Astrophys. J., 729, 90 (2011/March-2)
Gamma-ray emission from the globular clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752.
TAM P.H.T., KONG A.K.H., HUI C.Y., CHENG K.S., LI C. and LU T.-N.
Abstract (from CDS):
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, NGC 6624, and NGC 6752, and evidence of 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 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 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.
gamma rays: stars - globular clusters: individual (Liller 1, M80, NGC 6139, NGC 6441, NGC 6541, NGC 6624, NGC 6752) - pulsars: general
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