The Fundamental Plane of Gamma-ray Globular Clusters

We have investigated the properties of a group of gamma-ray-emitting globular clusters (GCs) that have recently been uncovered in our Galaxy. By correlating the observed gamma-ray luminosity L-gamma with various cluster properties, we probe the origin of the high-energy photons from these GCs. We report that L-gamma is positively correlated with the encounter rate Gamma(c) and the metallicity [Fe/H], which places an intimate link between the gamma-ray emission and the millisecond-pulsar population. We also find a tendency that L-gamma increases with the energy densities of the soft photon at the cluster location. Furthermore, the two-dimensional regression analysis suggests that L-gamma, soft-photon densities, and Gamma(c)/[Fe/H] possibly span fundamental planes; this finding could potentially provide better predictions for the gamma-ray properties of GCs.