Study of Pulsar Wind Nebulae in Very-High-Energy gamma-rays with H.E.S.S.

Pulsar wind nebulae consist of magnetised clouds of positrons and electrons accelerated to very high energies through the action of a central pulsar, often embedded within a host supernova remnant. They are the largest population of firmly identified sources of TeV gamma-rays within the Galaxy and are thought to be contributors to the leptonic Galactic cosmic-ray spectrum,and thus viable source candidates in the quest to interpret the origin of the cosmic-ray positron fraction excess. Their very-high-energy gamma-ray emission is of particular interest for spectral models, as their dominant radiation process is inverse Compton scattering on target photons present in interstellar radiation fields such as the visible, near and far infrared ambient photons in addition to the Cosmic Microwave Background.In this thesis dissertation I present the scope of my research work, which lies on pulsar wind nebula morphology and understanding their very-high-energy radiation.I report the latest morphological and spectral studies of the pulsar wind nebula within the MSH 15-52 composite supernova remnant through High Energy Stereoscopic System (H.E.S.S.) observations.In this phenomenological study I have conducted a fit of the very-high-energy emission morphology beyond ~ 0.3 TeV detected in HESS-I data using a template X-ray synchrotron map in the 4–7 keV band based on archival Chandra observations. In the model of the emission, the gamma-ray emission produced by inverse Compton scattering is thought to ensue from the same leptonic population responsible for the observed synchrotron emission, and thus the X-ray template represents the spatial distribution of these electrons and positrons, convolved with the spatial dependence of the magnetic field. Our best-fit results yield an additional extended TeV component located on the south-eastern region of the nebula, centered at ∼4 pc from the position of the associated pulsar PSR B1509-58, with an intrinsic radius of ∼7′(or 9 pc). We also detect a significant steepening in the spectral shape of the total emission from the pulsar wind nebula, occurring above∼10 TeV. This coincides with a shrinking of the emission observed in the sky map, as revealed by our energy-dependent morphological analyses. Several scenarii are presented so as to explain our morphological and spectral results concerning the emission originating from MSH 15-52. Based on the Galactic radiation field characteristics derived from published models, the gamma-ray emission is well described by leptons scattering on the far infrared and cosmic microwave backgrounds, respectively at lower and higher energies than the detected spectral steepening energy, which suggests an interpretation of the energy dependence as a consequence of Klein-Nishina effects in the cross-section.During the last year of my thesis I have worked with collaborators on a study of drivers behind the observed significant offsets of TeV-emitting pulsar wind nebulae with respect to their pulsar. We performed relativistic (magneto)-hydrodynamical numerical simulations and tested physical setups in one-dimensional studies and are pursuing an ongoing two-dimensional investigation to quantify the effect of the pulsar proper motion in comparison to ambient medium density gradients so as to derive constrains on these physical factors leading to asymmetrical evolution in pulsar wind nebulae.