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Fast radio bursts (FRBs) are extragalactic radio transients of extraordinary luminosity. Studying the diverse temporal and spectral behaviour recently observed in a number of FRBs may help determine the nature of the entire class. For example, a fast spinning or highly magnetised neutron star might generate the rotation-powered acceleration required to explain the bright emission. Periodic, sub-second components, suggesting such rotation, were recently reported in one FRB, and potentially in two more. Here we report the discovery of FRB 20201020A with Apertif, an FRB showing five components regularly spaced by 0.415 ms. This sub-millisecond structure in FRB 20201020A carries important clues about the progenitor of this FRB specifically, and potentially about that of FRBs in general. We thus contrast its features to the predictions of the main FRB source models. We perform a timing analysis of the FRB 20201020A components to determine the significance of the periodicity. We compare these against the timing properties of the previously reported CHIME FRBs with sub-second quasi-periodic components, and against two Apertif bursts from repeating FRB 20180916B that show complex time-frequency structure. We find the periodicity of FRB 20201020A to be marginally significant at 2.5$\sigma$. Its repeating subcomponents cannot be explained as a pulsar rotation since the required spin rate of over 2 kHz exceeds the limits set by typical neutron star equations of state and observations. The fast periodicity is also in conflict with a compact object merger scenario. These quasi-periodic components could, however, be caused by equidistant emitting regions in the magnetosphere of a magnetar. The sub-millisecond spacing of the components in FRB 20201020A, the smallest observed so far in a one-off FRB, may rule out both neutron-star rotation and binary mergers as the direct source of quasi-periodic FRBs., Comment: 13 pages, 6 figures, 3 tables, supplementary material. Submitted to A&A

The Green Bank Telescope (GBT) is the largest fully steerable radio telescope in the world and is one of our greatest tools for discovering and studying radio pulsars. Over the last decade, the GBT has successfully found over 100 new pulsars through large-area surveys. Here I discuss the two most recent---the GBT 350 MHz Drift-scan survey and the Green Bank North Celestial Cap survey. The primary science goal of both surveys is to find interesting individual pulsars, including young pulsars, rotating radio transients, exotic binary systems, and especially bright millisecond pulsars (MSPs) suitable for inclusion in Pulsar Timing Arrays, which are trying to directly detect gravitational waves. These two surveys have combined to discover 85 pulsars to date, among which are 14 MSPs and many unique and fascinating systems. I present highlights from these surveys and discuss future plans. I also discuss recent results from targeted GBT pulsar searches of globular clusters and Fermi sources., Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and Opportunities after 80 years", J. van Leeuwen (ed.); 6 pages, 2 figures

LOFAR, the Low Frequency Array, is an innovative new radio telescope currently under construction in the Netherlands. With its continuous monitoring of the radio sky we expect LOFAR will detect many new transient events, including GRB afterglows and pulsating/single-burst neutron stars. We here describe all-sky surveys ranging from a time resolution of microseconds to a cadence span of years., Proceedings of the "Frontiers of Space Astrophysics: Gamma Ray Bursts & Neutron Stars" meeting in Cairo/Alexandria, Egypt, April 2009. 4 Pages

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in Northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting panoramic surveys of the astrophysical sky. The fundamental scientific program of this new telescope is varied and exciting; we here discuss some of the first astronomical results., Comment: Invited review, proceedings of the "Panoramic Radio Astronomy: Wide-field 1-2 GHz research on galaxy evolution" meeting in Groningen, the Netherlands, June 2009. 8 Pages

The thesis focuses on different aspects of radio pulsars: on the one hand it describes research on the pulsar emission mechanism (nulling, drifting subpulses, single-pulse polarisation and their mutual interaction). On the other it gives an account of investigations into the evolution of the pulsar population as a whole (birthrates, velocities, magnetic field evolution) and possible ways to probe this population (simulations of upcoming pulsar surveys)., PhD Thesis, Utrecht University 2004, with a summary in Dutch. 108 pages, 55 low-resolution figures. High-resolution versions are available at http://dspace.library.uu.nl/handle/1874/716

Fast Radio Bursts (FRBs) are cosmological transients of unknown origin. While first discovered just over 10 years ago, the enigmatic nature of FRBs is yet to be understood. As exceedingly bright radio emitters, FRBs are visible over immense, cosmological distances. Many are over a billion light years from Earth. If we knew more about FRBs, we could use such bursts to study the evolution of the universe and the extremes of space. This thesis focuses on determining properties of FRBs. Rather than investigating the properties of specific FRBs, we chose to determine properties of the full FRB population. To do so, we use a computational and statistical method called population synthesis. This method involves modelling an intrinsic population, applying selection effects, and then trying to match simulated results to real detections. By taking all of the factors behind a single FRB detection into account, this type of simulation can help to uncover the true nature of FRBs. Recent research suggests FRBs could be powered by neutron stars with strong magnetic fields. We also study the behaviour of neutron stars in other systems, called Low Mass X-ray Binaries (LMXBs). Such systems feature a neutron star accreting mass from a companion star via a so-called accretion disk.

Modern radio-pulsar catalogues contain a large diversity of objects: young solitary pulsars and old ones in binary systems, periodic or sporadic emitters; bright and magnetized, or faint and even eclipsing. We study these by means of their radio pulses, a descriptive characteristic of magnetic field strength and emission mechanism. The most exotic pulsars can inform about the neutron-star equation of state; and about general relativity in the strong field regime. Finding new such objects requires very powerful telescopes coupled with advanced search techniques and computation. While there are nearly two thousand known Galactic radio pulsars, no such sources have been discovered in other, neighbouring galaxies. Pulsar discoveries in nearby galaxies could link Galactic and extragalactic pulsar populations, more reliably measure intergalactic medium and distances, and possibly explain fast radio bursts. This thesis focuses on targeted multi-frequency searches for radio pulsars and fast radio transients in afar regions of the Milky Way and nearby galaxies. We primarily employ two Dutch radio telescopes: LOFAR (at 150 MHz) and Westerbork (at 1400 MHz). Multi-frequency observations can tackle deleterious propagation effects (dispersion, scintillation, scattering) and remain sensitive to bright distant sources. The thesis addresses the following topics: real-time search pipeline auto-tuning optimization, telescope sensitivity limits for periodic and single pulses from extragalactic sources, systematic classification of the radio pulsars population, and their evolution.

Massive stars end their lives in supernova explosions, leaving behind compact remnants that enclose the ember of the star, often a pulsar. The supernova remnant (SNR) and pulsar harbor vital information on the explosion mechanism that destroyed their predecessor. Studying these systems helps us to unravel e.g., the violent fates of massive stars and the formation of neutron stars. The youngest and most energetic pulsars create a pulsar wind nebulae (PWNe), which acts as a calorimeter. Finding younger and more energetic systems will thus lead us to pin down the initial energetics of pulsars, also elucidating the young-pulsar/Fast Radio Burst (FRB) connection. This thesis focuses on finding young pulsars and understanding their environments. First, in Chapter 2 we conclude on the nature of the high-energy source HESS J1943+213, compatible with being either a PWN or a BL Lac object. Next, in Chapter 3 we explore to what extent PWNe and SNRs can negatively impact pulsar signal propagation. We find that for Galactic pulsars this effect is minimal. In Chapter 4 we aim to find young pulsars in SNRs and PWNe and report on a pulsar candidate in PWN G141.2+5.0. Finally, in Chapter 5 we report on the flux calibration verification we carried out as part of the scientific commissioning of ALERT, the Apertif LOFAR Exploration of the Radio Transient Sky.

Six years ago, the discovery of Rotating Radio Transients (RRATs) marked what appeared to be a new type of sparsely-emitting pulsar. Since 2006, more than 70 of these objects have been discovered in single-pulse searches of archival and new surveys. With a continual inflow of new information about the RRAT population in the form of new discoveries, multi-frequency follow ups, coherent timing solutions, and pulse rate statistics, a view is beginning to form of the place in the pulsar population RRATs hold. Here we review the properties of neutron stars discovered through single pulse searches. We first seek to clarify the definition of the term RRAT, emphasising that "the RRAT population" encompasses several phenomenologies. A large subset of RRATs appears to represent the tail of an extended distribution of pulsar nulling fractions and activity cycles; these objects present several key open questions remaining in this field.

The main thrust of this thesis is the search for radio pulsars using the Low Frequency Array (LOFAR). LOFAR is part of a new generation of radio telescopes leading up to the building of the Square Kilometer Array. LOFAR uses a multitude of inexpensive antennas whose signals are combined digitally to create many sensitive, fully steerable, beams on the sky. As part of LOFAR's commissioning we performed two radio pulsar surveys, the LOFAR Pilot Pulsar Survey (LPPS) and the LOFAR Tied-Array Survey (LOTAS). The former survey put a limit on the fast radio transient rate and re-detected 65 pulsars, while the latter re-detected 27 pulsars and discovered 2 new ones. These discoveries bode well for future, more sensitive, pulsar surveys using LOFAR. We also describe extensions to standard single-pulse search techniques. This thesis furthermore describes two smaller projects; attempts to detect pulsars as binary companions to subdwarf-B stars and an attempt to detect pulsed radio emission from the nearby pulsar Geminga. We were able to exclude pulsed radio emission from the sdB binary companions to a high degree of certainty. Our very sensitive observation of Geminga failed to detect pulsed radio emission, despite earlier claims. We conclude that those earlier detections were likely spurious, but leave open the possibility that Geminga is an intermittent radio source that turned off.