PUB - Publikationen an der Universität Bielefeld

Pulsars are observed as sources that give off rapidly pulsed radiation and as such act like precise clocks in the sky, which allow for unique experiments and discoveries. For example, the first planets ever discovered outside of the Solar system were in fact discovered around the pulsar B1257+12 as early as 1992. Another important finding was the first indirect detection of gravitational wave emission in the binary pulsar B1913+16, as the orbital shrinking due to the emission of gravitational waves was detected as slight deviations in the pulse arrival times.
Today, a major application of pulsars lies in the so-called “pulsar timing arrays”, which aim to detect low-frequency gravitational waves by observing a large set of highly predictable pulsars and searching for correlations in their pulse arrival time variability.
As pulsars are mostly observed at radio frequencies, their radiation is affected by interstellar propagation effects like dispersion, which causes a frequency-dependent propagation delay that can vary with time when there is relative motion between the Earth, the pulsar and the gas between the two. These effects on the radiation are a double-edged sword: on one hand, we can learn about the interstellar medium, its density, magnetic field and turbulence. On the other hand, these effects act like a source of noise in other experiments that rely on the clock-like behaviour of pulsars, making measurements less precise or creating biases if not properly dealt with.
The LOw Frequency ARray (LOFAR) telescope is part of a new generation of radio telescopes, which can point in any direction without moving parts, but rather by combining the signals received by a large number of antennas with high-performance computers. This allows for very large collecting areas and a very high flexibility in telescope usage. LOFAR is particularly interesting for studies of the interstellar medium, as it observes at relatively low radio frequencies, where the propagation effects are much stronger than at more commonly used higher frequencies.
With this thesis, I aim to improve our understanding of interstellar propagation effects, as well as quantify and monitor propagation effects to potentially improve on experiments at higher frequencies, which can usually measure pulse arrival times more precisely (due to a stronger signal), but have less sensitivity to the interstellar propagation effects. This study is the first of its kind, studying a large sample of pulsars at low radio frequencies. The measurement precision for interstellar propagation effects is unprecedented, allowing for new insights and demonstrating the scientific potential of low-frequency radio telescopes like LOFAR.