Modelling globular clusters as multi-wavelength diffuse emitters

Abstract

The future ground-based Cherenkov Telescope Array (CTA) is expected to detect more than half of the known Galactic globular cluster (GC) population. In this thesis I present results from an emission code that assumes millisecond pulsars (MSPs) to be sources of relativistic particles in GCs that will give broad-band radiation due to their interaction with the cluster magnetic and soft-photon fields. I perform a parameter study to investigate the GC model's behaviour and study the detectability of Galactic GCs for the High Energy Stereoscopic System (H.E.S.S.) and CTA. H.E.S.S. has plausibly1 detected only one GC (Terzan 5) at 5:3σ significance that has an unexpectedly asymmetric VHE morphology, offset from the optical source position. I therefore present new Fermi Large Area Telescope (LAT) data on this source and model its updated broadband spectral energy distribution (SED). I furthermore derive constraints on the embedded MSPs' luminosity function. Stacking upper limits by H.E.S.S. on the -ray ux of a population of 15 Galactic GCs are very constraining for leptonic emission models. I therefore show that uncertainty in model parameters leads to a large spread in the predicted ux, and there are indeed regions in parameter space for which the stacking upper limits are satisfied. It is thus important to increase measurement accuracy on key model parameters in order to improve predictions of cluster uxes so as to better guide the observational strategy of CTA. The various applications considered in this thesis indicate that continued multiwavelength observations of GCs will facilitate model refnement, and this will represent a substantial contribution to the Galactic science of CTA and that of satellites operating in lower-energy bands.