The Nature of Fast Radio Bursts and Their Potential as Probes of the Universe

Abstract

Fast Radio Bursts (FRBs) are fascinating flashes of radio waves stemming from distant galaxies. Recently discovered, their origins are still mysterious. Most FRBs only appear for the split second of their duration and are not found again, while about 5 % of FRB sources emit bursts repeatedly. Even though the origins are unclear, FRBs are the first extragalactic, short-duration radio signals, which makes them a unique probe of the Universe. Crafting FRBs into the right tools requires a detailed understanding of FRBs, their population, and the source environment. The aim of this thesis is to gain a better understanding of FRBs and to develop FRBs as tools to study astrophysical and cosmological problems. One challenge in our investigations of repeaters is that they repeat very irregularly. Several bursts within an observation can follow months without detections. Understanding the repetition patterns of single FRBs could tell us about the whole population. Additionally, the burst energies and changes in the spectra over the burst durations could lead a way to decipher their origins. Most methods using FRBs to address astrophysical or cosmological problems depend on additional observations with optical telescopes. This is known to be a problem, as observing time is highly contested, but the scope for future FRB science has not been examined. Another challenge is the joint influence of several cosmological quantities, which would ideally be inferred separately but are intertwined. This thesis gives a comprehensive overview of the proposed applications of FRBs and addresses all the above topics. Observations with the 305-m Arecibo Telescope address the unknown nature of FRBs. The target, FRB 121102, is the longest known and one of the most active repeating FRBs. The search yielded 849 bursts in ten observations, one of the largest set of bursts, including the highest burst rate as yet. The analysis of burst arrival times revealed their habit of sometimes arriving in groups around a characteristic separation time. Apart from this, the arrival times can purely be described by their variable rate. A novel model of the burst spectra led to a new spectro-temporal effect, providing a new diagnosis for emission models. Simulations of FRBs and their host galaxies investigate the problem of required optical telescope time. Inference of cosmological parameters from the artificial data confirmed that optical observations constitute the bottleneck of future FRB science. Some of the FRB applications entirely depend on whether considerable amounts of optical telescope time can be acquired. To separate the intertwined contributions of cosmological parameters, the thesis presents a new Bayesian framework. This framework exploits a proposed connection between FRBs and gravitational waves. Through the framework, future associated gravitational waves can be combined with FRB observables to untangle the different parameters.