Gamma-ray emission from high Galactic latitude globular clusters

Lloyd, Sheridan J.; Chadwick, Paula M.; Brown, Anthony M.

Abstract

We analyse 8 years of PASS 8 Fermi-LAT data, in the 60 MeV - 300 GeV energy range, from 30 high Galactic latitude globular clusters. Six of these globular clusters are detected with a TS > 25, with NGC 6254 being detected as gamma-ray bright for the first time. The most significant detection is of the well-known globular cluster 47 Tuc, and we produce a refined spectral fit for this object with a log parabola model. NGC 6093, NGC 6752 and NGC 6254 are fitted with hard, flat power law models, NGC 7078 is best fitted with a soft power law and NGC 6218 is best fitted with a hard, broken power law. This variety of spectral models suggests that there is a variety of γ-ray source types within globular clusters, in addition to the traditional millisecond pulsar interpretation. We identify a correspondence between diffuse X-ray emission in globular cluster cores and gamma-ray emission. This connection suggests that gamma-ray emission in globular clusters could also arise from unresolved X-ray sources or a relativistic electron population, perhaps generated by the millisecond pulsars. X-ray observations of further gamma-ray bright globular clusters would allow a functional relationship to be determined between diffuse X-ray and gamma-ray emission.

Citation

Lloyd, S. J., Chadwick, P. M., & Brown, A. M. (2018). Gamma-ray emission from high Galactic latitude globular clusters. Monthly Notices of the Royal Astronomical Society, 480(4), 4782-4796. https://doi.org/10.1093/mnras/sty2150

• Journal Article Type: Article
• Acceptance Date: Aug 6, 2018
• Online Publication Date: Aug 9, 2018
• Publication Date: Nov 11, 2018
• Deposit Date: Aug 15, 2018
• Publicly Available Date: Aug 22, 2018
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 480
• Issue: 4
• Pages: 4782-4796
• DOI: https://doi.org/10.1093/mnras/sty2150
• Public URL: https://durham-repository.worktribe.com/output/1322918
• Related Public URLs: https://arxiv.org/abs/1808.03792