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.