Gamma-ray emission from the globular clusters liller 1, M80, NGC6139, NGC6541, NGC6624, and NGC6752

Abstract

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 detail before. In this paper, we report the discovery of a number of new γ-ray GCs: Liller 1, NGC6624, and NGC6752, and evidence of γ-ray emission from M80, NGC6139, and NGC6541, in which γ-rays were found within the GC tidal radius. With one of the highest metallicities 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 a significant γ-ray emitting region next to NGC6441. 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. © 2011. The American Astronomical Society. All rights reserved.