Astrophysical Neutrinos and Blazars

TL;DR

This paper reviews recent evidence linking astrophysical neutrinos to blazars, especially IHBLs, highlighting their distinct properties and potential as neutrino sources due to high-energy proton acceleration.

Contribution

It identifies two main blazar sub-classes, LBLs and IHBLs, and provides evidence suggesting IHBLs are more likely neutrino counterparts, based on multi-frequency data analysis.

Findings

IHBLs show properties consistent with neutrino emission.

Evidence favors IHBLs over LBLs as neutrino sources.

Distinct spectral and polarization features differentiate blazar sub-classes.

Abstract

We review and discuss recent results on the search for correlations between astrophysical neutrinos and gamma-ray-detected sources, with many extra-galactic studies reporting potential associations with different types of blazars. We investigate possible dependencies on blazar sub-classes by using the largest catalogues and all the multi-frequency data available. Through the study of similarities and differences in these sources we conclude that blazars come in two distinct flavors: LBLs and IHBLs (low-energy-peaked and intermediate-high-energy-peaked objects). These are distinguished by widely different properties such as the overall spectral energy distribution shape, jet speed, cosmological evolution, broad-band spectral variability, and optical polarization properties. Although blazars of all types have been proposed as neutrino sources, evidence is accumulating in favor of IHBLs being the counterparts of astrophysical neutrinos. If this is indeed the case, we argue that the peculiar observational properties of IHBLs may be indirectly related to proton acceleration to very high energies.