CANGAROO-III Search for TeV Gamma-rays from Two Clusters of Galaxies

As 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. Recently, a prediction was presented for gamma-ray emission from a cluster of galaxies at a detectable level with modern 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 have 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. By comparing the upper limit for the north-west radio relic region of Abell 3667 with the model prediction, we can derive a lower limit of the magnetic field of the region of ∼ 0.1μG. This shows the potential of gamma-ray observations in studies of the cluster environment. We also discuss the upper limit from cluster center regions with a model of gamma-ray emission from neutral pion 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 was one order higher than that of our Galaxy.