Neutrino Constrains to the Diffuse Gamma-Ray Emission from Accretion Shocks

TL;DR

This paper uses IceCube neutrino data to constrain gamma-ray emission from accretion shocks in large scale structure formation, limiting their contribution to the extragalactic gamma-ray background and providing insights into cosmic ray acceleration.

Contribution

It presents the strongest constraints to gamma-ray emission from accretion shocks by linking neutrino observations to gamma-ray production, assuming an extragalactic origin of IceCube neutrinos.

Findings

Accretion shocks can contribute at most ~20% to the gamma-ray background.

High-energy neutrino flux constrains cosmic ray acceleration efficiency in galaxy clusters.

Neutrino data limits gamma-ray emission models of large scale structure formation.

Abstract

Accretion of gas during the large scale structure formation has been thought to give rise to shocks that can accelerate cosmic rays. This process then results in an isotropic extragalactic gamma-ray emission contributing to the extragalactic gamma-ray background observed by the Fermi-LAT. Unfortunately this emission has been difficult to constrain and thus presents an uncertain foreground to any attempts to extract potential dark matter signal. Recently, IceCube has detected high-energy isotropic neutrino flux which could be of an extragalactic origin. In general, neutrinos can be linked to gamma rays since cosmic-ray interactions produce neutral and charged pions where neutral pions decay into gamma rays, while charged pions decay to give neutrinos. By assuming that isotropic high-energy IceCube neutrinos are entirely produced by cosmic rays accelerated in accretion shocks during the process of structure formation, we obtain the strongest constraint to the gamma-ray emission from large scale structure formation (strong) shocks and find that they can make at best ~20% of the extragalactic gamma-ray background, corresponding to neutrino flux with spectral index 2, or ~10% for spectral index 2.46. Since typical objects where cosmic rays are accelerated in accretion shocks are galaxy clusters, observed high-energy neutrino fluxes can then be used to determine the gamma-ray emission of a dominant cluster type and constrain acceleration efficiency, and thus probe the process of large scale structure formation.