Innovative Pulsar Searching Techniques

Abstract

Pulsars are rapidly rotating, highly magnetised neutron stars which emit beams of electromagnetic radiation from their magnetic poles, most commonly in the radio spectrum. These massive and extremely compact objects have emerged as fantastic physical tools suitable to a wide variety of scientific applications, perhaps the most important being their role in testing and constraining of General Relativity and other alternative theories of gravity. This thesis presents the results of an ongoing search for new and scientifically-interesting pulsars in the High Time Resolution Universe South Low Latitude (HTRU-S LowLat) pulsar survey, and includes the discovery of PSR J1757-1854, the first relativistic binary pulsar to be discovered as part of this survey. Also presented in this thesis is a detailed study of the Fast Folding Algorithm (FFA), an additional pulsar searching technique capable of overcoming some of the limitations of more commonly used search techniques such as the Fast Fourier Transform (FFT), presenting a new avenue by which further pulsars discoveries may be made.<br /> Chapter 1 of this thesis summarises the essential ideas surrounding the formation, structure and behaviour of pulsars in the radio regime and their interaction with the interstellar medium (ISM). Also discussed are some of the specific scientific investigations to which pulsars have been applied and which motivate ongoing pulsar searches, with particular emphasis on the milestones achieved to date in the study of gravitational theories through the use of relativistic binary pulsars. Chapter 2 then details the fundamentals of standard pulsar searching methodology, with extra attention given both to binary search techniques and the basic principles behind the FFA, a time-domain pulsar searching technique. A summary is also provided of the techniques involved in pulsar timing, one of the primary tools by which scientific information is extracted from discovered pulsars, with an additional focus once again given to the timing of relativistic binary pulsars. In order to set the context of the HTRU-S LowLat pulsar survey, Chapter 2 concludes with a summary of previous and current pulsar searches conducted of the Galactic-plane region.<br /> The HTRU-S LowLat pulsar survey, along with its particular searching and processing methodology, is introduced in Chapter 3. This survey, conducted with the Parkes 64-m Radio Telescope in Australia, covers a region between -80° < l < 30° and |b| < 3.5° and was conducted with the specific goal of discovering new relativistic binary pulsars which can be applied to future tests of gravitational theories. To date, approximately 79% of the survey has been processed through a novel `partially-coherent segmented acceleration search' pipeline designed to optimise the detection of tight binary systems with short orbital periods. From the ~44% of the survey processed for this thesis, a total of 40 new pulsar discoveries are reported, at least 7 of which are in binary systems. Selected highlights include PSR J1618-4624, a millisecond pulsar (MSP) orbiting a carbon-oxygen-white dwarf (CO-WD) companion which, with an orbital period of 1.78 days and a spin period of only 5.93 ms, presents a challenge to current binary formation models. Two further MSPs with helium-white dwarf (He-WD) companions, PSRs J1537-5312 and J1547-5709, are also reported, along with a fourth MSP, the black widow system PSR J1745-23. Additionally, PSRs J1812-15 and J1831-04 are a pair of ~1 s, nulling/intermittent pulsars which show evidence of acceleration, indicating possible binary systems. PSR J1706-4434 meanwhile exhibits glitching behaviour, while PSR J1653-45 is an eclipsing binary system with a likely orbital period on the order of months to years, appearing similar to the previously known PSR B1259-63. Finally, the full set of 100 pulsars discovered to date in the HTRU-S LowLat survey is compared to the background population of Galactic-plane pulsars, indicating that HTRU-S LowLat has uncovered a population of older, lower-luminosity pulsars. Evaluations of the survey yield and performance are also presented, indicating that while the ~44% of the HTRU-S LowLat survey presented in this thesis has been processed to a standard consistent with the earlier processing conducted by Ng et al. (2015), the survey appears to fall below its expected pulsar yield by ~25% in the case of young, slow pulsars and by as much as ~150-250% in the case of MSPs.<br /> The flagship discovery of the HTRU-S LowLat survey, PSR J1757-1854, is presented in Chapter 4. A 21.5-ms pulsar in a 4.4-h orbit around a neutron star (NS) companion, this double neutron star (DNS) system exhibits some of the most extreme relativistic qualities of any known radio pulsar, including the strongest observed relativistic effects due to gravitational wave (GW) damping. PSR J1757-1854 represents precisely the type of pulsar the HTRU-S LowLat survey and its search pipeline were designed to find. A 1.6-yr, multi-frequency, multi-telescope timing campaign has resulted in the measurement of five post-Keplerian (PK) parameters, allowing for three immediate tests of gravitational theories, and also allowing for the masses of the pulsar and its companion neutron star to be separated. The larger mass of the companion neutron star and the high orbital eccentricity provide important clues regarding the system's binary formation. Timing simulations suggest that a 3-sigma measurement of both the contribution of Lense-Thirring precession to the rate of change of the semi-major axis as well as the relativistic deformation of the orbit will be possible within ~7-9 years. Both of these quantities have remained poorly constrained in other relativistic binary pulsars, including both the Hulse-Taylor pulsar PSR B1913+16 and the Double Pulsar PSR J0737-3039, such that PSR J1757-1854 stands out as a unique laboratory for new tests of gravitational theories, particularly general relativity (GR).<br /> Chapter 5 presents an in-depth study of the behaviour of the FFA, an alternative pulsar searching technique to the FFT. Although the FFT forms the backbone of most pulsar-searching efforts, including the pipeline currently employed on the HTRU-S LowLat survey, weaknesses in the FFT (including a susceptibility to red noise) leave it insensitive to pulsars with long rotational periods (P > 1 s). This sensitivity gap may result in a biased understanding of the true underlying pulsar population. The FFA, a coherent time-domain search technique, has the potential to overcome some of these biases, although many aspects of the behaviour of this search technique remain poorly understood, including its responsiveness to variations in pulse shape and to the presence of red noise. This chapter documents an extensive evaluation of the behaviour of the FFA with respect to variations in noise content and pulse shape using a custom software package `<i>ffancy</i>', including a comparison of the performance of the FFA against the FFT on real data taken from the HTRU-S LowLat survey. While the superiority of the FFA to the FFT in the long-period regime is demonstrated in both simulated and real data, there remains significant room for improvement in terms of the implementation and evaluation of the FFA and its evaluation algorithms.<br /> Lastly, Chapter 6 closes with a summary of the most important scientific results derived as part of this thesis project. Additional discussion is given to the future of the HTRU-S LowLat survey, including improvements to be included in future survey re-processing and the next steps required in the further scientific exploitation of the pulsar discoveries reported in this thesis. Future goals for PSR J1757-1854, and their reliance on the next generation of radio telescopes and instrumentation, are also discussed. An overview is also given of the future role of the FFA and its ongoing development and implementation as part of the next generation of pulsar surveys, where it is already breaking ground in the discovery of long-period pulsars. Finally, the chapter concludes with a brief summary of the future telescopes and pulsar surveys that will be the drivers of pulsar science over the coming decades.