Extreme blazars as counterparts of IceCube astrophysical neutrinos

TL;DR

This study finds a potential link between extreme blazars and IceCube astrophysical neutrinos, suggesting that a significant fraction of neutrinos may originate from these high-energy gamma-ray sources, with implications for future neutrino detection.

Contribution

The paper introduces a new observable, $N_{ u}$, and provides the first evidence of a correlation between extreme blazars and IceCube neutrinos, indicating a possible source connection.

Findings

Positive fluctuation of $N_{ u}$ at 0.4-1.3% significance for extreme blazars.

Approximately 10-20% of IceCube neutrinos may originate from extreme blazars.

Identification of about 5 candidate sources linked to neutrinos.

Abstract

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, $N_{\nu}$. In all three catalogues we consistently observe a positive fluctuation of $N_{\nu}$ 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 $\sim 10 - 20$ per cent. An investigation of the hybrid photon -- neutrino spectral energy distributions of the most likely candidates reveals a set of $\approx 5$ 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.