The contribution of Fermi-2LAC blazars to the diffuse TeV-PeV neutrino flux

TL;DR

This study uses IceCube data to analyze the contribution of 2LAC blazars to the diffuse high-energy neutrino flux, setting upper limits and constraining models of neutrino emission from these sources.

Contribution

It provides the first large-scale likelihood analysis of cumulative neutrino emission from the entire 2LAC blazar population, setting new upper limits on their contribution.

Findings

2LAC blazars contribute less than 27% to the observed neutrino flux between 10 TeV and 2 PeV.

The analysis excludes blazars emitting more than 50% of neutrinos up to a spectral index of -2.2.

The results constrain models predicting neutrino emission from blazars.

Abstract

The recent discovery of a diffuse cosmic neutrino flux extending up to PeV energies raises the question of which astrophysical sources generate this signal. One class of extragalactic sources which may produce such high-energy neutrinos are blazars. We present a likelihood analysis searching for cumulative neutrino emission from blazars in the 2nd Fermi-LAT AGN catalogue (2LAC) using an IceCube neutrino dataset 2009-12 which was optimised for the detection of individual sources. In contrast to previous searches with IceCube, the populations investigated contain up to hundreds of sources, the largest one being the entire blazar sample in the 2LAC catalogue. No significant excess is observed and upper limits for the cumulative flux from these populations are obtained. These constrain the maximum contribution of the 2LAC blazars to the observed astrophysical neutrino flux to be $27 \%$ or less between around 10 TeV and 2 PeV, assuming equipartition of flavours at Earth and a single power-law spectrum with a spectral index of $-2.5$. We can still exclude that the 2LAC blazars (and sub-populations) emit more than $50 \%$ of the observed neutrinos up to a spectral index as hard as $-2.2$ in the same energy range. Our result takes into account that the neutrino source count distribution is unknown, and it does not assume strict proportionality of the neutrino flux to the measured 2LAC $\gamma$-ray signal for each source. Additionally, we constrain recent models for neutrino emission by blazars.