München, Univ., Diss., 2001. / Computerdatei im Fernzugriff.

München, Techn. Universiẗat, Diss., 2001.

This thesis is concerned with the detection of very high energy 7-rays from isolated pulsars and objects that are not known to emit pulsed radiation, using the atmospheric Cerenkov technique. The first chapter summarises the processes by which celestial 7-rays can be generated and absorbed. A discussion of the criteria for selecting objects for study is given. The second chapter describes the atmospheric Cerenkov technique and gives a brief outline of its historical development. Details of the design and operation of the current Durham university V.H.E. 7- ray telescopes are given. The third chapter describes the standard analysis techniques used to search for pulsed and unpulsed emission. The fourth chapter presents results of new observations of four isolated pulsars. Upper limits to the V.H.E. 7-ray flux are derived for each pulsar. A summary of the results is given and conclusions drawn regarding the consequences of the observations for the most popular pulsar model. The fifth chapter deals with new analysis techniques that have been developed to test data for the presence of a signal which is not necessarily periodic. A number of different approaches is described. The results of these techniques when applied to simulated data are presented and conclusions are drawn regarding the relative effectiveness of the tests. The sixth chapter applies the most successful test to data from three objects. Upper limits to the flux from Centaurus A and SN1987aare derived from our observations. Evidence for V.H.E. 7-ray emission from Scorpius X-1 is presented, together with evidence that this emission is modulated with the period of the binary orbit of this system. The final chapter summarises the preceding results and conclusions and indicates areas where current research may lead to substantial improvements in telescope design and analysis techniques.

University, Diss., 2001--München.

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.

Un pulsar est une étoile à neutrons en rotation rapide et dotée d'un fort champ magnétique qui peut se manifester en émettant sur tout le spectre électromagnétique. Dans cette thèse, je m’intéresse au rayonnement radio produit par l’étoile. Je commence ainsi par décrire l'instrumentation de dédispersion cohérente à base de GPUs installée au Radiotélescope de Nançay avec deux autres modes d'observation que j'ai développés : un mode pour la recherche de nouveaux pulsar et un mode spectromètre. Une autre partie de ce travail détaille le retraitement en cours du sondage Foster fait à la fin des années 90 à Nançay ainsi que de nouvelles observations ciblées, sur des candidats HESS notamment. Je présente ensuite les résultats obtenus sur les pulsars relativistes J0737-3039A et J1906+0746 avec respectivement des tests de la Relativité Générale et la mesure de la précession géodétique. Des données de polarimétrie ont ainsi permis de déterminer la géométrie du système de PSR J1906+0746. Enfin, je termine par l'analyse des temps d'arrivées de 20 pulsars millisecondes observés à Nançay dans le cadre de l'EPTA, une collaboration européenne pour un réseau de chronométrage pulsar avec pour objectif la détection d'un fond d'ondes gravitationnelles, possible d'ici 5 à 10 ans. / A pulsar is a highly magnetized and fast-spinning neutron star which can be seen in radio waves. ln this thesis, l first describe the new GPU-based coherent dedispersion backend installed at the Nançay Radio Telescope with two others observing modes l have developped : a filterbank for(pulsar surveysjand a high resolution spectrometer. The second part deals with the ongoing reprocessing of the Foster survey done at Nançay in the late 90s and with new targeted pointings on HESS sources. l then present the results l got on the relativistic binaries J0737-3039A et J1906+0746, with new tests of General Relativity and the measurement of geodetic precession respectively. Polarimetric data has allowed to deduce the geometry of the PSR J1906+0746 system. ln the framework of the EPTA (European Pulsar Timing Array), l finish with the analysis of the times of arrivaIs from 20 millisecond pulsars, which aims to make a detection of the Gravitational Wave Background, which could succeed in the next 5 to 10 years.

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.

Pulsars are laboratories for extreme physics unachievable on Earth. As individual sources and possible orbital companions can be used to study magnetospheric, emission, and superfluid physics, general relativistic effects, and stellar and binary evolution. As populations they exhibit a wide range of sub-types, with parameters varying by many orders of magnitude signifying fundamental differences in their evolutionary history and potential uses. There are currently around 2200 known pulsars in the Milky Way, the Magellanic clouds, and globular clusters, most of which have been discovered with radio survey observations. These observations, as well as being suitable for detecting the repeating signals from pulsars, are well suited for identifying other transient astronomical radio bursts that last just a few milliseconds that either singular in nature, or rarely repeating. Prior to the work of this thesis non-repeating radio transients at extragalactic distances had possibly been discovered, however with just one example status a real astronomical sources was in doubt. Finding more of these sources was a vital to proving they were real and to open up the universe for millisecond-duration radio astronomy. The High Time Resolution Universe survey uses the multibeam receiver on the 64-m Parkes radio telescope to search the whole visible sky for pulsars and transients. The temporal and spectral resolution of the receiver and the digital back-end enable the detection of relatively faint, and distant radio sources. From the Parkes telescope a large portion of the Galactic plane can be seen, a rich hunting ground for radio pulsars of all types, while previously poorly surveyed regions away from the Galactic plane are also covered. I have made a number of pulsar discoveries in the survey, including some rare systems. These include PSR J1226-6208, a possible double neutron star system in a remarkably circular orbit, PSR J1431-471 which is being eclipsed by its companion with each orbit, PSR J1729-2117 which is an unusual isolated recycled pulsar, and PSR J2322-2650 which has a companion of very low mass - just 7 x 10⁻⁴M_⊙, amongst others. I begin this thesis with the study of these pulsars and discuss their histories. In addition, I demonstrate that optical observations of the companions to some of the newly discovered pulsars in the High Time Resolution Universe survey may result in a measurement of their age and that of the pulsar. I have discovered five new extragalactic single radio bursts, confirming them as an astronomical population. These appear to occur frequently, with a rate of 1.0^+0.6_-0.5 x 10⁴ sky⁻¹ day⁻¹. The sources are likely at cosmological distances - with redshifts between 0.45 and 1.45, making them more than half way to the Big Bang in the most distant case. This implies their luminosities must be enormous, 10³¹ to 10³³ J emitted in just a few milliseconds. Their source is unknown but I present an analysis of the options. I also perform a population simulation of the bursts which demonstrates how their intrinsic spectrum could be measured, even for unlocalised FRBs: early indications are that the spectral index of FRBs < 0.

This thesis reports on the recent results of a continuing, high-precision pulsar timing project, currently focused on the nearby, binary millisecond pulsar, PSR J0437_4715. Pulse arrival time analysis has yielded a remarkable series of constraints on the physical parameters of this system and evidence for the distortion of space-time as predicted by the General Theory of Relativity. Owing to the proximity of the PSR J0437_4715 system, relative changes in the positions of the Earth and pulsar result in both annual and secular evolution of the line of sight to the pulsar. Although the changes are miniscule, the effects on the projected orbital parameters are detectable in our data at a high level of significance, necessitating the implementation of an improved timing model. In addition to producing estimates of astrometric parameters with unparalleled precision, the study has also yielded the first three-dimensional orbital geometry of a binary pulsar. This achievement includes the first classical determination of the orbital inclination, thereby providing the unique opportunity to verify the shape of the Shapiro delay and independently confirm a general relativistic prediction. With a current post-fit arrival time residual RMS of 130 ns over four years, the unrivaled quality of the timing data presented herein may eventually contribute to the most stringent limit on the energy density of the proposed stochastic gravitational wave background. Continuing the quest for even greater timing precision, a detailed study of the polarimetry of PSR J0437_4715 was undertaken. This effort culminated in the development of a new, phase-coherent technique for calibrating the instrumental response of the observing system. Observations were conducted at the Parkes 64-m radio telescope in New South Wales, Australia, using baseband recorder technologies developed at York University, Toronto, and at the California Institute of Technology. Data were processed off-line at Swinburne University using a beowulf-style cluster of high-performance workstations and custom software developed by the candidate as part of this thesis.

L'existence des ondes gravitationnelles, fluctuations de l'espace-temps lui-même, a été prédite sans, pour l'instant, qu'une détection directe n'ait été encore possible. A l'heure actuelle, des méthodes consistant en des détecteurs interférométriques de plusieurs kilomètres de long sont à l'oeuvre pour permettre une première détection. Nous proposons, dans cette thèse, d'étudier une autre méthode : la chronométrie d'un réseau de pulsars milliseconde. Elle consiste en l'observation régulière et la datation précise des impulsions radio en provenance de pulsars ultrastables. L'onde gravitationnelle produisant retards ou avances des impulsions sur Terre, nous recherchons sa présence sous forme d'un signal corrélé entre les observations faites des différents pulsars du réseau. Dans un premier temps, nous détaillons les processus d'observation et de chronométrie des pulsars, pour nous pencher sur un cas particulier avec le pulsar J1614-2230. Puis, nous présentons les ondes gravitationnelles et leurs sources ainsi que les différentes méthodes de détection. Nous décrivons tout particulièrement la méthode de chronométrie d'un réseau de pulsars appliquée à la recherche d'un signal en provenance d'un système binaire de tous noirs supermassifs. Ensuite, après avoir détaillé les outils statistiques et numériques utilisés, nous appliquons notre méthode à l'injection d'un tel signal dans les observations réelles faites dans le cadre de l'EPTA. Enfin, nous présentons les limites supérieures sur l'amplitude d'un signal en provenance d'un système binaire obtenues sur ces données sans injection grâce à notre méthode en fonction de la fréquence et de la position de la source. / The existence of gravitational waves, ripples in space-time itself, has been predicted but their detection remains elusive. Multiple techniques exist for searching for them, including ground-based kilometer long inteferometers. In this thesis, we present an alternative approach, based on the monitoring and precise timing of radio pulses from an array of millisecond pulsars. A gravitational wave will perturb the propagation of those radio pulses, causing them to reach the Earth with a certain delay. By searching for correlations in the arrival times of the pulsations from different pulsars, we can in principle infer the presence of gravitational waves from observations. We begin by giving an overview of pulsar observations and timing. We illustrate those principles with a practical example : the study of the millisecond pulsar J1614-2230. In the second part we describe gravitational waves, the sources that create them, and the various detection methods. Then, we focus on the pulsar timing array technique, and its potential application to the search for gravitational waves from supermassive black hole binary system. We pursue with a detailed description of the statistical and numerical tools that we used in the present work, and present the results of a search ofr an injected signal in the real EPTA data set. Finally, we employ our new method to derive upper limits on the amplitude of a putative signal in the same EPTA data set, as a function of the frequency and sky location of the supermassive black hole binary system.