Gamma-ray Emission from the Globular Clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752

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