Extreme blazars as counterparts of IceCube astrophysical neutrinos

We explore the correlation of gamma-ray emitting blazars with IceCube neutrinos by using three very recently completed, and independently built, catalogues and the latest neutrino lists. We introduce a new observable, namely the number of neutrino events with at least one gamma-ray counterpart, N<SUB>nu</SUB>. In all three catalogues we consistently observe a positive fluctuation of N<SUB>nu</SUB> with respect to the mean random expectation at a significance level of 0.4-1.3 per cent. This applies only to extreme blazars, namely strong, very high energy gamma-ray sources of the high energy peaked type, and implies a model-independent fraction of the current IceCube signal ˜10-20 per cent. An investigation of the hybrid photon - neutrino spectral energy distributions of the most likely candidates reveals a set of ≈5 such sources, which could be linked to the corresponding IceCube neutrinos. Other types of blazars, when testable, give null correlation results. Although we could not perform a similar correlation study for Galactic sources, we have also identified two (further) strong Galactic gamma-ray sources as most probable counterparts of IceCube neutrinos through their hybrid spectral energy distributions. We have reasons to believe that our blazar results are not constrained by the gamma-ray samples but by the neutrino statistics, which means that the detection of more astrophysical neutrinos could turn this first hint into a discovery.