Understanding the gamma-ray
emission from Globular
Clusters

Abstract

Observations with the Large Area Telescope (LAT) on-board the Fermi Gamma-ray
Space Telescope have led to the discovery of gamma-ray emission from many Globular
Clusters. The high stellar encounter rates in Globular Clusters favour the formation
of millisecond pulsars, which have been postulated as the source of the gamma-ray
emission. However, the evidence for this is indirect and there have been limited studies
attempting detailed spectral comparison between Globular Clusters and their millisecond
pulsar populations. The detection of pulsed gamma-ray emission would constitute direct
evidence of millisecond pulsars contributing to the Globular Cluster emission.
In this work, I use 13 years of Fermi-LAT data to perform a detailed spectral analysis of 6 Globular Clusters where no pulsed gamma-ray emission has been reported, but
millisecond pulsars have been detected using radio telescopes. I also analyse the Globular Cluster NGC 6624, where pulsed gamma-ray emission has already been observed
and find that a single millisecond pulsar can account for the majority of NGC 6624’s
gamma-ray emission over 13 years. I then find no evidence of any statistically significant pulsed emission in any of the Globular Clusters I selected. Based on these findings
and motivated by previous studies, I consider the possibility of dark matter annihilation contributing to the Globular Cluster emission. I perform likelihood fits of three
scenarios for the Globular Cluster emission: dark matter only, millisecond pulsars only,
and a combination of millisecond pulsars and dark matter. I use these results to place
upper limits on the millisecond pulsar contribution to the gamma-ray emission in each
Globular Cluster. I find evidence that the millisecond pulsars in some of the Globular
Clusters are fainter than those in the Galactic neighbourhood. Finally, I conclude that a
dark matter component cannot be ruled out for any of the Globular Clusters I studied.