Search

Pulsars are famed for their highly precise frequency of rotation and periodic pulses. The time of arrival of their periodic pulses can generally be predicted to a high precision, but not perfectly. Deviation of the times of arrival (TOA) of the pulses from prediction is common to many known pulsars. One type of the deviation known as timing noise is characterised by unexplained wandering of pulse phases relative to a simple model. It is believed that this timing noise holds clues to the physics of the stellar interior. Also, if the timing noise is properly modelled, the times of arrival can be improved to allow pulsars to be used as precise clocks. Observations of some pulsars have shown that the spin-down varies with time and it is also linked with perturbations of the conditions in the pulsar’s magnetosphere. This link, in the case of PSR B0329+54, was investigated in this work. PSR B0329+54 is known to undergo mode changes, in which a sequence of pulses has one of the two stable shapes. Firstly, it was demonstrated that the systematic offset of the abnormal mode TOAs from normal mode TOAs is associated with magnetospheric mode changing, rather than the effect of template matching. Secondly, the shape of each pulse profile was quantified using the fractional occurrence of one of the stable pulse shapes and the variations of the spin-down rate was obtained by modelling the timing noise with Gaussian process regression. There was a strong correlation between the fractional occurrence and the timing noise during the section of periodic modulation for which a correlation coefficient of 0.6 was found. There was also a strong correlation between the fractional occurrence and the spin-down variation during the periodic modulation. These results show that the rate of change between these pulse shapes is itself changing with time, and corresponds to variations in the spin-down rate.

Pulsars are known to emit radiation which is observed to vary in a vast range of frequencies, from very low radio frequencies (e.g. Pilia et al., 2016) even up to gamma rays (e.g Thompson, 2004), mainly due to the randomly disturbed around the universe ionised matter. However, lately it has been noted, particularly in the work of Bilous et al. (2016), that, when observed in very low radio frequencies, a few pulsars show fluctuations in their flux densities and spectra in magnitudes that require further explanation than just scintillation effects. Driven by their results, a supplementary monitoring campaign was conducted with the LOFAR telescope. For a total of four months, a set of thirteen pulsars were observed with a ∼2-weeks interval for a maximum of 30-minutes for each observation during the transit of each source. With the intention of getting a more thorough understanding of the large variability in their flux densities and spectra on timescales of both minutes and weeks, imaging data were acquired simultaneously to beam-formed. In this thesis, two pulsars, PSR B2016+28 and PSR B2020+20, were chosen from the observed set, due to their brightness and close proximity. The beam-formed and imaging data were processed to obtain flux density measurements and spectra, while additional analysis was done for the RFI mitigation process, the dependence of the results from the weighting scheme as well as the influence of the beam model on the measurements. Observational and intrinsic effects are being taken into account, as well as calibration errors, and discussed in an effort to explain the phenomenon. Our major concern, however, revolves around a large variability appearing simultaneously in beam-formed and imaging measurements for both sources, leading us to conclude that a better calibration scheme, which takes into consideration ionospheric effects and large contaminating sources in the field of the target, needs to be included in the standard analysis of the imaging LOFAR data. Furthermore, the results from image processing are deemed essential, as it can account for any misconceptions beam-formed data analysis may lead to.

It is estimated that there are ∼100,000 radio emitting pulsars in the Galaxy. The characteristic narrow beams of pulsars means that only some of these will be visible from Earth due to necessary alignment of the radiation beam across our line of sight. Over 2500 pulsars have been discovered since their initial discovery fifty years ago, this year. Such a small sample of the total population can provide only limited knowledge of the different groups, properties and physics of pulsars. Some of the most basic questions surrounding their origin and radiation processes remain open but pulsar surveys provide a way of discovering new sources capable of answering them. The discovery of pulsars has always required innovative hardware and software. Their discovery, although serendipitous, relied on high time resolution, large collecting area, and plenty of on-sky time. In this thesis, I present results from two major surveys using a latest generation telescope and one of the most technically advanced surveys yet undertaken, and discuss their results, challenges and opportunities. The LOFAR Tied Array All Sky (LOTAAS) survey is an ongoing all-Northern sky search for pulsars and transients using the LOw Frequency ARray (LOFAR). It it the first large-scale pulsar survey at the low frequency of 135 MHz using a multi-beam interferometer. The survey uses 222 beams, generated in software, in a single one hour observation of mixed tied-array beams (TABs) and sub-array pointings (SAPs). Together they simultaneously provide a large field of view of ∼ 60 square degrees and achieving sub mJy sensitivity. The sky will be observed in full with the TABs such that the SAPs will cover the sky three times over. In this thesis, I present the results of the first LOTAAS sky pass (of three). Using a pipeline that I co-developed I have efficiently processed more than 2.5 PB of data using the Dutch national supercomputer Cartesius. Processing of the survey has resulted in the redetection of 155 known pulsars that I use to analyse the sensitivity of the survey by comparing the detected fluxes to the expected fluxes of those known pulsars. I show that the LOTAAS survey fluxes mostly agree to within a factor of two of the expected fluxes extrapolated from the published values at 400 MHz. The 155 redetected known 13 pulsars include 5 millisecond pulsars as well as 22 pulsars that were blindly and independently discovered in the LOTAAS and Green Bank North Celestial Cap surveys, demonstrating the discovery potential of the survey. I present the basic parameters of the first 20 pulsars discovered in the LOTAAS survey. I demonstrate how LOFAR’s multi-beaming capabilities can be exploited for the localisation and confirmation of discoveries. All pulsars discovered in the survey are monitored in long term timing programmes with the LOFAR Core and the Lovell Telescope as well as with the UK and German international LOFAR stations for the brightest sources. I present the phase-coherent timing solutions of 17 pulsars including those of PSR J0140+56 and PSR J0614+37 that were first discovered in the LOFAR Tied-Array Survey (LOTAS), a pre-cursor to LOTAAS. I show that the spin proper- ties of the LOFAR pulsars suggest a possible over abundance of LOFAR discovered sources towards the death line in the P − P ̇ diagram. From this I infer that the LOTAAS survey is preferentially discovering pulsars that are older (τc > 10 Myr) with low magnetic field strengths (B < 1 TG). I present the discovery and average profiles of the 22 pulsars discovered with LO- FAR and, with the exception of PSR J1529+40, all pulsars were found to have a mea- sured duty cycle less than 10% and more than half have duty cycles less than 5%. This is true for all LOFAR pulse profiles at 148 and 1520 MHz. There is no clear evidence to suggest a preferred radius to frequency mapping, although we find that it is definitely not the case that pulse widths are typically broader at lower frequencies. We have observed all of these 22 pulsars with the Lovell Telescope at 1520 MHz, of which only eight are detected regularly. For these pulsars the average profiles at 1520 MHz are presented and their width evolution with frequency examined. PSR J1529+40 displays significant profile evolution between the two frequencies. This pulsar also has a very small period derivative (10−19) compared to pulsars with similar periods (0.5 seconds). In this thesis, I also present a summary of results and discoveries of the High Time Resolution Universe (HTRU) High Latitude survey. The processing of survey data was performed at University of Manchester, UK and Swinburne University in Australia using two different pipelines named DTSC and Morello respectively. Analysis with the the DTSC pipeline led to the discovery of 7 new pulsars (Thornton, 2013) and processing with the Morello pipeline (Morello, 2016) led to the discovery of 6 pulsars presented here. Five other pulsars were discovered in the high latitude survey bringing the total number to 18 discoveries (P> 100 ms). This is greater than the 11 new pulsars 14 estimated from population synthesis simulations by Keith et al. (2010). This is an increase of 60% and the 18 pulsars presented here account for 8% of the known high latitude population. I present the phase coherent timing solutions for five of these 18 pulsars and their average profiles. The two HTRU pipelines described in this thesis, DTSC and Morello, are used as a comparison of search techniques. I show that the performance of the Morello pipeline is better and that the DTSC pipeline fails to detect three of the 18 pulsars discovered in the high latitude survey. Both surveys presented in this thesis have generated tens of millions of candidates. I explore the different methods for filtering candidate numbers to reduce the number that are needed to be viewed by eye. One of these methods is machine learning classification. We present, with Lyon et al. (2016), a set of 8 new features that are, in combination with a Decision Tree classifier, applied to both the LOTAAS and HTRU survey candidates. For LOTAAS, the classifier reduces the 20,000 candidates produced per pointing to just 500 for visual inspection. In the case of HTRU, of the 1.5 million candidates generated with periods greater than 100 ms, 350,000 are predicted to be positive, i.e. a pulsar. I test the performance of the classifier and show that it achieves 94% recall on the LOTAAS dataset and 100% recall on the HTRU dataset. However, for both surveys, we found the false positive rate to be high, up to 80% for HTRU. We demonstrate the ability of the classifier to separate pulsars from candidates arising from noise but show that radio frequency interference now presents the next challenge in candidate selection.

One of the most fundamental unknowns about pulsars is the mechanism bywhich they radiate. Although many steps have been taken towards understandingthis in the 48 years since their discovery, there is still much to learn. The recentlaunch of the Fermi γ-ray satellite has led to an astonishing increase in thenumber of pulsars detected in γ-rays, making this an ideal time to consider theemission at both low energies (radio) and high energies (γ-rays).In this thesis we use radio polarisation data and a novel method to determine the viewing geometries of two samples of pulsars, one consisting of youngpulsars which have been detected in γ-rays and the other consisting of comparable young pulsars which are not detected in γ-rays. We find that the magneticinclination angle distribution of the γ-ray-detected pulsars does not match thecommonly-expected random distribution for such young pulsars as these, insteadbeing skewed towards lower values. We find two possible explanations for this.The first is that the expectation of a random distribution is erroneous, a situationwhich will have important consequences for pulsar population studies, considerations of the origins of magnetic fields in neutron stars and models of the supernovaprocess. The alternative is that the radio beams are larger than expected by afactor which depends on the magnetic inclination, which would have implicationsfor the magnetic field structure.We also find that the γ-ray- and non-γ-ray-detected pulsars differ in radioprofile width. This leads us to present a unified model of these young pulsarswhere the γ-ray detectability is highly dependent on magnetic inclination, whichdetermines if our line of sight samples the γ-ray emission. We report for the firsttime ever a significant correlation between the radio and γ-ray profile morphology. This is independent confirmation that the existing models of the radio andγ-ray emission regions are, at least qualitatively, realistic, although significantquantitative problems are reported.

The geometry of pulsar beams is one of the intrinsic properties of neutron stars, governing the pulse-profile phenomenon and other aspects of pulsar astron- omy. With a number of pulsars in our dataset, their beam geometry is derived from the polarisation position angle (PPA) using the simple polar cap emission and dipole field model. This includes the rotating vector model (RVM), for which the solutions can hardly be constrained or fail to be consistent because of the lim- itations of the model itself. The inconsistencies in the results suggest that the initial PPAs can be strongly perturbed by additional parameters above the emis- sion altitude, such as the plasma medium or rotational aberration effects, after which their characteristic shape is no longer related to the geometry via the RVM. We investigate further into the effects of wave propagation in the pulsar magne- tosphere, and find an indication that, in most cases, the RVM-calculated PPAs are likely to be altered by plasma effects.In recent years, there have been an increasing number of intermittent and mode-switching pulsars observed to have their radio pulse profiles correlated with the change in pulsar spin frequency (ν ̇) (e.g. Lorimer et al. 2012, Lyne et al. 2010). These two phenomena are understood to be related via the states of plasma in the magnetosphere. As one such pulsar, and also one with known geometry and other astonishing behaviour, PSR B1822–09 is studied in terms of the mode- switching properties, the hollow-cone model and the wave propagation in the magnetosphere. We also study the model for explaining the intermittent pulsars PSRs B1931+24, J1841+0500 and J1832+0029, and find it can be consistently applied for PSRs B1822–09 and B0943+10, and other profile-switching pulsars. However, aspects of the conclusions are limited because of the lack of understand- ing of the connection between the radio flux and the states of plasma. We are also able to use the difference in the PPAs between two states of PSR B0943+10 to predict the change in plasma states and ν ̇, which cannot be measured directly from timing analysis as its switching timescale is too short.