No description available.

The origins of ultra-high-energy cosmic rays (UHECRs) are one of the open puzzles of astrophysics. A number of plausible candidates, such as active galactic nuclei (AGNs) have been discussed, but no clear consensus has been reached. One way to assess the different hypotheses is by analyzing the UHECR arrival directions. Recently, a small number of studies have begun applying Bayesian methodologies to this problem, forming the first steps in the development of a comprehensive Bayesian framework for the study of UHECRs. In this work, we have developed two Bayesian methods to study this question, and have applied them to UHECRs from the Pierre Auger Observatory (PAO). The first method was a Bayesian approach to studying the catalogue-independent clustering of UHECRs. Previously, this had been difficult as there is no well motivated clustered model that can be used in a Bayesian model comparison. We have resolved this difficulty by developing a multi-step approach that derives such a model from a sub-set of the data. This approach could have broad applications for anisotropy searches in other areas of astronomy. Our results were consistent with both isotropic and clustered models. The second was a Bayesian method that was aimed to find associations between UHECR arrival directions and source catalogues. It was an extension of a previous Bayesian study, but analyzed a greater data set, used a more refined UHECR model, and was generalized to be applicable to a greater variety of source catalogues. Our results were broadly consistent with previous work, with the purely isotropic UHECR models being disfavoured for reasonable parameter ranges. It will be of great interest to apply our methods to samples of greater size. The extended UHECR samples that will be available in the near future should be sufficient for our methods to determine the origins of the UHECRs.

The age of gravitational-wave astronomy has begun. Gravitational waves are propagating spacetime perturbations ($\textit{“ripples in the fabric of space-time”}$) predicted by Einstein’s theory of General Relativity. These signals propagate at the speed of light and are generated by powerful astrophysical events, such as the merger of two black holes and supernova explosions. The first detection of gravitational waves was performed in 2015 with the LIGO interferometers. This constitutes a tremendous breakthrough in fundamental physics and astronomy: it is not only the first direct detection of such elusive signals, but also the first irrefutable observation of a black-hole binary system. The future of gravitational-wave astronomy is bright and loud: the LIGO experiments will soon be joined by a network of ground-based interferometers; the space mission eLISA has now been fully approved by the European Space Agency with a proof-of-concept mission called LISA Pathfinder launched in 2015. Gravitational-wave observations will provide unprecedented tests of gravity as well as a qualitatively new window on the Universe. Careful theoretical modelling of the astrophysical sources of gravitational-waves is crucial to maximize the scientific outcome of the detectors. In this Thesis, we present several advances on gravitational-wave source modelling, studying in particular: (i) the precessional dynamics of spinning black-hole binaries; (ii) the astrophysical consequences of black-hole recoils; and (iii) the formation of compact objects in the framework of scalar-tensor theories of gravity. All these phenomena are deeply characterized by a continuous interplay between General Relativity and astrophysics: despite being a truly relativistic messenger, gravitational waves encode details of the astrophysical formation and evolution processes of their sources. We work out signatures and predictions to extract such information from current and future observations. At the dawn of a revolutionary era, our work contributes to turning the promise of gravitational-wave astronomy into reality.

This thesis firstly describes efforts to characterise large-area, high-gain, Avalanche Photodiodes (APDs), manufactured by Radiation Monitoring Devices (RMD) inc. of Massachusetts. These are relatively new devices in the field of X-ray spectroscopy and the research presented here attempts to increase our understanding of their behaviour as X-ray detectors and their underlying internal physical processes. Models are suggested for Quantum Detection Efficiency and for Photopeak Fraction in these devices. Measurements of these properties as a function of energy constrain the models, revealing new information about the internal structure of APDs and providing powerful predictive tools for detector response. The intrinsic silicon dead layer of a typical device is found to be 2 microm thick, whilst the sensing layer is 21 microm thick. Secondly, this thesis provides detailed accounts of how the new tools mentioned above have been utilised to characterise an ensemble of APDs and how that characterisation has been used to simulate the behaviour of an APD-based astrophysical instrument: CATSAT's Soft X-ray Spectrometer (SXR). This work includes simulated SXR observations of the diffuse soft X-ray background, the crab nebula and CATSAT's target objects: Gamma-ray Bursts. The results of these simulations are presented, leading to an analysis of CATSAT's ability to meet its scientific objectives. It is estimated that the SXR will observe approximately 7 bursts per year above the five-sigma significance level. In approximately half of these cases, it should be possible to discriminate between the hypothesis that there is an absorbing hydrogen column of density 1 x 1022 cm-2 and the hypothesis that there is no column.

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In this work, two rather different experiments with their respective analysis have been described. The first experiment concerned the 20Ne(d,p)21 Ne transfer reaction and the second the 12 C+12C fusion reaction. An experiment of 20 Ne(d,p)21 Ne transfer reaction was performed in Munich, Germany using the Quadrupole Dipole Dipole Dipole (Q3D) magnetic spectrograph. This experiment has aimed at the investigation of spectroscopic information around the Gamow window of the 17O(α,γ)21 Ne located between Ex = 7.65-8.05 MeV. The stated experiment was performed using an 21Ne implanted target on carbon and covered the excitation energies varying from 6.9 MeV to 8.5 MeV. Sixteen states were identified, three of which, the 8.328(6) MeV, 6.977(17) MeV and 6.960(2) MeV have first been observed in this work. Within the Gamow window however, only one state, the 7.955(2) MeV by correspondence to the 7.9603(10) Mev as recorded in literature, has been identified. Out the sixteen states, this work made eleven firm assignments and two “tentative” assignments. The 12 C+12 C fusion reaction was performed at TRIUMF, Canada using TUDA, the TRIUMF UK Detector Array. The experiment covered the centre of mass located between 3.4 to 4.02 MeV. The aim of this work was the determination of the cross section, which would help, at theses energies, reduce uncertainties observed in present data sets. Analysis such ADCs and TDCs calibrations, different cuts selection necessary for particle identification are presented.

In this thesis I present measurements of the physical properties of the nuclei of Jupiter Family comets (JFCs), based on time-series observations. These data were collected in four observing runs; two using the 3.6m NTT in Chile, and two using the 2.5m INT on the island of La Palma. From the time-series photometry rotation rates and elongations were measured, and from these constraints were placed on the bulk density and porosity of nuclei. Multi-filter imaging was performed to enable measurement of their surface colours. In addition, a large amount of ‘snap-shot’ imaging was performed during these observing runs, and taken with the time-series data is used to measure nuclei sizes. These results are compared with other data from the literature to study the general properties of JFC nuclei. A size distribution is measured which is consistent with that predicted for a population of collisional fragments, while the distribution in rotation rates is found to be flat and non-collisional. The low minimum densities measured for all comets imply that the true bulk density of nuclei is low, and the porosity is high. These properties are shown to have similar values in the Kuiper Belt Object (KBO) population, which is the supposed parent population for JFCs. The surface colours of JFCs are shown to match the blue end of the KBO distribution, and can be derived from the observed KBO distribution under the assumption of a de-reddening function that preferentially depletes the reddest surfaces.

No description available.

In this thesis, we present a number of atomic transitions associated with three astrophysically important systems S9+, Ar2+, and Co2+ . The latter is especially prominent in the study of particular classes of Supernovae (SNe). We also provide an additional collisional-radiative modelling procedure to analyze electron collisions involving Co2+ by implementing an isolated atom approximation. We consider typical temperature and density diagnostics to constrain properties such as SNe evolution and mass-loss processes. Large evaluations for these three systems have been considered using the well studied R-matrix method, where appropriate parallel computer codes DARC and BP have been used to incorporate relativistic effects accordingly. We have studied in detail the photoionization of S8+, both valence and L2-shell photoionization of Ar+, above threshold photoionization of Co+ and the applicable extension to the electron-impact excitation of Co2+ required for the computation of Maxwellian averaged collision strengths. To describe these systems, we have considered various approaches to model such many-body problems. Two computer packages CIV3 and GRASPO have been executed to solve the target state problem, and we apply the theory of configuration-interaction to include sufficient correlation effects to enhance the wavefunctions. Energy levels, oscillator strengths and transition probabilities have been presented here and comparisons are conducted with other works. A systematic analysis has been carried out, and theoretical predictions for our current set of scattering results agree well with other theoretical and experimental measurements. Therefore, we conclude that in this thesis we have presented accurate and extremely useful atomic data for transitions involving electrons and photons essential for spectral modelling.

This thesis concerns the study of energetic MHD waves in the solar atmosphere using high- resolution observations from the Dunn Solar Telescope, New Mexico, USA. The purpose of this study is to provide a greater understanding of the mechanisms that heat the upper solar atmosphere to temperatures ~ 1 million K above the solar surface. Compressible sausage modes are studied in magnetic pores as candidates for energy transport through the lower solar atmosphere. For the first time, sausage mode waves are shown to propagate from the solar surface into higher regions, exhibiting a reduction in wave energy that indicates that they may contribute to the enhanced heating of the upper solar atmosphere. Alongside this, the first verification of surface and body modes of sausage waves is made through the study of 7 photospheric pores. The surface modes are more prevalent than the body modes, which exist only in smaller, less magnetic pores. The surface modes also exhibit greater energies than the body modes. In addition to this, the effects of waves that steepen into plasma shocks in the centre of sunspots were studied. The high-resolution data on offer allowed for an unprecedented study of the heating potential of these Umbral Flashes using magnetic field extrapolations and thermal inversions. Two populations of shocks were identified, one close to spot centre and the other in more inclined fields, and it was shown that shocks formed in more magnetic regions were more efficient heaters of local plasma. Their complex motions were also seen to not be related to large scale magnetic configurations, indicating that behaviour below the resolution of the data is present. This thesis provides evidence to further the hypothesis that waves have a part in both energy transport and dissipation into heat in the upper solar atmosphere.