A Cross-correlation Study between IceCube Neutrino Events and the Fermi Unresolved Gamma-ray Sky

TL;DR

This study investigates the correlation between IceCube neutrino events and the unresolved gamma-ray background from Fermi LAT data, setting upper limits on the contribution of unresolved sources to the observed neutrino flux.

Contribution

It presents the first cross-correlation analysis between IceCube neutrinos and the unresolved gamma-ray background, providing new constraints on source populations contributing to high-energy neutrinos.

Findings

Up to 60% of the unresolved gamma-ray source population may contribute to IceCube neutrinos.

No significant correlation was detected between neutrino events and the unresolved gamma-ray background.

The maximum contribution to the neutrino flux from unresolved sources is about 1% at 100 TeV.

Abstract

With the coincident detections of electromagnetic radiation together with gravitational waves (GW170817) or neutrinos (TXS 0506+056), the new era of multimessenger astrophysics has begun. Of particular interest are the searches for correlation between the high-energy astrophysical neutrinos detected by the IceCube Observatory and gamma-ray photons detected by the Fermi Large Area Telescope (LAT). So far, only sources detected by the LAT have been considered in correlation with IceCube neutrinos, neglecting any emission from sources too faint to be resolved individually. Here, we present the first cross-correlation analysis considering the unresolved gamma-ray background (UGRB) and IceCube events. We perform a thorough sensitivity study and, given the lack of identified correlation, we place upper limits on the fraction of the observed neutrinos that would be produced in proton-proton or proton-gamma interactions from the population of sources contributing to the UGRB emission and dominating its spatial anisotropy (aka blazars). Our analysis suggests that, under the assumption that there is no intrinsic cutoff and/or hardening of the spectrum above Fermi-LAT energies, and that all gamma-rays from the unresolved blazars dominating the UGRB fluctuation field are produced by neutral pions from p-p (p-gamma) interactions, up to 60% (30%) of such population may contribute to the total neutrino events observed by IceCube. This translates into a O(1%) maximum contribution to the astrophysical high-energy neutrino flux observed by IceCube at 100 TeV.