Probing the Interstellar Medium and Dark Matter with Pulsars

Abstract

Pulsars are rapidly rotating, highly magnetised neutron stars which emit electromagnetic radiation from their magnetic poles in the form of highly collimated beams. Pulsars are known as a powerful tool to probe the interstellar medium (ISM) and its constituents in the Miky Way. In this thesis we focus on probing the non-baryonic entities in the Milky Way, namely interstellar magnetic fields and dark matter. <br /> The first part of the thesis is dedicated to the investigation of Galactic magnetic fields, which are a major agent in the dynamics and energy balance of the ISM, and general evolution of the Galaxy. Small-scale turbulent magnetic fields in the Milky Way can be probed by monitoring variations in the Faraday rotation of linearly polarised radiation of pulsars. Following this idea, we use high-cadence, low-frequency observations from a set of selected pulsars carried out with German LOw-Frequency ARray (LOFAR) stations. The method that is used to determine the Faraday rotation measures (RMs) of pulsar signals is the Bayesian generalised Lomb-Scargle periodogram technique, developed in this thesis. We find that measured RMs are strongly affected by the highly time-variable terrestrial ionosphere. We have mitigated the ionospheric contribution assuming a thin-layer model of the ionosphere. We conclude that within this approximation the ionospheric RM corrections are accurate to ~ 0.06 - 0.07 rad/m², which defines our sensitivity towards long-term astrophysical RM variations. Following these results, we investigate the sensitivity to the turbulence in the magnetised ISM between the pulsar and observer. No astrophysically credible signal has been detected. We discuss implications of the non-detection and analyse the possibilities for future investigations.<br /> The second part of this thesis deals with dark matter - a matter which accounts for about a quarter of the energy density of the Universe, and the nature of which is still under debate. The ultralight scalar field dark matter is one of the compelling dark matter candidates, which leaves characteristic imprints in the times of arrival of radio pulses from pulsars. We search for traces of ultralight scalar-field dark matter in the Galaxy using the latest Parkes Pulsar Timing Array dataset that contains the times of arrival of 26 pulsars. No statistically significant signal has been detected. Therefore, we set an upper limit on the local dark matter density. The most stringent constraints are still one order of magnitude above the local dark-matter density inferred from kinematics of stars in the Milky Way. We conclude by discussing the prospects of detecting the fuzzy dark matter with future radio astronomical facilities.