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Title: Searching for pulsars : from multi-beam receivers to interferometers
Author: Cooper, Sally
ISNI:       0000 0004 7429 6932
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2017
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It is estimated that there are âˆ1⁄4100,000 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 âˆ1⁄4 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 LOFARâ€TMs 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 (Ï„c > 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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: pulsar ; astronomy ; machine learning