Understanding the gamma-ray emission from Globular Clusters

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.