HESS J1741−302 ist eine nicht identifizierte Quelle sehr hochenergetischer Gammastrahlen, welche circa 1,7 Grad vom Zentrum der Milchstraße entfernt liegt. Diese Quelle ist eines der schwächsten Objekte im TeV-Bereich mit einem Photonfluss von Φ(>1 TeV) = (1.65 ± 0.28stat ± 0.33sys) × 10^−13 cm^−2 s^−1, was ~1% des Krebsnebelflusses im gleichen Energiebereich entspricht. Die Analyse des aktuellen H.E.S.S. Datensatzes von 145 Stunden Beobachtungen mit hoher Qualität gibt Einblicke in die Morphologie von HESS J1741−302. Das Energiespektrum von HESS J1741−302 geht über 10 TeV hinaus, ohne dabei ein klares Anzeichen für einen spektralen Abbruch zu zeigen. Das Spektrum kann durch ein Potenzgesetz mit einem spektralen Index von Γ = 2.28 ± 0.16stat ± 0.20sys und einer Normierung bei 1 TeV von Φ0 = (2.12 ± 0.42stat ± 0.42sys) × 10^−13 cm^−2 s^−1 TeV^−1 beschrieben werden. In der vorliegenden Arbeit werden verschiedene Szenarien für die beobachtete Gammastrahlung und deren Entstehung in Betracht gezogen. Diese beinhalten die Wechselwirkung von Protonen der kosmischen Strahlung mit Molekülwolken entlang der Sichtlinie, IC Streuung an Infrarot-Photonen eines nahe gelegenen OH/IR Sterns und die Präsenz eines Pulsarwindnebels, welcher möglicherweise zu PSR B1737−30 gehört. / HESS J1741−302 is an unidentified very-high-energy (VHE) γ-ray source located in the Galactic Plane at about 1.7° away from the Galactic Center. It is one of the faintest TeV objects detected so far, with a flux Φ(>1 TeV) = (1.65 ± 0.28stat ± 0.33sys) × 10^−13 cm^−2 s^−1 corresponding to ~ 1% of the Crab Nebula flux at the same energies. The data analysis of an updated high-quality dataset of ~145 hours of VHE H.E.S.S. data taken between 2004 and 2013 has revealed the morphology of HESS J1741−302. The γ-ray spectrum of HESS J1741−302 extends beyond 10 TeV without showing any clear evidence of a cut-off. The source spectrum is well described by a power-law model with a spectral index of Γ = 2.28 ± 0.16stat ± 0.20sys and a normalization at 1 TeV of Φ0 = (2.12 ± 0.42stat ± 0.42sys) × 10^−13 cm^−2 s^−1 TeV^−1. Different scenarios will be considered in this thesis, including the interaction of cosmic-ray protons with molecular clouds found along the line of sight, inverse Compton scattering of infra-red photons provided by a nearby OH/IR star and the presence of a nearby pulsar wind nebula possibly related to PSR B1737−30, in order to explain the observed VHE gamma-ray emission.

Diese Arbeit beschreibt die Identifikation der bisher unidentifizierten TeV Gammastrahlungsquelle, HESS J1303-631 als Pulsarwind-Nebel, angetrieben von dem Pulsar PSR J1301-6305. Dieses Ergebnis wird durch den Nachweis von energieabhängiger Morphologie in den vom High Energy Stereoscopic System (H.E.S.S.) genommenen Daten und durch die Detektion eines neuen Röntgen-Pulsarwind-Nebels in XMM-Newton Daten erreicht. Zudem wird eine obere Schranke auf den Fluss von Radiostrahlung aus Beobachtungen mit dem Parkes Radioteleskop bei 4.48 GHz abgeleitet. Diese Ergebnisse können in einem leptonischen Modell des Pulsarwind-Nebels verstanden werden, wo Elektronen und Positronen in der Nähe des Termination Shocks des Pulsarwindes auf ultrarelativistische Energien beschleunigt werden. Diese Leptonen bilden einen ausgedehnten Pulsarwind-Nebel, der auf Grund des inversen Compton-Effekts und Synchrotronstrahlung TeV Gammastrahlung beziehungsweise Röntgen- und Radiostrahlung erzeugt. Da nur eine obere Grenze auf den Radiofluss abgeleitet wurde, erfolgte die Modellierung im Rahmen eines einfachen ``one zone models'''', wo angenommen wird, dass die Radio-, Röntgen- und Gammastrahlung alle von derselben Leptonenpopulation erzeugt werden. Das Modell wird aber trotzdem von den Daten schon eingeschränkt und liefert ein schwaches Magnetfeld von ungef 0.9 Microgauss. Diese Magnetfeldstärke ist überraschend niedrig, da in ähnlichen Systemen die Magnetfeldstärken eher bei 10 Microgauss liegen. Andererseits passt das Ergebnis gut zu dem sehr niedrigen Synchrotronstrahlungsfluss. Ein derart schwaches Magnetfeld wird im theoretischen Szenario eines ausgedehnten, beziehungsweise entwickelten Pulsarwind-Nebels erklärt. / This work represents the identification of the very high energy, E > 100 GeV (VHE), Gamma-ray source HESS J1303-631 as a pulsar wind nebula (PWN) powered by the pulsar PSR J1301-6305. This is achieved through the detection of energy dependent morphology in the High Energy Stereoscopic System (H.E.S.S.) data, the detection of a new X-ray PWN in archival XMM-Newton X-ray observations, as well as multi-wavelength modeling of the source and its energetics. An upper limit on the radio synchrotron flux is obtained from observations made by the Parkes telescope at 4.48 GHz. The combined Gamma-ray, X-ray and radio measurements are used to constrain a leptonic emission model, where strong winds of relativistic electrons and positrons from the pulsar power the acceleration of particles to ultrarelativistic energies at the wind termination shock region, and these shock accelerated leptons then form a nebula which emits in the X-ray and radio bands via synchrotron emission in the ambient magnetic field and Gamma-rays through the inverse Compton mechanism. One surprising result of this analysis is the anomalously low magnetic field derived for the PWN. Typical values for PWNe are on the order of 10 microgauss. For this source, however, the low synchrotron levels predict an average magnetic field of approximately 0.9 microgauss. The low magnetic field is explained in the scenario of an expanded/evolved PWN.

This study focusses on modelling important aspects of the evolution of pulsar wind nebulae using two different approaches. The first uses a hydrodynamic model to simulate the morphological evolution of a spherically-symmetric composite supernova remnant that is expanding into a homogeneous interstellar medium. In order to extend this model, a magnetic field is included in a kinematic fashion, implying that the reaction of the fluid on the magnetic field is taken into account, while neglecting any counter-reaction of the field on the fluid. This approach is valid provided that the ratio of electromagnetic to particle energy in the nebula is small, or equivalently, for a large plasma β environment. This model therefore allows one to not only calculate the evolution of the convection velocity but also, for example, the evolution of the average magnetic field. The second part of this study focusses on calculating the evolution of the energy spectra of the particles in the nebula using a number of particle evolution models. The first of these is a spatially independent temporal evolution model, similar to the models that can be found in the literature. While spatially independent models are useful, a large part of this study is devoted to developing spatially dependent models based on the Fokker-Planck transport equation. Two such models are developed, the first being a spherically-symmetric model that includes the processes of convection, diffusion, adiabatic losses, as well as the non-thermal energy loss processes of synchrotron radiation and inverse Compton scattering. As the magnetic field geometry can lead to the additional transport process of drift, the previous model is extended to an axisymmetric geometry, thereby allowing one to also include this process. / PhD (Space Physics), North-West University, Potchefstroom Campus, 2014

This study focusses on modelling important aspects of the evolution of pulsar wind nebulae using two different approaches. The first uses a hydrodynamic model to simulate the morphological evolution of a spherically-symmetric composite supernova remnant that is expanding into a homogeneous interstellar medium. In order to extend this model, a magnetic field is included in a kinematic fashion, implying that the reaction of the fluid on the magnetic field is taken into account, while neglecting any counter-reaction of the field on the fluid. This approach is valid provided that the ratio of electromagnetic to particle energy in the nebula is small, or equivalently, for a large plasma β environment. This model therefore allows one to not only calculate the evolution of the convection velocity but also, for example, the evolution of the average magnetic field. The second part of this study focusses on calculating the evolution of the energy spectra of the particles in the nebula using a number of particle evolution models. The first of these is a spatially independent temporal evolution model, similar to the models that can be found in the literature. While spatially independent models are useful, a large part of this study is devoted to developing spatially dependent models based on the Fokker-Planck transport equation. Two such models are developed, the first being a spherically-symmetric model that includes the processes of convection, diffusion, adiabatic losses, as well as the non-thermal energy loss processes of synchrotron radiation and inverse Compton scattering. As the magnetic field geometry can lead to the additional transport process of drift, the previous model is extended to an axisymmetric geometry, thereby allowing one to also include this process. / PhD (Space Physics), North-West University, Potchefstroom Campus, 2014

This thesis concerns with the study of X-ray binaries which are gravitationally bound systems consisting of a compact object (either a neutron star or a black hole) and usually a non degenerate companion star, both rotating around the common centre of mass. The compact star shines brightly in the X-ray regime. Emission from these systems are powered by accretion which is the most radioactively efficient mechanism known in the universe by the release of gravitational potential energy when matter from the companion star falls on the compact object. Accretion onto high magnetic field neutron stars are special as the magnetic field plays a crucial role in governing the dynamics of gas flow and the flow of the matter close to the compact object. The radiation emitted from these systems are anisotropic and for a distant observer, the intensity is modulated at the spin period of the neutron star, hence these objects are called accretion powered pulsars. The angular pattern of the emitted radiation is also highly anisotropic and depends on the mass accreted and hence the luminosity. The beaming pattern commonly known as the pulse profiles exhibit a wide variety in the pulse shape and pulse fraction and vary with energy as well as intensity. They also exhibit cyclotron absorption features in their energy spectrum which are a direct probe to the magnetic field geometry of these systems. This thesis is dedicated to the study of the magnetic field and emission geometry of accretion powered pulsars through the pulse phase resolved studies of the cyclotron absorption features which are a direct probe of the magnetized plasma. In order to study these features in detail broadband continuum modeling of the energy spectrum is done, taking care of all other factors which may smear the pulse phase dependence. Another prerequisite for detailed continuum modeling is accounting for the low absorption dips in the pulse profiles of many these sources. The dips are presumably formed by phase locked accretion stream causing partial covering absorption when the stream is along our line of sight towards the emission region. Studying the pulse phase dependence of this partial covering absorber also provides us with important clues on the local environment of the neutron star and the structure of the accretion stream. All of these studies are performed with data from the broadband and most sensitive instruments onboard the Japanese satellite Suzuki. Lastly we provide estimates of the polarization expected to be detected from these sources by a Thomson scattering polarimeter being developed to observe the polarization of X-rays in the energy range of 5--30 keV. Along with the X-ray pulsars, we also make an estimate of the likelihood of detection of X-ray polarization from black hole X-ray binaries in different spectral states. This is a particularly interesting topic as it will play a crucial role in providing additional handles on the magnetic field geometry in accretion powered pulsars as well as constrain the fundamental parameters of a black hole like its spin.

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