On the sources of high energy neutrinos

The discovery of a diffuse flux of high energy neutrinos has opened a new era in the field of neutrino astronomy. Up to now only one high energy neutrino has an identified astrophysical counterpart, the blazar TXS 0506+056. However the origin of the diffuse neutrino flux remains still a mystery, even if many possible explanations have been proposed in the last few years. The most natural hypothesis was that high energy neutrinos are produced by blazars, since these powerful objects dominate the γ -ray sky above 100 TeV. However the IceCube stacking limit shows that resolved blazars cannot contribute more than 20%. Other natural sources are the ones rich of gas, in which the proton-proton interaction dominates. In this scenario an issue would be the over-production of γ -rays associated to neutrinos, if the neutrino spectrum were too soft. In this work we summarize the present knowledge and we discuss the role of low luminosity BL Lacs, showing that it is still possible to power the sub-PeV neutrino flux with blazars. Moreover we also discuss the role of pp sources, showing that they are still into the game and they can saturate the sub-PeV neutrino emission, giving also a contribution larger than 50% in the energy range between 10 TeV and 100 TeV.