The Transient Radio Sky

The high time-resolution radio sky represents unexplored astronomical territory. This thesis presents a study of the transient radio sky, focussing on millisecond scales. As such, the work is concerned primarily with neutron stars. In particular this research concentrates on a recently identified group of neutron stars, known as RRATs, which exhibit radio bursts every few minutes to every few hours. After analysing neutron star birthrates, a re-analysis of the Parkes Multibeam Pulsar Survey is described which has resulted in the discovery of 19 new transient radio sources. Of these, 12 have been seen to repeat and a follow-up campaign of observations has been undertaken. These studies have greatly increased our knowledge of the rotational properties of RRATs and enable us to conclude that they are pulsars with extreme nulling and/or pulse-to-pulse modulation. Although the evolution of neutron stars post-supernova is not yet understood, it seems that RRATs fit into the emerging picture in which pulsar magnetospheres switch between stable configurations.

While synoptic surveys in the optical and at high energies have revealed a rich discovery phase space of slow transients, a similar yield is still awaited in the radio. Majority of the past blind surveys, carried out with radio interferometers, have suffered from a low yield of slow transients, ambiguous transient classifications, and contamination by false positives. The newly-refurbished Karl G. Jansky Array (Jansky VLA) offers wider bandwidths for accurate RFI excision as well as substantially-improved sensitivity and survey speed compared with the old VLA. The Jansky VLA thus eliminates the pitfalls of interferometric transient search by facilitating sensitive, wide-field, and near-real-time radio surveys and enabling a systematic exploration of the dynamic radio sky. This thesis aims at carrying out blind Jansky VLA surveys for characterizing the radio variable and transient sources at frequencies of a few GHz and on timescales between days and years. Through joint radio and optical surveys, the thesis addresses outstanding questions pertaining to the rates of slow radio transients (e.g. radio supernovae, tidal disruption events, binary neutron star mergers, stellar flares, etc.), the false-positive foreground relevant for the radio and optical counterpart searches of gravitational wave sources, and the beaming factor of gamma-ray bursts. The need for rapid processing of the Jansky VLA data and near-real-time radio transient search has enabled the development of state-of-the-art software infrastructure. This thesis has successfully demonstrated the Jansky VLA as a powerful transient search instrument, and it serves as a pathfinder for the transient surveys planned for the SKA-mid pathfinder facilities, viz. ASKAP, MeerKAT, and WSRT/Apertif.

Most of what is currently known about slow radio transients (supernovae, gamma-ray bursts, tidal disruption events, stellar flares, etc.) has come via radio follow-up of objects identified by synoptic survey telescopes at optical, X-ray or gamma-ray wavelengths. However, with the ability to capture obscured, unbeamed and magnetically-driven phenomena, radio surveys offer an opportunity to find unique cosmic transients and study their diagnostic features. This thesis describes the first systematic exploration of the dynamic radio sky on timescales between one day to several years using multi-epoch widefield surveys with the Karl G. Jansky Array (Jansky VLA). It announces the discovery of several fascinating transients that appear to be unique to the radio, and thus establishes the Jansky VLA as an efficient transient discovery machine. This thesis has enormous implications for designing efficient transient surveys during the era of the Square Kilometer Array (SKA), the demographics of variable radio sources including AGN, future searches of the radio counterparts of gravitational-wave sources, and the search for new classes of radio transients.

Although many of the most energetic phenomena in the universe display changes in their brightness over time, the study of these variable and transient events in the radio sky is a relatively new field with many questions waiting to be answered. Within this thesis, I began tackling some of these inquiries using a new, deep-field survey, COSMOS HI Large Extragalactic Survey (CHILES) being conducted at the Karl G. Jansky Very Large Array (VLA). CHILES serves as a unique data set from which we can statistically understand the radio transient and variable universe. Within this thesis, I give a general overview of what was observed within the CHILES time domain and focus on the uniqueness of a few of the most variable sources. These findings provide previously unprecedented data with which to explore low-luminosity active galactic nuclei, the evolution of active galactic nuclei, and introduce some potentially unique radio sources for future studies. In addition to my work with CHILES, I also explored potential new radio transients: gamma-ray bursts (GRBs) that have an associated supernova (SN). As the GRB radio emission begins to fade away over time, it has been theorized that the SN's ejecta will begin to sweep up the interstellar medium and the remnant will eventually outshine the GRB within tens of years after the initial explosion. A detection of this rebrightening would allow us to not only better understand the phenomena of GRB-SNe, but would also be a connection between the death of a star and its evolution into a supernova remnant. Although I was unable to detect the emission of this rebrightening, my measurements allow for a better understanding of the environments of these sources and constrains multiple parameters that are vital in our current models. Together, this information about individual sources and a summary of the variable sources within a blind radio study, help to address what can be found in upcoming surveys and the faint radio time domain.

Neutron stars are the most compact astronomical objects in the universe which are accessible by direct observation. Studying neutron stars means studying physics in regimes unattainable in any terrestrial laboratory. Understanding their observed complex phenomena requires a wide range of scientific disciplines, including the nuclear and condensed matter physics of very dense matter in neutron star interiors, plasma physics and quantum electrodynamics of magnetospheres, and the relativistic magneto-hydrodynamics of electron-positron pulsar winds interacting with some ambient medium. Not to mention the test bed neutron stars provide for general relativity theories, and their importance as potential sources of gravitational waves. It is this variety of disciplines which, among others, makes neutron star research so fascinating, not only for those who have been working in the field for many years but also for students and young scientists. The aim of this book is to serve as a reference work which not only reviews the progress made since the early days of pulsar astronomy, but especially focuses on questions such as: "What have we learned about the subject and how did we learn it?", "What are the most important open questions in this area?" and "What new tools, telescopes, observations, and calculations are needed to answer these questions?". All authors who have contributed to this book have devoted a significant part of their scientific careers to exploring the nature of neutron stars and understanding pulsars. Everyone has paid special attention to writing educational comprehensive review articles with the needs of beginners, students and young scientists as potential readers in mind. This book will be a valuable source of information for these groups.

This book presents lecture materials from the Third LOFAR Data School, transformed into a coherent and complete reference book describing the LOFAR design, along with descriptions of primary science cases, data processing techniques, and recipes for data handling. Together with hands-on exercises the chapters, based on the lecture notes, teach fundamentals and practical knowledge. LOFAR is a new and innovative radio telescope operating at low radio frequencies (10-250 MHz) and is the first of a new generation of radio interferometers that are leading the way to the ambitious Square Kilometre Array (SKA) to be built in the next decade. This unique reference guide serves as a primary information source for research groups around the world that seek to make the most of LOFAR data, as well as those who will push these topics forward to the next level with the design, construction, and realization of the SKA. This book will also be useful as supplementary reading material for any astrophysics overview or astrophysical techniques course, particularly those geared towards radio astronomy (and radio astronomy techniques).