Constraining baryon loading efficiency of AGNs with diffuse neutrino flux from galaxy clusters

TL;DR

This paper constrains the efficiency of baryon loading in AGNs by analyzing diffuse neutrino flux from galaxy clusters, providing tighter limits than previous gamma-ray observations, and enhancing understanding of cosmic ray acceleration.

Contribution

It introduces a method to constrain the baryon loading factor in AGNs using diffuse neutrino flux data from galaxy clusters, improving existing limits.

Findings

Upper limit on baryon loading factor: η_p,grav ≲ 2×10⁻³ to 0.1

Neutrino flux constraints are more stringent than gamma-ray limits for the Coma cluster

Provides new insights into cosmic ray acceleration efficiency in AGNs

Abstract

The active galactic nuclei (AGNs) are widely believed to be one of the promising acceleration sites of ultrahigh-energy cosmic rays (CRs). Essentially, AGNs are powered by the gravitational energy of matter falling to supermassive black holes. However, the conversion efficiency of gravitational to kinetic energy of CRs in AGNs, which is defined as baryon loading factor $\eta_p$, is not well known yet. After being accelerated, high-energy CRs could escape the host galaxy and enter the intra-cluster medium (ICM). These CRs can be confined within the galaxy cluster and produce $\gamma$-rays and neutrinos through proton-proton collisions with the ICM. In this paper, we study the diffusion of CRs in galaxy clusters and calculate the diffuse neutrino flux from galaxy cluster population. Using the latest upper limits on the cumulative unresolved TeV-PeV neutrino flux from galaxy clusters posed by the IceCube Neutrino Observatory, we derive the upper limit of the average baryon loading factor as $\eta_{p,\mathrm{grav}} \lesssim 2 \times 10^{-3} - 0.1$ for the population of galaxy clusters. This constraint is more stringent than the one obtained from $\gamma$-ray observation on the Coma cluster.