Pulsar Scintillation and Interstellar Lenses
Creators: name: Sprenger, Tim
Publisher: name: Universitäts- und Landesbibliothek Bonn
Year: 2024
Subjects: subject: Pulsar
subject: Szintillation
subject: ISM
subject: Radioastronomie
subject: Streuung
subject: scintillation
subject: radio astronomy
subject: scattering
subject: ddc:500
subject: ddc:520
subject: ddc:530

description: The radio emission of pulsars is scattered within the interstellar medium due to variations of the electron column density. Pulsars are very compact sources of pulsed radiation and, thus, emit spatially coherent radiation leading to observable interference of scattered paths. The corresponding amplitude variations are called scintillation. Due to the sensitivity to tiny differential phase shifts, the interstellar medium can be probed on angular scales below milliarcseconds. <br> The focus of this thesis lies on the analysis and modelling of scintillation phenomena in the Fourier domain. Scintillation arcs reveal the presence of scattered images of the pulsar. Resolving these images is a form of holography. The curvature of scintillation arcs as well as their evolution over time allows for the measurement of geometrical parameters such as distances and transverse velocities of the Earth, a thin scattering screen, and the pulsar. The aim of this study is to improve the theoretical understanding of pulsar scintillation and to advance analysis methods, for which two pulsars are studied. <br> An influential archival data set of PSR B0834+06 is used. Beside a new interferometric method amounting to beamforming, new data transformations are developed. Of these, the most important innovation is the $theta$--$theta$ transformation, which transforms secondary spectra into the space of angles on the sky. This enables a mapping of the image amplitude distribution as well as new methods to measure the curvature of scintillation arcs, which is a central observable. <br> The results of an observing campaign of PSR B1508+55 with the Effelsberg 100-m telescope are presented. This pulsar in the past showed the unexplained properties of stripes in secondary spectra as well as echoes in its pulse profile at low frequencies. These echoes correspond to structures on larger scales and were expected to cross the direct line of sight in late 2020. Indeed, a transition was observed between weak and strong scattering. The curvature of two scintillation arcs and the movements of features along the arc are measured. Application of the methods developed for PSR B0834+06 led to the discovery of a visible modulation of the image amplitudes on the time scale of hours. <br> The amplitude modulation strongly implies the presence of a second screen that introduces scintillation even before the radio waves reach the closer screen. Based on the observational findings, a two-screen theory is formulated and analytically solved for the three observables of arc curvature, feature movement, and modulation speed. This model is shown to agree to all aspects of the data, including the stripes. It ascribes the transition to a change of relative image amplitudes. Two inferred properties of the transition are independently confirmed by the localized echo components. <br> In conclusion, powerful new data transformations and a working two-screen model have been developed, and a new observable has been discovered.