Systematic effects and mitigation strategies in observations of cosmic re-ionisation with the Hydrogen Epoch of Reionization Array
- Authors: Charles, Ntsikelelo
- Date: 2024
- Subjects: Cosmology , Astrophysics , Radio astronomy , Hydrogen Epoch of Reionization Array , Epoch of reionization
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/432605 , vital:72886 , DOI 10.21504/10962/432605
- Description: The 21 cm transition from neutral Hydrogen promises to be the best observational probe of the Epoch of Reionisation (EoR). It has driven the construction of the new generation of lowfrequency radio interferometric arrays, including the Hydrogen Epoch of Reionization Array (HERA). The main difficulty in measuring the 21 cm signal is the presence of bright foregrounds that require very accurate interferometric calibration. However, the non-smooth instrumental response of the antenna as a result of mutual coupling complicates the calibration process by introducing non-smooth calibration errors. Additionally, incomplete sky models are typically used in calibration due to the limited depth and resolution of current source catalogues. Combined with the instrumental response, the use of incomplete sky models during calibration can result in non-smooth calibration errors. These, overall, impart spectral structure on smooth foregrounds, leading to foreground power leakage into the EoR window. In this thesis we explored the use of fringe rate filters (Parsons et al., 2016) as a mean to mitigate calibration errors resulting from the effects of mutual coupling and the use of an incomplete sky model during calibration. We found that the use of a simple notch filter mitigates calibration errors reducing the foreground power leakage into the EoR window by a factor of ∼ 102. Thyagarajan et al. (2018) proposed the use of closure phase quantities as a means to detect the 21 cm signal, which has the advantage of being independent (to first order) from calibration errors and, therefore, bypasses the need for accurate calibration. In this thesis, we explore the impact of primary beam patterns affected by mutual coupling on the closure phase. We found that primary beams affected by mutual coupling lead to a leakage of foreground power into the EoR window, which can be up to ∼ 104 times and is mainly caused by the unsmooth spectral structure primary of primary beam sidelobes affected by mutual coupling. This power leakage was confined to k < 0.3 pseudo h Mpc−1. Lastly, we also proposed and demonstrated an analysis technique that can be used to derive a flux scale correction in post-calibrated HERA data. We found that after applying flux scale correction to calibrated HERA data, the bandpass error reduces significantly, with an improvement of 6%. The derived flux scale correction was antenna-independent, and it can be applied to fix the overall visibility spectrum scale of H4C data post-calibration in a fashion similar to Jacobs et al. (2013). , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2024
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- Date Issued: 2024
Modifcations to gravitational waves due to matter shells
- Authors: Naidoo, Monogaran
- Date: 2021-10-29
- Subjects: Gravitational waves , General relativity (Physics) , Einstein field equations , Cosmology , Matter shells
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/191118 , vital:45062 , 10.21504/10962/191119
- Description: As detections of gravitational waves (GWs) mount, the need to investigate various effects on the propagation of these waves from the time of emission until detection also grows. We investigate how a thin low density dust shell surrounding a gravitational wave source affects the propagation of GWs. The Bondi-Sachs (BS) formalism for the Einstein equations is used for the problem of a gravitational wave (GW) source surrounded by a spherical dust shell. Using linearised perturbation theory, we and the geometry of the regions exterior to, interior to and within the shell. We and that the dust shell causes the gravitational wave to be modified both in magnitude and phase, but without any energy being transferred to or from the dust. This finding is novel. In the context of cosmology, apart from the gravitational redshift, the effects are too small to be measurable; but the effect would be measurable if a GW event were to occur with a source surrounded by a massive shell and with the radius of the shell and the wavelength of the GWs of the same order. We extended our investigation to astrophysical scenarios such as binary black hole (BBH) mergers, binary neutron star (BNS) mergers, and core collapse supernovae (CCSNe). In these scenarios, instead of a monochromatic GW source, as we used in our initial investigation, we consider burst-like GW sources. The thin density shell approach is modified to include thick shells by considering concentric thin shells and integrating. Solutions are then found for these burst-like GW sources using Fourier transforms. We show that GW echoes that are claimed to be present in the Laser Interferometer Gravitational-Wave Observatory (LIGO) data of certain events, could not have been caused by a matter shell. We do and, however, that matter shells surrounding BBH mergers, BNS mergers, and CCSNe could make modifications of order a few percent to a GW signal. These modifications are expected to be measurable in GW data with current detectors if the event is close enough and at a detectable frequency; or in future detectors with increased frequency range and amplitude sensitivity. Substantial use is made of computer algebra in these investigations. In setting the scene for our investigations, we trace the evolution of general relativity (GR) from Einstein's postulation in 1915 to vindication of his theory with the confirmation of the existence of GWs a century later. We discuss the implications of our results to current and future considerations. Calculations of GWs, both analytical and numerical, have normally assumed their propagation from source to a detector on Earth in a vacuum spacetime, and so discounted the effect of intervening matter. As we enter an era of precision GW measurements, it becomes important to quantify any effects due to propagation of GWs through a non-vacuum spacetime Observational confirmation of the modification effect that we and in astrophysical scenarios involving black holes (BHs), neutron stars (NSs) and CCSNe, would also enhance our understanding of the details of the physics of these bodies. , Thesis (PhD) -- Faculty of Science, Mathematics (Pure and Applied), 2021
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- Date Issued: 2021-10-29
Occult forces -- lived identities: witchcraft, spirit possession and cosmology amongst the Mayeyi of Namibia's Caprivi Strip
- Authors: Von Maltitz, Emil Arthur
- Date: 2007
- Subjects: Yeye (African people) , Yeye (African people) -- Namibia -- History , Occultism -- Namibia , Witchcraft -- Namibia , Spirit possession -- Namibia , Cosmology
- Language: English
- Type: Thesis , Masters , MA
- Identifier: vital:2121 , http://hdl.handle.net/10962/d1013279
- Description: Around Africa there seems to be an increasing disillusion with 'development', seen under the rubric of teleological 'progress', which is touted by post-colonial governments as being the cure for Africa's ailments and woes. Numerous authors have pointed out that this local disillusion, and the attempt to manage the inequities that arise through development and modernity, can be seen to be understood and acted upon by local peoples through the idiom of witchcraft beliefs and fears (see Geschiere & Fisiy 2001; Geschiere 1997; Nyamnjoh 2001; Comaroff & Comaroff 1993; Ashforth 2005) and spirit possession nanatives (see Luig 1999; Gezon 1999), or more simply, occult beliefs and praxis (Moore & Sanders 2001). The majority of the Mayeyi of Namibia's Eastern Caprivi perceive that development is the only way their regiOn and people can survive and succeed in a modernising world. At ~he same time there is also a seeming reluctance to move towards perceiving witchcraft as a means of accumulation (contra Geschiere 1997). This notion of the 'witchcraft of wealth' is emerging, but for the most part witches are seen as the enemies of development, while spirit possession narratives speak to the desire for development and of the identity of the group vis-a-vis the rest of the world. The thesis presented argues that, although modernity orientated analyses enable occult belief to be used as a lens through which to 1..mderstand 'modernity's malcontents' (Comaroff & Comaroff 1993), they can only go so far in explaining the intricacies of witchcraft and spirit possession beliefs themselves. The dissertation argues that one should return to the analysis of the cosmological underpinnings of witchcraft belief and spirit possession, taken together as complementary phenomena, in seeking to understand the domain of the occult. By doing so the thesis argues that a more comprehensive anthropological understanding is obtained of occult belief and practice, the ways in which the domain of the occult is constituted and the ways in which it is a reflection or commentary on a changing world.
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- Date Issued: 2007