The constant flux of cosmic rays that bombard Earth from within our own galaxy are understood to come from both shell-type supernova remnants and pulsar wind nebulae (PWNe). Multiwavelength study of these objects can help us to understand what types of particles are accelerated, and gamma-ray emission is key to understanding the highest energy cosmic rays. In this thesis, I analyze and interpret observations made with the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a gamma-ray telescope located in Southern Arizona. LS 5039 and HESS J1825-137 occupy the same field of view on the sky and were observed for about 8 hours with VERITAS. LS 5039 is a gamma-ray binary, and the observations supports theories that the compact object hosts a PWN which is continuously interacting with the nearby star. HESS J1825-137 is a very extended PWN with an extent of diameter greater than 1 degree on the sky. Using the VERITAS observations, I am able to measure the radial profile and compare the gamma-ray luminosity to other PWN. DA 495, a "Crab-like" PWN with unusually strong magnetic fields, was observed for about 70 hours with VERITAS. In this study, results are combined with radio and X-ray spectral information to allow for detailed astrophysical modeling of the region. This broadband spectral modeling places constraints on the properties of the particle population in this PWN and allows for both leptonic and hadronic emission scenarios to be evaluated. Hadronic scenarios instil doubt on the pure PWN interpretation and favor a previously undetected shell-type remnant being present.

Compact Objects (Neutron Stars) form in the last moments of a star's life, during the violent events known as supernovae. As the star's core fusion falters, matter undergoes a dramatic gravitational compression resulting in internal densities rivaling subatomic particles. Ever since their discovery in the mid-twentieth century, these highly magnetized and rapidly rotating balls of condensed matter have provided a bountiful playground for astronomers seeking out exotic physics. Neutron Stars that emit electromagnetic radiation are seen by observers as Pulsars, named such for the pulse of intensity as the pulsar's radiation beam passes into our line of sight. These beams are comprised of two unique regions with differing phenomenology; core emission that arises close to the pulsar polar cap and centered within the radiation beam, and higher altitude conal emission that lies along the beam's periphery. While pulsars can and do emit over a wide frequency range, most known pulsars are seen as radio sources, at sensitivities where studies of the pulsar single-pulses allows us to probe the rich details of the plasma-filled pulsar magnetosphere. Even then, the radio emission often has a steep spectra, restricting the frequencies in which radio telescopes can study pulsars. We have utilized the unmatched sensitivity of Arecibo Observatory to conduct a multi-frequency single-pulse survey, between 327 MHz and the novel 4.5 GHz, of Arecibo's brightest high-frequency pulsars. The broad frequency range and single-pulse resolutions have allowed us to set accurate beam classifications for these nearly two dozen pulsars while extending constraints on important population trends to higher frequencies. Several of the pulsars in our survey exhibit deviant behavior, and are thus useful as followup case studies to further our understanding of pulsar radio emission. One of the most interesting cases involves pulsar B0823+26, where we find evidence for an age-dependent death-line separating core and conal dominated pulsars, suggesting that the plasma generating capabilities of a pulsar changes as they age. For the other three, they fall in the ``Partial Cone'' class; a type of pulsar that is characterized by strong delays in their emission. We find that all three of these pulsars show evidence of core emission.

The main topic of this thesis is analysis of an unidentified Galactic TeV gamma-ray source, MGRO J1908+06, discovered by Milagro instrument in 2007. We analyzed 54 hours of observational data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a ground-based gamma-ray observatory in southern Arizona comprised of an array of four Cherenkov Telescopes that reconstructs the energy and direction of astrophysical gamma-rays by imaging Cherenkov light emitted by energetic particles in air showers produced by the primary gamma-rays. MGRO J1908+06 is located between a supernova remnant SNR G40.5-0.5 and a young, energetic pulsar PSR J1907+0602. We studied the energy dependent morphology of the TeV emission from the source and measured the source extent and spectrum. The source extends well past the boundary of the SNR and is not correlated with strong radio continuum or molecular line emission which likely excludes an origin for the emission as solely due to the SNR. While emission in the 0.5-1.25 TeV band was centered around the pulsar, higher energy emission was observed near the supernova remnant. This morphology is opposite that observed in other pulsar wind nebulae. We proposed two models for the high energy emission located well away from the pulsar but close to the SNR: (1) shock acceleration at the shock front created by an interaction between the pulsar wind and the dense gas at the edge of the SNR or (2) molecular clouds around the SNR provides seed photons with energies higher than those from Cosmic Microwave Backgrounds for inverse Compton scattering. The former model can be tested by looking for molecular emission lines that trace shocks and by measuring the pulsar velocity. In addition, we investigated the gamma-ray emission from the nova explosion of V407 Cygni that occurred in March 2010. The Fermi-LAT observed this event in the energy range of E >100 MeV. The origins of the gamma-ray emission that the Fermi-LAT team proposed are either protons (hadronic model) or electrons (leptonic model), both of which were accelerated at the nova shock via the Fermi acceleration mechanism. We did not consider their leptonic model because no TeV gamma-ray emission is predicted. Their hadronic model can generate TeV gamma-rays with the modeled parameters. We found no evidence for TeV emission. We showed that with the flux upper limit calculated using the VERITAS data imposes constraints on the extension of the proton spectrum at high energies.

Nous tentons dans cette thèse d?interpréter les Extreme Scattering Events (ESE?s) par la turbulence naturelle du milieu interstellaire (MIS) ionisé. Il est bien connu que les inhomogénéités du MIS ionisé suivent un spectre de puissance de Kolmogorov. Dans l?approximation de l?écran mince, on montre d?abord en optique géométrique qu?on peut produire le phénomène d?ESE. On effectue ensuite le calcul dans le cadre de la diffraction de Fresnel. On obtient des ?pseudo?-ESEs qui ne sont pas très profonds et n?apparaissent pas au même moment aux trois fréquences simulées comme dans le cas des observations du pulsar B1937+21 au radiotélescope de Nançay. On montre par contre que les paramètres obtenus de la scintillation simulée sont en très bon accord avec les observations et la théorie. Cette étude conduite sur un ?ALPHA server? est une première étape nécessaire et importante dans l?étude de la scintillation interstellaire.

The aim of this study is to determine the free electron density distribution of the Milky Way Galaxy using dispersion measures of pulsars. By making use of 1893 Galactic pulsar, 274 supernova remnant and 543 HII region data, the overall free electron density map of the Galaxy is obtained by using a 3D mesh-like structure of irregular size. The main idea behind the study is to treat each 3D section of the Galaxy privately considering the distance versus dispersion measure graphs of the pulsars that fall into those sections. This sectioning procedure is followed using a trial and error method and results in 348 sections through which free electron densities can be calculated. Using linear fits of distance versus dispersion measure graphs, pulsars that deviate from the curves are investigated and new distances are adopted to 140 of them that are decided to have wrong distance estimates. By this way both distance values and the free electron densities of the sections are improved. In the end, by using the free electron density values of 348 sections, a projected and cumulative free electron density map of the Galaxy is plotted in polar coordinates. This map is compared with three different spiral arm models and it is seen that the best accordance is with Hou et al. 2009 spiral arm model.

Pulsars, as rotating magnetised neutron stars got much attention during the last 40 years since their discovery. Observations revealed them to be gamma-ray emitters with energies continuing up to the sub 100 GeV region. Better observation of this upper energy cut-off region will serve to enhance our theoretical understanding of pulsars and neutron stars. The H-test has been used the most extensively in the latest periodicity searches, whereas other tests have limited applications and are unsuited for pulsar searches. If the probability distribution of a test statistic is not accurately known, it is possible that, after searching through many trials, a probability for uniformity can be given, which is much smaller than the real value, possibly leading to false detections. The problem with the H-test is that one must obtain the distribution by simulation and cannot do so analytically. For such simulations, random numbers are needed and are usually obtained by utilising so-called pseudo-random number generators, which are not truly random. This immediately renders such generators as useless for the simulation of the distribution of the H-test. Alternatively there exists hardware random number generators, but such devices, apart from always being slow, are also expensive, large and most still don't exhibit the true random nature required. This was the motivation behind the development of a hardware random number generator which provides truly random U(0,l) numbers at very high speed and at low cost The development of and results obtained by such a generator are discussed. The device delivered statistically truly random numbers and was already used in a small simulation of the H-test distribution. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.

Les travaux présentés dans ce mémoire s'inscrivent dans le cadre de la détection d'évènements impulsionnels intermittents en provenance de pulsars. Ces objets astrophysiques sont des étoiles à neutrons hautement magnétisées en rotation rapide, qui émettent un faisceau radio balayant l'espace comme la lentille d'un phare. Ils sont détectables grâce à une instrumentation spécifique. Depuis quelques années, on a découvert de nouvelles catégories de ces pulsars, aux caractéristiques extrêmes, avec en particulier des impulsions individuelles plus intenses et irrégulières comparé à la moyenne. Il faut pouvoir les détecter en temps réel dans un environnement radio perturbé à cause des signaux de télécommunications. Cette étude propose des algorithmes de traitement d'interférences radio fréquence (RFI) adaptés à ce contexte. Plusieurs méthodes de traitement de RFI sont présentées et comparées. Parmi elles, deux ont été retenues et comparées au moyen de simulations Monte Carlo, avec un jeu de paramètres simulant le pulsar et un signal BPSK avec des puissances et des durées différentes. Pour la recherche de nouveaux pulsars, une méthode alternative est proposée (SIPSFAR), combinant capacité de recherche en temps réel et robustesse contre les RFI. Elle est basée sur la transformée de Fourier 2D et la transformée de Radon. Une étude comparative théorique a permis de confronter et comparer la sensibilité de cette nouvelle méthode avec celle communément utilisée par les radioastronomes. La méthode a été implantée sur un GPU GTX285 et testée sur un grand relevé du ciel effectué au radiotélescope de Nançay. Les résultats obtenus ont donné lieu à une comparaison statistique complémentaire à partir de données réelles.

Pulsars, as rotating magnetised neutron stars got much attention during the last 40 years since their discovery. Observations revealed them to be gamma-ray emitters with energies continuing up to the sub 100 GeV region. Better observation of this upper energy cut-off region will serve to enhance our theoretical understanding of pulsars and neutron stars. The H-test has been used the most extensively in the latest periodicity searches, whereas other tests have limited applications and are unsuited for pulsar searches. If the probability distribution of a test statistic is not accurately known, it is possible that, after searching through many trials, a probability for uniformity can be given, which is much smaller than the real value, possibly leading to false detections. The problem with the H-test is that one must obtain the distribution by simulation and cannot do so analytically. For such simulations, random numbers are needed and are usually obtained by utilising so-called pseudo-random number generators, which are not truly random. This immediately renders such generators as useless for the simulation of the distribution of the H-test. Alternatively there exists hardware random number generators, but such devices, apart from always being slow, are also expensive, large and most still don't exhibit the true random nature required. This was the motivation behind the development of a hardware random number generator which provides truly random U(0,l) numbers at very high speed and at low cost The development of and results obtained by such a generator are discussed. The device delivered statistically truly random numbers and was already used in a small simulation of the H-test distribution. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.

Pulsar wind nebulae (PWNe), nebulae harbouring a rotation-powered neutron star that was born in a supernova, provide opportunities to study highly relativistic pulsar winds and their interaction with the surrounding medium. Particularly interesting are PWNe that do not show any sign of the expected surrounding SNR shell and were thought to be born in subenergetic explosions or with unusual progenitors. The detection of a shell around one such PWN suggested that shells are indeed produced but may be faint due to unseen shocked ejecta, a low density environment, and/or a young age that has not yet allowed the shell to brighten and become visible. Here, by using observational X-ray data from modern telescopes with excellent spatial and energy resolution (Chandra and XMM-Newton), we target PWNe that do not have prominent SNR shells, and are known to be in varied environments, to further explore the characteristics of this growing, but poorly explored, class of PWNe. By combining imaging and spectroscopic results, we study the morphology of the PWNe, search for thermal emission from shock-heated material, investigate the energetics of the nebulae, and search for candidates for the neutron stars powering the nebulae. We find that while the faint shell surrounding G21.5–0.9 can be explained as a young PWN evolving in a low density medium, CTB 87 (G74.9+1.2) appears to be in an advanced stage of evolution, and G63.7+1.1 appears to be both in an advanced stage of evolution and in a dense environment. By performing spatially resolved spectroscopy, we have shown how the spectral characteristics vary across the PWNe, and note that more data will place better constraints on possible thermal emission in these remnants. The imaging portion of these studies has revealed intriguing large-scale morphologies for CTB 87 and G63.7+1.1, as well as a torus-jet structure in CTB 87 and neutron star candidates in both CTB 87 and G63.7+1.1. We conclude that both CTB 87 and G63.7+1.1 are likely interacting with the supernova remnant reverse shock, and CTB 87 may be additionally influenced by the motion of its neutron star.

Searches for pulsars during the past fifty years, have been characterised by two problems making their discovery difficult: i) an increasing volume of data to be searched, and ii) an increasing number of `candidate' pulsar detections arising from that data, requiring analysis. Whilst almost all are caused by noise or interference, these are often indistinguishable from real pulsar detections. Deciding which candidates should be studied is therefore difficult. Indeed it has become known as the `candidate selection problem'. This thesis presents an interdisciplinary study of the selection problem, with the aim of developing a new method able to mitigate it. Specifically for future pulsar surveys undertaken with the Square kilometre Array (SKA). Through a combination of critical literature evaluations, theoretical modelling exercises, and empirical investigations, the selection problem is described in-depth here for the first time. It is shown to be characterised by the dominance of Gaussian distributed noise signals, a factor that no existing selection method accounts for. It also reveals the presence of a significant trend in survey data rates, which suggest that candidate selection is transitioning from an off-line processing procedure, to an on-line, and real-time, decision making process. In response, a new real-time machine learning based method, the GH-VFDT, is introduced in this thesis. The results presented here show that a significant improvement in selection performance can be achieved using the GH-VFDT, which utilises a learning procedure optimised for data characterised by skewed class distributions. Whilst the principled development of new numerical features that maximise the separation between pulsars and Gaussian noise, have also greatly improved GH-VFDT pulsar recall. It is therefore concluded that the sub-optimal performance of existing selection systems, is due to a combination of poor feature design, insensitivity to noise, and an inability to deal with skewed class distributions.