Neutron stars are amongst the most exotic objects known in the universe; more than a solar mass of material is squeezed into an object the size of a city, leading to a density comparable to that of an atomic nucleus. They have a surface magnetic field which is typically around a trillion times stronger than the magnetic field here on Earth, and we have observed them to spin up to around 700 times per second. The existence of neutron stars was first proposed by Baade and Zwicky in 1934 but later graduated from theory to fact in 1967 as the first pulses were detected by Jocelyn Bell-Burnell, a then graduate student at the University of Cambridge. There are now well over 2000 neutron stars whose radio emission beams point at, and have been detected on Earth. We call these objects pulsars. Because of their remarkable properties, pulsars are very useful to physicists, who can employ them as precision timing tools due to the unwavering nature of their emission and of their rotation. Having an array of ultra-accurate clocks scattered throughout our galaxy is very useful for performing astrophysical experiments. In particular, precise pulsar timing measurements and the models that explain them, will permit the direct detection of gravitational radiation; a stochastic background initially, and potentially the individual signals from supermassive black hole binaries. Our models of pulsar behaviour are so precise that we are now able to notice even slight departures from them; we are starting to see that unmodelled variability in pulsars occurs over a broad range of timescales, both in emission and in rotation. Any unmodelled variability is, of course, detrimental to the pulsar's utility as a precision timing tool, and presents a problem when looking for the faint effects of a passing gravitational wave. We are hoping that pulsar timing arrays will detect gravitational radiation in the coming decade, but this depends, in part, on our ability to understand and mitigate the effects of the unmodelled intrinsic instabilities that we are observing. One important clue as to the nature of the variability in pulsar emission and rotation, is the emerging relationship between the two; we sometimes observe correlation on timescales of months and years. We have been observing pulsars for almost fifty years and our expanding datasets now document decades of pulsar behaviour. This gives us the ability to investigate pulsar variability on a range of timescales and to gain an insight into the physical processes that govern these enigmatic objects. In this thesis I describe new techniques to detect and analyse the emission and rotational variability of radio pulsars. We have employed these techniques on a 24 year pulsar dataset to unearth a striking new example of a dramatic and simultaneous shift in a pulsar's emission and rotation. We hypothesise that this event was caused by an asteroid interaction, although other explanations are also possible. Our variability techniques have also been used to analyse data from 168 young, energetic pulsars. In this thesis we present results from the nine most interesting. Of these, we have found some level of correlated variability in seven, one of which displays it very strongly. We have also assessed the emission stability of the NANOGrav millisecond pulsars and have found differing degrees of variability, due to both instrumental and astrophysical causes. Finally, we propose a method of probing the relationship between emission and rotation on short-timescales and, using a simulation, we have shown the conditions under which this is possible. Throughout the work, we address the variability in pulsar emission, rotation and links between the two, with the aim of improving pulsar timing, attaining a consolidated understanding of the diverse variable phenomena observed and elucidating the evolutionary path taken by pulsars.

The Neutron-star Interior Composition Explorer is an X-ray astrophysics payload that will be placed on the International Space Station. Its primary science goal is to measure with high accuracy the pulse profiles that arise from the non-uniform thermal surface emission of rotation-powered pulsars. Modeling general relativistic effects on the profiles will lead to measuring the radii of these neutron stars and to constraining their equation of state. Achieving this goal will depend, among other things, on accurate knowledge of the source, sky, and instrument backgrounds. We use here simple analytic estimates to quantify the level at which these backgrounds need to be known in order for the upcoming measurements to provide significant constraints on the properties of neutron stars. We show that, even in the minimal-information scenario, knowledge of the background at a few percent level for a background-to-source countrate ratio of 0.2 allows for a measurement of the neutron star compactness to better than 10% uncertainty for most of the parameter space. These constraints improve further when more realistic assumptions are made about the neutron star emission and spin, and when additional information about the source itself, such as its mass or distance, are incorporated.

The aim of this thesis is to characterise a sample of High Mass X-ray Binaries (HMXBs) formed by: IGR J00370+6122, XTE J1855-026, AX J1841.0-0535 and AX J1845.0-0433. These objects are composed of pulsars (rotating neutron stars) accreting material from the wind of their supergiant companions. The X-rays are produced in the interaction of the accreted material with the strong gravitational field of the neutron star that accelerates this material and heats it up to ~ 107 K. The study of HMXBs has strong implications in several areas of Physics and Astrophysics. They contain neutron stars whose study is essential to constrain the equation of state of nuclear dense matter, and provides insights on the astrophysical models of core collapse and Supernovae explosions. HMXBs considered as a population give information on the properties of the galaxy. In addition they are excellent test-beds to study accretion physics and outflows. The X-ray behaviour of these systems determines the class of system (classical HMXBs, Supergiant Fast X-ray Transients, Be/X-ray Binaries). The differences in the X-ray emission are supposed to be due to the different properties of the binary systems, such as the orbital properties, the magnetic field of the neutron star or the spectral type of the donor star. HMXBs in this thesis are wind-fed systems, therefore, the properties of the wind (which depend on the spectral type) and the interaction of this wind with the gravitational field of the compact object are key elements to understand the X-ray emission. Therefore, in this thesis an orbital solution for each target of study has been determined using optical spectra of the donor star. Moreover, to check if wind variability is related to the orbit of the binary system, analysis of Ha variations have been carried out. Furthermore, in the case of IGR J00370+6122 and XTE J1855-026 we have obtained an atmosphere model for each of the donor stars allowing us to characterise the atmospheres of these stars, and consequently to determine physical parameters such as the Teff or the log g. Finally publicly available X-ray light curves have been analysed to study the X-ray emission of the different sources against their orbital periods. As a general conclusion, it seems there is a continuum of properties of these systems more than a strict classification. A combination of factors, of which some of them could be unknown, might be the cause of their different X-ray flux behaviours. The outline of this thesis is as follows: the scientific context is given in Chapter 1 an overview of the analysis performed for each of the sources of study is presented in Chapter 2; Chapter 3 is dedicated to the description of a pipeline optimised for the reduction of FRODOSpec spectra of obscured red sources (donor stars of the targets of study); Chapters (4, 5 and 6) present the characterization of the four sources in this thesis, which are different kind of wind-fed systems; and finally general conclusions and future work are given in Chapter 7.

This thesis considers the various morphologies of radio supernova remnants (SNRs), and attempts to determine whether their appearance results from the properties of the progenitor star and its supernova explosion, or from the structure of the interstellar medium (ISM) and ambient magnetic field into which a SNR consequently expands. High-resolution observations of Supernova 1987A show a young remnant whose appearance and evolution are completely dominated by the structure of its progenitor wind. A statistical study of the Galactic population of bilateral SNRs demonstrates that the symmetry axes of these remnants run parallel to the Galactic Plane. This result can be explained by the interaction of main sequence stellar wind-bubbles with the ambient magnetic field; expansion of SNRs into the resulting elongated cavities results in a bilateral appearance with the observed alignment. Radio observations of SNR G296.8-00.3 show a double-ringed morphology which is best explained by expansion either into an anisotropic main-sequence progenitor wind or into multiple cavities in the ISM. Data on SNRs G309.2-00.6 and G320.4-01.2 (MSH 15-52) make a strong case that the appearance of both remnants is significantly affected by collimated outflows from a central source; for G309.2-00.6 the source itself is not detected, but for G320.4-01.2 there is now compelling evidence that the remnant is associated with and is interacting with the young pulsar PSR B1509-58. I conclude that, while the youngest SNRs are shaped by their progenitor's circumstellar material, the appearance of most SNRs reflects the properties of the local ISM and magnetic field. Remnants which interact with an associated pulsar or binary system appear to be rare, and are easily distinguished by their unusual and distorted morphologies.

Le Large Area Telescope à bord du satellite Fermi, lancé le 11 juin 2008, est un télescope spatial observant l'univers des hautes énergies. L'instrument couvre l'intervalle en énergie de 20MeV à 300 GeV avec une sensibilité nettement améliorée et la capacité de localiser des sources ponctuelles. Il détecte les photons gamma par leur conversion en paire électron-positron, et mesure leur direction et leur énergie grâce à un trajectographe et un calorimètre. Cette thèse présente les courbes de lumières et les mesures spectrales résolues en phase des pulsars radio et gamma détectés par le LAT. La mesure des paramètres spectraux (flux, indice spectral, et énergie de coupure) dépend des fonctions de réponse de l'instrument (IRFs). Une méthode développée pour la validation en orbite de la surface ecace est présentée en utilisant le pulsar de Vela. Les efficacités des coupures entre les données du LAT et les données simulées sont comparées à chaque niveau de la rejection du fond. Les résultats de cette analyse sont propagés vers les IRFs pour évaluer les systématiques des mesures spectrales. La dernière partie de cette thèse présente les découvertes de nouveaux pulsars individuels tels que PSR J0205+6449, J2229+6114, et J1048-5832 à partir des données du LAT et des éphémérides radio et X. Des analyses temporelles et spectrales sont investies dans le but de contraindre les modèles d'émission gamma. Finalement, nous discutons les propriétés d'une large population de pulsars gamma détectés par le LAT, incluant les pulsars normaux et les pulsars milliseconde.

PoGOLite is a balloon-borne experiment, which will study polarized soft γ-ray emissionfrom astrophysical targets in the 25-80 keV energy range by applying well-typephoswich detector technology. Polarized γ-rays are expected from a wide variety of sources including rotation-powered pulsars, accreting black holes and neutron stars,and jet-dominated active galaxies. Polarization measurements provide a powerfulprobe of the γ-ray emission mechanism and the distribution of magnetic and radiation fields around the source. The polarization is determined using Compton scattering and photoelectric absorption in an array of 217 plastic scintillators. The sensitive detector is surrounded by a segmented Bismuth Germanium Oxide (BGO) anticoincidence shield. The function of this shield is to reduce backgrounds from charged cosmic rays, primary and atmospheric γ-rays, and atmospheric and instrumenta lneutrons. The anticoincidence shield consists of 427 BGO crystals with three different geometries. The characteristics of the BGO crystals of the bottom anticoincidence shield have been studied with particular focus on the light yield.The maiden flight of PoGOLite will be with a reduced detector volume “pathfinder” instrument. The flight, lasting about 24 hours, is foreseen from Esrange, Sweden in August 2010. The performance of the pathfinder has been studied using computer simulations. The effect of atmospheric attenuation, both on the signal of theastronomical target and on the background, are studied. These allow an observationstrategy to be developed for the forthcoming flight. A polarization analysis method is described and applied to an observation example. The method sets anupper limit on the accuracy with which the polarimeter will be able to detect polarization the angle and degree. The PoGOLite polarimeter has a relatively small field of view (2.4◦×2.4◦) which must be kept aligned to objects of interest on the sky. A star tracker forms part of the attitude control system. The star trackersystem comprises a CCD camera, a lens, and a baffle system. Preliminary studiesof the star identification performance are presented and are found to be compatible with the environment around the Crab, which is the main observational target for the first flight. / QC20100629

Prior to this thesis no serious attempt has been made within binary system population synthesis research to model the selection effects of observational surveys. Conversely, many pulsar population models have accounted for radio survey selection effects but not detailed binary evolution. Such modelling becomes especially important when comparing theory directly to observations. In examining the factors that influence pulsar evolution, both in binary systems and as single stellar objects, we have bridged this existing gap between these two research fields. This thesis populates a model Galaxy with binary systems and evolves the population forward in time. A prediction of the Galactic pulsar population characteristics is produced, at the assumed age of the Galaxy, after we have accounted for detailed changes in stellar and binary evolution and Galactic kinematics. Synthetic observational surveys mimicking a variety of radio pulsar surveys are then performed on this population. The population synthesis synthetic survey (PS3) package is comprised of three components: stellar/binary evolution (binpop), Galactic kinematics (binkin), and survey selection effects (binsfx). The resultant pulsar populations, assuming the magnetic-dipole decay and accretion induced magnetic decay models, can compare well to many of the detected pulsar population characteristics. The comparisons between models and observations have lead to the conclusions described below. The models exclude short (∼ 5 Myr) timescales for standard pulsar exponential field decay and find that ablation of low-mass millisecond pulsar companions can redress both the lack of synthetic isolated pulsars and their excessive distances in height from the Galactic plane. Coalescing double neutron star and collapsar Galactic populations, evolved owing to standard binary evolutionary assumptions, are too centrally concentrated owing to the typical merger timescale of double neutron stars being a few million years. Dwarf galaxy models of coalescing double neutron stars and collapsars produce equally good agreement with long gamma-ray burst projected distances. Therefore our models cannot provide any distinction between which of these populations (coalescing double neutron stars or collapsars) are the progenitor of long gamma-ray bursts. The Galactic birth rate of double neutron star binaries in our model is 8.2 Myr−1 and the merger rate is 6.8 Myr−1. Scintillation is found to be an important aspect in the detection of low flux density pulsars. The assumed pulsar luminosity law is found to require an inverse trend with spin period and our favoured models suggest that there are one million radio active pulsars within the Galaxy.

This thesis considers the various morphologies of radio supernova remnants (SNRs), and attempts to determine whether their appearance results from the properties of the progenitor star and its supernova explosion, or from the structure of the interstellar medium (ISM) and ambient magnetic field into which a SNR consequently expands. High-resolution observations of Supernova 1987A show a young remnant whose appearance and evolution are completely dominated by the structure of its progenitor wind. A statistical study of the Galactic population of bilateral SNRs demonstrates that the symmetry axes of these remnants run parallel to the Galactic Plane. This result can be explained by the interaction of main sequence stellar wind-bubbles with the ambient magnetic field; expansion of SNRs into the resulting elongated cavities results in a bilateral appearance with the observed alignment. Radio observations of SNR G296.8-00.3 show a double-ringed morphology which is best explained by expansion either into an anisotropic main-sequence progenitor wind or into multiple cavities in the ISM. Data on SNRs G309.2-00.6 and G320.4-01.2 (MSH 15-52) make a strong case that the appearance of both remnants is significantly affected by collimated outflows from a central source; for G309.2-00.6 the source itself is not detected, but for G320.4-01.2 there is now compelling evidence that the remnant is associated with and is interacting with the young pulsar PSR B1509-58. I conclude that, while the youngest SNRs are shaped by their progenitor's circumstellar material, the appearance of most SNRs reflects the properties of the local ISM and magnetic field. Remnants which interact with an associated pulsar or binary system appear to be rare, and are easily distinguished by their unusual and distorted morphologies.

M.Sc. / The content of this dissertation is directed at a clearer understanding of pulsar behaviour. Pulsars are neutron stars in relatively early stages of their evolution. The intervals between observed arrival times of pulsar beams have been observed to suddenly decrease implying an increase in the star’s angular velocity Ω. This phenomenon is called a glitch. The first observed glitch occurred in the Vela pulsar (PSR 0833 -45) in 1969. The largest observed glitch also occurred in the Vela pulsar and exhibited a relative spin-up, Ω∕Ω~10−6 [1]. These relative pulsar spin-ups generally fall within the range 10−9 to 10−6 [2]. Pulsar glitches occur as a result of the processes in the interior of a neutron star [3]. Thus, a rigorous study of neutron star interiors is justified. Neutron star interiors are composed of hadronic matter: mostly neutrons, with a few per cent of protons (and, of course, non-hadronic electrons to conserve electric charge). The density of a neutron star interior increases with increasing depth as illustrated in 1.1.

The dissertation presents the computational technique Particle-In-Cell, or PIC for short, and its applications in studying the magnetospheres of neutron stars, modeled as conducting rotators with strong magnetic fields. Pigeon, an open-source PIC simulator written by the author in modern C++, is anatomically examined as an instrument to illustrate the principles, algorithms and engineering difficulties of the PIC technique. Two types of rotators are studied using Pigeon. The monopolar rotator, which has an exact solution in the force free limit, serves as a tester for the code, as well as an example of the PIC's capability. The main application of Pigeon is on the ab initio simulation of an (axisymmetric) dipolar rotator with self-consistent gamma ray photon emission and pair creation, the study of which could reveal valuable information of the mechanism of the pulsars. Thanks to the performance boost brought by Pigeon's dynamic load balancing functionality, we are able to perform the simulation with a 4096x4096 high resolution grid. The high resolution is critical in obtaining a Lorentz factor of 10000 of the polar cap potential drop, which in turn enables good separations of energy levels and hence makes the simulation closer to representing the real-life pulsars. With the high resolution, we are also able to study the Y point more closely, where we find that the angular momentum conservation dictates the process of magnetic flux surface crossing that is responsible for the release of electromagnetic energies into the plasma.