Mon. Not. R. Astron. Soc., 480, 4782-4796 (2018/November-2)
Gamma-ray emission from high Galactic latitude globular clusters.
LLOYD S.J., CHADWICK P.M. and BROWN A.M.
Abstract (from CDS):
We analyse 8 yr 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 gamma-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.
© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
astroparticle physics - pulsars: general - globular clusters: general - gamma-rays: general
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