The Origin of Galactic TeV Gamma Ray Sources

Abstract

The High Energy Stereoscopic System (H.E.S.S.) is a ground-based telescope which detects gigaelectronvot (GeV) and teraelectronvolt (TeV) -rays. Many TeV -ray sources are associated with high-energy environments such as pulsar wind nebulae (PWNe) and supernova remnants (SNRs). However, the majority of these TeV -ray sources are ‘unidentified’ or ‘dark’ sources as no clear counterpart to be powering the -rays has been discovered. One of these ‘unidentified’ TeV -ray sources is HESS J1804−216, which is the focus of this thesis. HESS J1804−216 has several plausible counterparts investigated in detail in this thesis. It is vital to have an understanding of the interstellar medium (ISM) surrounding a very high energy (VHE) Galactic -ray source as it can lead to constraining the nature of the source. Several scenarios for the origin of VHE -ray emission from HESS J1804−216 have been considered. For a hadronic scenario, SNRG8.7−0.1 is a plausible candidate for the acceleration of CR protons. Sufficient interstellar gas as a target for CR collisions and -ray production is present in the corresponding velocity component. Assuming an isotropic diffusion model, the CR proton spectra from SNRG8.7−0.1 showed that this interpretation requires slow diffusion to match the observed values. For an alternate hadronic scenario, the undetected progenitor SNR of PSR J1803−2137 is another promising candidate. This is due to the derived CR enhancement factors for this case matching the -ray observations well. For a leptonic scenario, the TeV emission is produced by highly energetic electrons from PSR J1803−2137 as a PWNe. A TeV -ray efficiency of 3% supports the PWN scenario from an energetics point of view. Expanding on this further, modelling of the diffusive energy-dependent escape of CR protons and interaction with the surrounding ISM in the hadronic scenario is performed for the two plausible SNR counterparts. The spatial and spectral distributions of CRs are generated for a range of model parameters, describing, amongst others, the diffusion and the injection spectrum of CRs. Gamma-ray morphology maps and spectral distributions are created based on these CR distributions and the ISM distribution for the two nearby plausible CR accelerators: SNRG8.7−0.1 and the progenitor SNR of PSR J1803−2137. Finally, all the available H.E.S.S. data is analysed to produce spatial morphology maps and spectral distributions. The analysis shows that the spectrum of HESS J1804−216 extends up to tens of TeV with a cutoff of Ecutoff = 24.7±18.4TeV. This is indicative of a PeVatron source. The morphology of -ray emission towards HESS J1804−216 is investigated over different energy bands to perform an energydependent morphology study. Both PWNe and SNR scenarios are investigated for any energy-dependent morphology. The PWNe scenario is expected to exhibit clear energy-dependent morphology, which is not consistent with the morphology from this study. No clear evidence of energy-dependent morphology was found, as supported by statistical tests. The observations for the SNR scenarios showed no compelling evidence of energy-dependence. The modelled -ray emission, however, indicated the presence of energy-dependent morphology. Therefore, the observations and model for the SNR scenario were significantly different from one another.