CANGAROO-III search for TeV Gamma-rays from two clusters of galaxies

TL;DR

This study used the CANGAROO-III telescope to search for TeV gamma-ray emission from galaxy clusters Abell 3667 and Abell 4038, setting upper limits and exploring implications for magnetic fields and cosmic-ray energy densities.

Contribution

First observational constraints on gamma-ray emission from galaxy clusters using CANGAROO-III, providing limits on magnetic fields and cosmic-ray energy densities.

Findings

No significant gamma-ray excess detected.

Lower limit of ~0.1 micro G for magnetic field in Abell 3667.

Cosmic-ray energy density upper limit exceeds that of the Milky Way.

Abstract

Because accretion and merger shocks in clusters of galaxies may accelerate particles to high energies, clusters are candidate sites for the origin of ultra-high-energy (UHE) cosmic-rays. A prediction was presented for gamma-ray emission from a cluster of galaxies at a detectable level with the current generation of imaging atmospheric Cherenkov telescopes. The gamma-ray emission was produced via inverse Compton upscattering of cosmic microwave background (CMB) photons by electron-positron pairs generated by collisions of UHE cosmic rays in the cluster. We observed two clusters of galaxies, Abell 3667 and Abell 4038, searching for very-high-energy gamma-ray emission with the CANGAROO-III atmospheric Cherenkov telescope system in 2006. The analysis showed no significant excess around these clusters, yielding upper limits on the gamma-ray emission. From a comparison of the upper limit for the north-west radio relic region of Abell 3667 with a model prediction, we derive a lower limit for the magnetic field of the region of ~0.1 micro G. This shows the potential of gamma-ray observations in studies of the cluster environment. We also discuss the flux upper limit from cluster center regions using a model of gamma-ray emission from neutral pions produced in hadronic collisions of cosmic-ray protons with the intra-cluster medium (ICM). The derived upper limit of the cosmic-ray energy density within this framework is an order of magnitude higher than that of our Galaxy.