Interpretation of the diffuse astrophysical neutrino flux in terms of the blazar sequence

Abstract

We study if the diffuse astrophysical neutrino flux can come from blazar jets -- a subclass of Active Galactic Nuclei (AGNs) -- while it, at the same time, respects the blazar stacking limit based on source catalogs and is consistent with the observation from TXS 0506+056. We compute the neutrino flux from resolved and unresolved sources using an averaged, empirical relationship between electromagnetic spectrum and luminosity, known as the {\em blazar sequence}, for two populations of blazars (BL Lacs and FSRQs). Using a source model with realistic neutrino flux computations, we demonstrate that blazars can indeed power the diffuse neutrino flux at the highest energies and obey the stacking limit at the same time, and we derive the conditions for the baryonic loading (proton versus $\gamma$-ray luminosity) evolving over the blazar sequence. Under the hypothesis that low-luminosity blazars power the diffuse astrophysical neutrino flux, we find that the dominant contribution of the diffuse flux up to PeV energies must come from unresolved BL Lacs with baryonic loadings larger than about $10^5$ -- while only a very small contribution may come from resolved high-luminosity BL Lacs or FSRQs, which can be directly tested by the stacking limit. We find that the blazar TXS 0506+056 is on the verge of these populations in our baseline scenario, at a relatively high luminosity and redshift; as a consequence we predict about 0.3 $\gamma$-ray-neutrino associations per year from the whole population, dominated by BL Lacs with $L_\gamma \simeq 10^{45} \, \mathrm{erg/s}$ and $z \sim 0.1$.