# Can BL Lac emission explain the neutrinos above 0.2 PeV?

## TL;DR

This study tests whether BL Lac objects can explain the highest energy neutrinos detected by IceCube, finding that the hypothesis is disfavored at 3.7 sigma based on correlation analysis.

## Contribution

It provides a statistical analysis of the correlation between IceCube neutrino events and BL Lacs, challenging the hypothesis that BL Lacs are the main sources of the highest-energy neutrinos.

## Key findings

- Only 1 out of expected 10.2 correlated events was observed.
- The null result is not statistically significant yet.
- BL Lacs are disfavored as primary sources at 3.7 sigma.

## Abstract

Multi-messenger astronomy can help to investigate the sources of the high-energy neutrinos observed by the high-energy neutrino telescope IceCube. We consider the hypothesis that the highest energy neutrinos are produced by BL Lacs, arguing that this is not contradicted severely by any known fact. We check the BL Lac hypothesis by searching for correlations between the through-going muon events of IceCube and the BL Lacs of the second catalog of Fermi-LAT (2FHL). We expect 10.2 $\pm$ 2.4 correlated events but we find that just 1 event has a BL Lac as counterpart. We also assess the probability of observing one multiplet from the same source, finding that the present null result is not yet of critical significance. We conclude that the hypothesis that the BL Lacs are the main emitters of the highest-energy neutrinos observed by IceCube is disfavored at 3.7{\sigma}. We discuss implications and possible ways out; for example, this could work if the angular resolution was 4{\deg}, which is much more than expected.