Gamma - ray emission from the globular clusters

Globular clusters (GCs) are emerging as a new class of γ-ray emitters, thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now, eight GCs are known to emit γ-rays at energies >100 MeV. Based on the stellar encounter rate of the GCs, we identify potential γ-ray emitting GCs out of all known GCs that have not been studied in details before. In this paper, we report the discovery of a number of new γ-ray GCs: Liller 1, NGC 6624, and NGC 6752, and evidence for γ-ray emission from M80, NGC 6139, and NGC 6541, in which γ-rays were found within the GC tidal radius. With one of the highest metallicity among all GCs in the Milky Way, the γ-ray luminosity of Liller 1 is found to be the highest of all known γ-ray GCs. In addition, we confirm a previous report of significant γ-ray emitting region next to NGC 6441. We briefly discuss the observed offset of γ-rays from some GC cores. The increasing number of known γ-ray GCs at distances out to ∼10 kpc is important for us to understand the γ-ray emitting mechanism and provides an alternative probe to the underlying millisecond pulsar populations of the GCs. Subject headings: Gamma rays: stars — Globular clusters: individual (Liller 1, M80, NGC 6139, NGC 6441, NGC 6541, NGC 6624, and NGC 6752) — Pulsars: general