Neutrinos are subatomic particles, very common yet elusive and enigmatic. They are produced by the decay of radioactive elements and they are almost massless, carrying no electric charge and interacting with other particles through the weak force. In the Universe, they can be created by some of Nature’s most energetic and violent phenomena, such as black holes and massive exploding stars. The detection of these ”ghostly” and very high-energetic particles, although being challenging because of their low cross section, opens a whole window on the Universe thus marking an important era in astronomy. Being neutral particles, they carry spatial information: they can indeed travel straight from the source to the Earth without being deflected by magnetic fields or being absorbed, thus providing information about the position of the astrophysical (exotic) source (mechanism) that has generated them. The primary aim of experiments as ANTARES and IceCube is to use neutrinos as cosmic messengers to study the particle acceleration mechanisms in energetic astrophysical objects such as Active Galactic Nuclei (AGNs) and Gamma-Ray Bursts (GRBs), among others. Neutrinos may also shed light on dark matter, if they are produced in the decay of gravitationally captured dark matter particles in the Earth or in the Sun cores.

 

Currently, the ongoing neutrino telescopes are IceCube [2], which is placed in the deep South Pole ice, and ANTARES [3], which is an underwater neutrino telescope in the Mediterranean Sea.

 

 

As stated before, the detection of neutrinos is a challenging task, since the Earth acts as a shield against all particles except neutrinos. Therefore, a neutrino telescope uses the detection of upward-going muons as a signature of muon neutrino interactions in the matter below the detector. ANTARES is a Cherenkov neutrino detector in sea water operating since 2008, optimized for the detection of muons from high-energy astrophysical neutrinos. Its location at a depth of 2475 m in the Mediterranean Sea allows to use the Earth as a filter when observing the Southern hemisphere, and therefore the sensitivities will be higher for neutrinos (with energies below 100 TeV) arriving from this region of the sky, in which many galactic sources lie.

 

IceCube is a much larger telescope, with a cubic kilometer volume, located in the Antarctic ice at depths ranging from 1450 m to 2450 m below the surface. Its larger size allows IceCube to collect a much larger amount of data compared to ANTARES, although, being located at the South Pole, it is not shielded against the large background of atmospheric muons from the Southern sky, resulting into a lower sensitivity for low-energy neutrinos.

 

The fact that the two experiments differ so much in size and location allows them to provide independent and complementary measurements of the neutrino flux, so that their combination is almost two times more precise than the individual results.

 

 

There are several theoretical proposals for the origin of high energy neutrinos, that include sources of both galactic and extra-galactic origin, such as AGN, GRBs, but even microquasars or supernova remnants. In order to test these models it is necessary to trace back where the neutrinos are coming from and verify whether the reconstructed trajectories point to any particular direction in the sky. Both ANTARES and IceCube can extract the needed spatial information from their data and, in this way, they can scan the full Southern sky looking for points that significantly stand out from the background. Both collaborations have performed this analysis separately [4,5], and then released the first results using their combined datasets [1], which enjoys a higher precision. The analysis is performed exploiting two different techniques: an unbiased search over the full Southern sky (see Fig. 1) and a targeted test of 40 pre-identified candidate sources. Unfortunately, no significant point-like source has been found, meaning that the resolution of these experiments is still too low to conclude that high-energy cosmic neutrinos are emitted by compact, identifiable objects. Nonetheless, this non-observation could be translated into an upper bound on the cosmic neutrino flux as a function of the angular position of a hypothetical source in the sky. Since the dependence of this flux on the neutrino energy is not established (there are several models suggesting different possible profiles), the bounds have been derived choosing two different hypotheses: one proportional to E^-2  and the other to E^-2.5. Figure 2 shows the result for the former case.

 

In the next few years, as new data will be collected, the sharpness of these great “eyes” into the sky will certainly improve, and we can hope that they will soon be able to bring a few neutrino sources into focus.

 

 

 

Figure 1: Skymap of the statistical analysis for the combined ANTARES 2007-2012 and IceCube 40, 59, 79 point-source analyses. The red circle indicates the location of the most significant cluster.

 

 

Figure 2: 90% CL sensitivities (blue, red) and limits (green) for the neutrino emission from point sources as a function of source declination in the sky, for an assumed energy spectrum of the source. As reference, the declination of the Galactic Center is approximately at sin(δ = 29º)≃ −0.48.

 

 

Text by Ilaria Brivio and Valentina De Romeri

 

References

[1] S. Adrian-Martinez et al. [ANTARES and IceCube Collaborations], arXiv:1511.02149 [hep- ex]. 

[2] A- Achterberg et al. [IceCube Collaboration], Astropart. Phys 26 (2006) 155, arXiv:0604450 [astro-ph]. 

[3] M. Ageron et al. [ANTARES Collaboration], Nucl. Instrum. Meth. A656 (2011) 11, arXiv:1104.1607 [astro-ph.IM] 

[4] G. Aartsen et al. [IceCube Collaboration], The Astrophysical Journal 779 (2013) 132, arXiv:1307.6669 [astro-ph] 

[5] A. Adrian-Martinez et al. [ANTARES Collaboration], The Astrophysical Journal Letters 786 (2014) L5, arXiv:1402.6182 [astro-ph]