Active Galactic Nuclei as sources of the diffuse neutrino background

Abstract

The detection of a high-energy (~290 TeV) neutrino, IceCube-170922A, by IceCube on September 22, 2017 was in directional and temporal coincidence with the blazar TXS0506+056. The discovery of this first neutrino source indicates that blazars are possible sources of high-energy neutrinos. Blazars are active galactic nuclei with the relativistic jet pointing towards the Earth. Neutrino production occurs in the jet because of hadronic processes. Since IceCube has previously detected a diffuse high-energy neutrino background, we try to model the contribution of the blazar population to it. Blazars are classiffed into two types, BL Lac objects and FSRQs. We study the neutrino flux of the population of each class of blazars separately, considering that all sources emit as TXS0506+056, with a lepto-hadronic modelling neutrino flux, and assuming the number density of BL Lacs and FSRQs derived from the Fermi Large Area Telescope data, respectively. The diffuse emission of the BL Lac objects fits in with the IceCube data, while the FSRQs' one is much higher than the observed neutrino flux. This can be explained if the neutrino production in FSRQs is somehow suppressed, suggesting an intrinsic difference between the two blazar subclasses. Furthermore, we study the cosmic ray diffuse emission of the BL Lac objects and we conclude that only a small fraction of protons can escape efficiently from the relativistic jet of the blazars