Constraining the baryon loading factor of AGN jets: implication from the gamma-ray emission of the Coma cluster

TL;DR

This study uses gamma-ray observations of the Coma cluster to constrain the baryon loading factor of AGN jets, providing new limits on cosmic ray acceleration efficiency in galaxy clusters.

Contribution

It introduces a novel method to constrain AGN jet baryon loading factors using gamma-ray data from galaxy clusters, linking cosmic ray physics with gamma-ray observations.

Findings

Upper limit of baryon loading factor is < 0.1.

Constraints are tighter than those needed for diffuse neutrino flux models.

Gamma-ray data effectively limits cosmic ray injection in galaxy clusters.

Abstract

High-energy cosmic rays (CRs) can be accelerated in the relativistic jets of Active Galactic Nuclei (AGNs) powered by supermassive black holes. The baryon loading efficiency onto relativistic CR baryons from the accreting black holes is poorly constrained by observations so far. In this paper, we suggest that the $\gamma$-ray emission of galaxy clusters can be used to study the baryon loading factor of AGN jets, since CRs injected by AGN jets are completely confined in the galaxy clusters and sufficiently interact with intra-cluster medium via hadronic process, producing diffuse $\gamma$-rays. We study the propagation of CRs in the galaxy clusters and calculate the radial distribution of the gamma-rays in the galaxy cluster with different injection rates from AGNs. By comparison with the $\gamma$-ray flux and upper limits of the Coma cluster measured by $Fermi$-LAT and VERITAS, we find the upper limit of the average baryon loading factor {(defined as the efficiency with which the gravitational energy is converted into relativistic particles)} to be $\eta_{p, \mathrm{grav}} < 0.1$. The upper limit is much lower than that required to account for diffuse neutrino flux in the conventional blazar models.