Origin of high energy neutrinos and multi-messenger connections

Andrea Palladino


The discovery of a diffuse flux of high energy neutrinos, provided by IceCube in 2013, has opened a new era in the field of astroparticle physics. Up to now only 1 of the IceCube neutrinos has a clear counterpart, identified in the blazar TXS0506+056, thanks to the coincident observations of a high energy neutrino and γ-rays from a flaring blazar in the same time window. Therefore all the other neutrinos remain still without any satisfying counterpart, although there are still several candidate sources.
The first candidate source is represented by blazars, that are the brightest objects in the γ-ray sky above 100 GeV. However the absence of correlations between the arrival directions of neutrinos and the positions of resolved blazars strongly constrains the contribution of these sources, that cannot contribute more than ∼ 20% according to the IceCube collaboration. Another natural candidate is represented by Starburst Galaxies, that are also the second contributors to the γ-ray flux in the GeV energy range. However in this case the flux of γ-rays associated to neutrinos would be too high and this constrains the contribution of these objects at level 10%-15%. What can we learn from these results? Is there really a crisis in finding the source of high energy neutrinos or alternative hypotheses on the production mechanisms have to be considered?