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Mathematisch-Naturwissenschaftliche Fakultät - Jahrgang 2015

 

Titel Investigating astrophysical plasmas with pulsars using LOFAR and other telescopes
Autor Charlotte Anne Sobey
Publikationsform Dissertation
Abstract Pulsars are rapidly-rotating, highly-magnetised neutron stars that produce radio emission in their magnetospheres, which is detected as pulses on Earth. The focus of this thesis is observing pulsars as the means to study the properties of astrophysical plasmas that span many orders of magnitude in magnetic field strength (B) and scale.
Chapter 1 provides an introduction to pulsars, their use as as probes of astrophysical plasmas, and our current knowledge of the Galactic magnetic field.
Chapter 2 presents the numerous magnetospheric (B ~ 1012 G) emission characteristics of a mode-changing pulsar, PSR B0823+26, the mechanism behind which is not yet well understood. Using Low Frequency Array (LOFAR) observations, I report on the discovery that PSR B0823+26 has a weak and sporadically-emitting 'quiet' mode that is over 100 times weaker than that of the more regularly-emitting 'bright' mode. The transition between emission modes is concurrent across the range of frequencies observed and occurs within one rotational period (0.531 seconds).
In Chapter 3 I review the practical methods for determining Faraday rotation measures (RMs) towards polarised sources, and collect the literature RMs measured towards pulsars and extragalactic (EG) sources, for the purpose of studying the Galactic magnetic field (GMF, B ~ μG).
Chapter 4 describes the effect of the ionosphere on radio observations of astronomical sources. Observations of pulsars using LOFAR demonstrate the high accuracy of a code used to calculate the amount of ionospheric Faraday rotation towards a specific line-of-sight using publicly available, GPS-derived total electron content maps and the geomagnetic reference field (B ~ 0.5 G). I show that this technique can confidently determine some of the highest-precision ionosphere-corrected RMs towards pulsars ever achieved, towards more accurately measuring and monitoring the GMF.
Chapter 5 presents new, or increased-accuracy, ionosphere-corrected RMs towards pulsars using polarisation observations with three different central radio frequencies. The RMs towards pulsars and EGs collected in Chapters 3 and 5 are used to reconstruct a map of the large-scale GMF using wavelet analysis, for which the additional RM data were beneficial. However, to more accurately reconstruct the three-dimensional GMF, more RMs and independent pulsar distances are required, especially towards the Galactic halo.
Chapter 6 summarises this thesis and provides prospects for future related work.
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© Universitäts- und Landesbibliothek Bonn | Veröffentlicht: 19.02.2015