and intensity of Cherenkov light from charged secondary particles, which are produced in interactions of the neutrinos with the nuclei in the medium. Two basic event topologies can be distinguished: track-like patterns of detected Cherenkov light (hits) which originate from muons produced in charged-current interactions of muon neutrinos (muon channel); spherical hit patterns which originate from the hadronic cascade at the vertex of neutrino interactions or the electromagnetic cascade of electrons from charged current interactions of electron neutrinos (cascade channel). If the charged current interaction happens inside the detector or in case of charged current tau-neutrino interactions, these two topologies overlap which complicates the reconstruction.
Because of the long lever arm of the muon hit-pattern, the direction of muons can be reconstructed significantly better than that of cascades reaching ∼ 0.1 • for cubic-kilometer sized detectors at high energies. At the relevant energies, the neutrino is approximately collinear with the muon and, hence, the muon channel is the prime channel for the search for point-like sources of cosmic neutrinos. On the other hand, cascades deposit all of their energy inside the detector and therefore allow for a much better energy reconstruction with a resolution of a few 10%. Figure 1: Upper part: Sensitivities of neutrino telescopes at 90% CL to a E -2 neutrino flux as a function of the source declination: ANTARES (dotted, predicted 1 yr) [1], ANTARES 304 days (dashed-dotted) [2], IceCube 86 strings (dashed, predicted 1 yr) [3], IceCube 40+59 strings (dashed-dotted-dotted, 375 + 348 d) [4], KM3NeT (solid, predicted 1 yr), [5]. Lower part: declination of Galactic objects with observed TeV gamma-ray emission. The position of the Galactic Center is marked with a star.
Currently, the most sensitive neutrino telescope in the Northern Hemisphere is the ANTARES detector, installed off the coast of South France in the Mediterranean Sea at a depth of 2500 m. In the Southern hemisphere, the currently worldwide largest and hence most sensitive neutrino telescope, IceCube, was completed in Dec. 2011. With an instrumented volume of 1 km 3 it is the first cubic-kilometer-class detector and marks a preliminary high point in neutrino telescope development, which have increased their sensitivity by a factor of 1000 in only 15 years. The sensitivities of IceCube and ANTARES to an E -2 neutrino flux from a point-like source are shown in Fig. 1. Source fluxes are expected to lie in the region below 10 -12 TeV cm -2 s -1 . It is apparent that in order to achieve a high-sensitivity coverage over the full sky a cubic-kilometer class detector in the Northern Hemisphere is mandatory which will cover most of the Galactic pla
This content is AI-processed based on open access ArXiv data.
