Dissertations / Theses on the topic 'Pulsar Search'

Pulsars are rapidly rotating neutron stars which emit a strong beam of energy through mechanisms that are not entirely clear to physicists. These very dense stars are used by astrophysicists to study many basic physical phenomena, such as the behavior of plasmas in extremely dense environments, behavior of pulsar-black hole pairs, and tests of general relativity. Many of these tasks require information to answer the scientific questions posed by physicists. In order to provide more pulsars to study, there are several large-scale pulsar surveys underway, which are generating a huge backlog of unprocessed data. Searching for pulsars is a very labor-intensive process, currently requiring skilled people to examine and interpret plots of data output by analysis programs. An automated system for screening the plots will speed up the search for pulsars by a very large factor. Research to date on using machine learning and pattern recognition has not yielded a completely satisfactory system, as systems with the desired near 100% recall have false positive rates that are higher than desired, causing more manual labor in the classification of pulsars. This work proposed to research, identify, propose and develop methods to overcome the barriers to building an improved classification system with a false positive rate of less than 1% and a recall of near 100% that will be useful for the current and next generation of large pulsar surveys. The results show that it is possible to generate classifiers that perform as needed from the available training data. While a false positive rate of 1% was not reached, recall of over 99% was achieved with a false positive rate of less than 2%. Methods of mitigating the imbalanced training and test data were explored and found to be highly effective in enhancing classification accuracy.

The aim of this Thesis is to study the development of pulsar wind nebulae in the TeV regime and in doing so uncover more sources which have as yet not been observed at these wavelengths. It is found that the extent of pulsar wind nebula in the TeV gamma-ray increases with its age while no developmental relationship is seen concerning the luminosity or spectral index of the nebulae when observed in the TeV gamma-ray regime due to uncertainties in the measurements available. TeV gamma-ray upper limits are calculated for several nebulae observed in the X-ray regime allowing the strength of their magnetic fields to be constrained but only one new source, which was previously confused with its companion, was discovered, the Eel Nebula. Predictions of the fluxes of many of the sources for which upper limits are derived in this work have been calculated from observations of their emission in X-rays and some of these sources should be uncovered with the next generation CTA instrument.

Searches for pulsars during the past fifty years, have been characterised by two problems making their discovery difficult: i) an increasing volume of data to be searched, and ii) an increasing number of `candidate' pulsar detections arising from that data, requiring analysis. Whilst almost all are caused by noise or interference, these are often indistinguishable from real pulsar detections. Deciding which candidates should be studied is therefore difficult. Indeed it has become known as the `candidate selection problem'. This thesis presents an interdisciplinary study of the selection problem, with the aim of developing a new method able to mitigate it. Specifically for future pulsar surveys undertaken with the Square kilometre Array (SKA). Through a combination of critical literature evaluations, theoretical modelling exercises, and empirical investigations, the selection problem is described in-depth here for the first time. It is shown to be characterised by the dominance of Gaussian distributed noise signals, a factor that no existing selection method accounts for. It also reveals the presence of a significant trend in survey data rates, which suggest that candidate selection is transitioning from an off-line processing procedure, to an on-line, and real-time, decision making process. In response, a new real-time machine learning based method, the GH-VFDT, is introduced in this thesis. The results presented here show that a significant improvement in selection performance can be achieved using the GH-VFDT, which utilises a learning procedure optimised for data characterised by skewed class distributions. Whilst the principled development of new numerical features that maximise the separation between pulsars and Gaussian noise, have also greatly improved GH-VFDT pulsar recall. It is therefore concluded that the sub-optimal performance of existing selection systems, is due to a combination of poor feature design, insensitivity to noise, and an inability to deal with skewed class distributions.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, February 2008.
Includes bibliographical references (p. 167-171).
Over the last several years the Laser Interferometer Gravitational Wave Observatory (LIGO) has been making steady progress in improving the sensitivities of its three interferometers, two in Hanford, Washington, and one in Livingston, Louisiana. These interferometers have reached their target design sensitivities and have since been collecting data in their fifth science run for well over a year. On the way to increasing the sensitivities of the interferometers, difficulties with increasing the input laser power, due to unexpectedly high optical absorption, required the installation of a thermal compensation system. We describe a frequency resolving wavefront sensor, called the phase camera, which was used on the interferometer to examine the heating effects and corrections of the thermal compensation system. The phase camera was also used to help understand an output mode cleaner which was temporarily installed on the Hanford 4 km interferometer. Data from the operational detectors was used to carry out two continuous gravitational wave searches directed at isolated neutron stars. The first, targeted RX J1856.5-3754, now known to be outside the LIGO detection band, was used as a test of a new multi interferometer search code, and compared it to a well tested single interferometer search code and data analysis pipeline. The second search is a targeted search directed at the Crab pulsar, over a physically motivated parameter space, to complement existing narrow time domain searches. The parameter space was chosen based on computational constraints, expected final sensitivity, and possible frequency differences due to free precession and a simple two component model.
(cont.) An upper limit on the strain of gravitational radiation from the Crab pulsar of 1.6 x 10-24 was found with 95% confidence over a frequency band of 6 x 10-3 Hz centered on twice the Crab pulsar's electromagnetic pulse frequency of 29.78 Hz. At the edges of the parameter space, this search is approximately 105 times more sensitive than the time domain searches. This is a preliminary result, presently under review by the LIGO Scientific Collaboration.
by Joseph Betzwieser.
Ph.D.

The high time-resolution radio sky represents unexplored astronomical territory where the discovery potential is high. In this thesis I have studied the transient radio sky, focusing on millisecond scales. As such, this work is concerned primarily with neutron stars, the most populous member of the radio transient parameter space. In particular, I have studied the well known radio pulsars and the recently identified group of neutron stars which show erratic radio emission, known as RRATs, which show radio bursts every few minutes to every few hours. When RRATs burst onto the scene in 2006, it was thought that they represented a previously unknown, distinct class of sporadically emitting sources. The difficulty in their identification implies a large underlying population, perhaps larger than the radio pulsars. The first question investigated in this thesis was whether the large projected population of RRATs posed a problem, i.e. could the observed supernova rate account for so many sources. In addition to pulsars and RRATs, the various other known neutron star manifestations were considered, leading to the conclusion that distinct populations would result in a 'birthrate problem'. Evolution between the classes could solve this problem - the RRATs are not a distinct population of neutron stars. Alternatively, perhaps the large projected population of RRATs is an overestimate. To obtain an improved estimate, the best approach is to find more sources. The Parkes Multi-beam Pulsar Survey, wherein the RRATs were initially identified, offered an opportunity to do just this. About half of the RRATs showing bursts during the survey were thought to have been missed, due to the deleterious effects of impulsive terrestrial interference signals. To remove these unwanted signals, so that we could identify the previously shrouded RRATs, we developed new interference mitigation software and processing techniques. Having done this, the survey was completely re-processed, resulting in the discovery of 19 new sources. Of these, 12 have been re-detected on multiple occasions, whereas the others have not been seen to re-emit since the initial discovery observations, and may be very low burst-rate RRATs, or, isolated burst events. These discoveries suggest that the initial population estimate was not over-estimated - RRATs, though not a distinct population, are indeed numerous. In addition to finding new sources, characterisation of their properties is vital. To this end, a campaign of regular radio observations of the newly discovered sources, was mounted, at the Parkes Observatory, in Australia. In addition, some of the initially identified RRATs were observed with the Lovell Telescope at Jodrell Bank. These have revealed glitches in J1819-1458, with anomalous post-glitch recovery of the spin-down rate. If such glitches were common, it would imply that the source was once a magnetar, neutron stars with the strongest known magnetic fields of up to 10¹⁵ gauss. The observations have also been used to perform 'timing' observations of RRATs, i.e. determination of their spin-down characteristics. At the beginning of this thesis, 3 of the original sources had 'timing solutions' determined. This has since risen to 7, and furthermore, 7 of the newly discovered sources now also have timing solutions. With this knowledge, we can see where RRATs lie in period-period derivative space. The Parkes RRATs seem to be roughly classifiable into three groupings, with high observed nulling fractions - normal pulsars, high magnetic field pulsars and old, 'dying' pulsars. It seems that RRATs and pulsars are one and the same. When a pulsar is more easily detected in searches for single bright pulses, as opposed to in periodicity searches, we label it a RRAT. Such searches impart a selection effect on the parameter space of possible sources, in both nulling fraction and rotation period. In this sense, an observational setup could be designed to make any pulsar appear as a RRAT. For realistic survey parameters however, this is not the case, and the groups mentioned above seem to be the most likely to appear as RRATs. In fact, we can utilise RRAT searches to identify neutron stars, difficult to find by other means, in particular high-magnetic field pulsars, and pulsars approaching the pulsar "death valley". Some of the RRATs are well explained as being distant/weak pulsars with a high modulation index, others seem to be nulling pulsars. This highlights the incomplete knowledge of nulling behaviour in the pulsar population. It seems that there may be a continuum of nulling durations, under a number of guises, from 'nulling pulsars' to 'RRATs' to 'intermittent pulsars'. In fact this nulling may fit into the emerging picture, whereby pulsar magnetospheres switch between stable configurations.

Technological advances coupled with a decline in digital storage costs have resulted in a profusion of data being created, collected and consumed. These data give rise to new challenges and opportunities in many disciplines ranging from science and engineering to biology and finance. An example of a future project in radio astronomy that promises both Big Data and Big Discoveries is the Square Kilometre Array (SKA) radio telescope project. Astrophysicists are confident that the Big Data amassed by the SKA will not only answer fundamental questions regarding the Universe but also contain big discoveries not yet postulated. The transformational potential of the SKA and its ensuing data and algorithmic challenges, in particular for the discovery and study of pulsars, drive the research of this thesis. Discovering all pulsars beaming towards Earth is one of the key science goals of the SKA. However, in addition to low signal strengths, searching for pulsars is extremely difficult due to the intrinsic weakness of their signals, propagation effects and the presence of anthropogenic interferences. Numerous techniques have been developed to overcome some of these difficulties and to assist in the quest to find more pulsars. However, despite the success of these techniques, the number of pulsars discovered in recent surveys (Swiggum et al. 2014, Lazarus et al. 2015) has fallen well short of the number predicted by pulsar population synthesis models (Lorimer 2011). This shortfall in pulsar detections can be attributed to radio frequency interference (RFI), red noise and scintillation (Lazarus et al. 2015). For this thesis, and in order to investigate and quantify these claims, I first developed a new technique to simulate pulsar search data that contain different types of RFI and varying noise baselines (i.e. red noise). This surrogate modelling technique was then used in a framework that I developed to inexpensively explore the sensitivity of pulsar search pipelines for different noise and RFI settings. The results from this framework highlight the necessity to develop algorithms that are able to identify and remove non-stationary variations from the data before RFI excision and searching is performed in order to limit false positive detections. To address the shortcomings identified with the framework which assessed the performance of existing pulsar search pipelines, I developed a new real-time algorithm for excising RFI while simultaneously normalising the variability in time and frequency inherent to pulsar observations. Processing synthetic data with the algorithm resulted in an expansion of the noise/pulsar spin period parameter space for which we are able to successfully detect pulsars. Furthermore, the algorithm is shown to reduce the number of false positive detections. In conclusion, the insights gained from the work presented in this thesis and the improvements achieved will contribute to the development of a new realtime pulsar search pipeline adept at dealing with the challenges posed by the SKA.

Graphics processing units (GPUs) have been used to accelerate computation in a broad range of fields; this work presents a GPU-accelerated search for pulsars. Pulsars are highly magnetised neutron stars with extremely stable rotational periods. These periods can be accurately measured, which makes them exceptionally powerful reference tools in the field of astrophysics. Pulsars have very weak emissions, making them difficult to find. Most pulsars are found in large-scale surveys, which generate a large amount of data, and require extensive data processing. This work describes a GPU-based solution, with implications for real-time processing of pulsar search data. Pulsar astronomy uses radio telescope observations with high spectral and temporal resolution, which produce very large data sets and require intensive Digital Signal Processing. Large-scale pulsar surveys using next-generation radio telescopes such as the Square Kilometre Array (SKA), will have to be performed in real time as the volumes of raw data produced will be too large to be stored for an extended period. These computational requirements are compounded when searching for binary pulsars as their orbital motion makes them difficult to detect using classic periodicity searches. However, these rare pulsars are of great interest to physicists, as they allow us to test general relativity. Acceleration searches are the most common technique for detecting signals from binary pulsars that may be missed by standard search techniques. One of these, the frequency domain acceleration search (FDAS), mitigates the effect of orbital acceleration by correlating a matched template with the spectrum of a signal. This method has been shown to be more efficient than the alternative time domain acceleration search (TDAS)s. Even so, it is extremely computationally intensive to perform on a large scale. The existing implementation, Accelsearch, is run on a central processing unit (CPU), which limits its performance. We address this problem by creating a GPU port of the FDAS. An analysis of the fundamental calculations on which the FDAS is based informs the design of a fully asynchronous pipeline that exploits multiple levels of parallelism. This entails developing a novel technique for calculating Fresnel integrals, which increases the speed and numerical accuracy of the calculations, in both single- and double-precision. Furthermore, we develop a new estimate which improves the numerical accuracy of filter coefficients for accelerations close to zero. The GPU-accelerated pipeline achieves speeds 30 to 70 times faster than the existing serial CPU implementation. Our results clearly show that GPU acceleration is effective at reducing the cost of processing the FDAS component, to the point at which the SKA1-mid survey data could be searched in real time using 340 to 675 desktop GPUs from the Pascal generation.

Gravitational waves (GWs) have been detected for the first time in 2015 by the LIGO-Virgo Scientific Collaboration. The source of the GWs was a binary black hole (BBH). The observation caught the final fraction of a second as the two black holes spiralled together and merged. This observation (and the others to follow) marked the beginnings of GW astronomy, ‘a new window on the dark universe’, providing a means to observe astronomical phenomena which may be completely inaccessible via other avenues as well as a new testing ground for Einstein’s theory of general relativity (GR). However, this is just the beginning – like electromagnetic astrophysics, there is a full spectrum of GW frequencies to explore. At very low frequencies, pulsar timing arrays (PTAs) are being used to search for the GW background from the merging population of massive black hole binaries (MBHBs). No detection has yet been made, but upper limits have been placed. Here we present results on what inference on the MBHB population can be learnt from present and possible future PTA results, and also compare current upper limits with astrophysical predictions, finding them to be fully consistent so far. We also present a generic method for testing the consistency of a theory against experimental evidence in the situation where there is no strong viable alternative (for example GR). We apply this to BBH observations, finding them to be fully consistent with GR and also to Newton’s constant of gravitation, where there is considerable inconsistency between measurements.

Methods for searching for periodic gravitational wave signals from triaxial pulsars using interferometric gravitational wave detectors have been developed. Since the gravitational wave signals from pulsars are expected to be weak, long stretches of data must be used for any detection. Over the course of a day, and a year, these periodic signals are Doppler shifted due to the motion of the Earth. The response of the interferometers to each polarisation of gravitational waves will also give rise to an amplitude modulation of the periodic signal. These effects are taken into account and an end-to-end Bayesian scheme for making inferences from the data is presented. Several software tests have been performed to validate the core routines, such as barycentring, using independent software. The GEO 600 and LIGO interferometers had their first scientific data run (S1) for 17 days between 23 August and 9 September 2002. An analysis was carried out to search for gravitational wave signals from pulsar B1937+21. While no signals were detected, a 95% upper limit of h0 < 1.4 x 10-22 was determined using S1 data where h0 is the amplitude of the gravitational waves. Given that pulsar B1937+21 is at a distance of 3.6 kpc, and assuming a moment of inertia of 1038kg m2, the corresponding upper limit on the equatorial ellipticity was determined to be = 2.9 x 10-4. The upper limit on gravitational waves from pulsar B1937+21 using S1 data was over an order of magnitude lower than the previous best limit at the time. Data from LIGO's second science run (S2) in the spring of 2003 was analysed with the sensitivity of each detector in the network being roughly an order of magnitude better than in S1 across a large range of frequencies. Upper limits were placed on a total of 28 isolated pulsars using the S2 data. The analysis procedure for S2 was more robust to interfering spectral lines and took advantage of the longer stationarity of the S2 data. Two hardware injections of hypothetical pulsars were injected in the LIGO interferometers during S2. The successful extraction of these signals from the LIGO S2 data significantly increased our confidence in the the overall data analysis pipeline. For four of the closest pulsars their equatorial ellipticities were constrained to less than = 10-5 with 95% confidence. These limits are beginning to reach interesting ellipticities which some exotic theories suggest could be supported in neutron stars. The third science run (S3) in which GEO 600 and LIGO participated took place from late October 2003 to early January 2004. Again, the improvement in sensitivity compared to the previous run (S2) was significant. Preliminary multi-detector results were determined for the same previous 28 pulsars using S3 data. The equatorial ellipticities for 11 of these pulsars are constrained to less = 10-5 with 95% confidence. With the S3 data, the upper limit on the gravitational wave emission from the Crab pulsar was only approximately a factor of four from the upper limits inferred from the spindown of the pulsar. When this barrier is overcome the prospects of detecting gravitational waves from the Crab pulsar will become more plausible. Future work based on these implementations will examine a larger set of missing known pulsars including binary systems. Studies in Markov Chain Monte Carlo techniques may also allow the expansion this method to a larger parameter space.

We used the first station of the Long Wavelength Array (LWA) to observe giant pulses (GPs) from pulsars and search for other radio transients. Using the LWA with a bandwidth of 16 MHz at 39 MHz, we made a 24-hour observation of pulsar radio pulses from PSR B0950+08. The average pulse ux density and pulse width (dominated by "normal" pulses) are consistent with previous studies by others. Using techniques we developed for searching for radio transients, in this observation we detected 119 giant pulses (with signal-to-noise ratios 10 times larger than for the mean pulse). The giant pulses have a narrower temporal width (17.8 ms, on average) than the mean pulse (30.5 ms). Giant pulses occur at a rate of about 5.0 per hour, or 0.035% of the total number of pulse periods. The strength and rate of giant pulses is less than observed by others at ~100 MHz. The probability distribution of the cumulative pulse strength is a power law, but deviates from the Gaussian distribution of normal pulses. These results suggest PSR B0950+08 produces less frequent and weaker giant pulses at 39 MHz than at 100 MHz. We detected no other transients in this observation within a dispersion measure (DM) range from 1 to 90 pc cm³. Furthermore, we conducted observations of giant pulses from PSR B0950+08 in a separate set of observations of 12 hours made simultaneously at 42 and 74 MHz. In these observations we detected a total of 275 at 42 MHz and a total of 465 giant pulses at 74 MHz. Giant pulses with double-peak temporal structure have a shorter peak-to-peak separation compared to the average pulse. Once again, PSR B0950+08 appears to produce less frequent and weaker giant pulses than reported at 100 MHz. Giant pulses are identified with signal-to-noise ratios 10 times larger than for the mean pulse, and the probability distribution of the cumulative pulse strength is a power law, but deviate from the Gaussian distribution of normal pulses, for both frequencies. There were only 128 giant pulses detected simultaneously at 42 and 74 MHz, which implies that more than half of them are narrow-band radio pulses. Using these observations we analyzed the effect of scattering due to the interstellar medium on pulses with signal-to-noise ratio > 7 and the average pulse using a CLEAN-based algorithm, assuming a thin-screen scattering model. The scatter-broadening time constant τ ∝ ν^α, where ν is the observing frequency. The resulting α as calculated from pulses with signal-tonoise ratio > 7 and for the average pulses is found to be α = −1.45±0.14 and −0.14±0.03, respectively. These results indicate differences along the line of sight from a Kolmogorov spectrum for electron density uctuations. We calculated the altitude of the emission region for the pulsar using the dipolar magnetic field model. We found a similar magnitude for the emission region altitudes of normal and giant pulses. We detected no other transient pulses in a wide DM range from 1 to 4990 pc cm⁻³. We also conducted another a 12-hour observational study of PSR B0031−07 at 38 and 74 MHz, simultaneously. Giant pulses were identified with ux densities of a factor of ≥ 90 and ≥ 80 times that of an average pulse, at 38 and 74 MHz. The cumulative pulse strength distribution follows a power law, and has a much more gradual slope than a Gaussian distribution for the normal pulses. We found 158 of the observed pulses at 38 MHz qualified as giant pulses. At 74 MHz a total of 221 of the observed pulses were giant pulses. Only 12 giant pulses were detected within the same pulse period at both 38 and 74 MHz, meaning that the majority of them are narrow-band radio pulses. No other radio transients were detected within a DM range 1 to 4990 pc cm⁻³. We used the same data processing pipeline for observations of pulsar GPs to search within the pulsar observations for fast radio bursts (FRBs). We did not detect any nonpulsar signals with signal-to-noise ratio larger than 10. When the radio transient signals propagate through the interstellar medium, they are affected by propagation effects such as dispersion and scattering. Scattering may limit the detectability of radio transients. By examination of archived pulsar profiles, we investigated the impact of scattering on observed pulses. We utilized a CLEAN-based strategy to decide the scatter-broadening time, τ , under both the thin-screen and uniform-medium scattering models and to determine the scatter-broadening time frequency scaling index, α, where τ ∝ ν^α. In most cases the scattering tail was not large compared to the pulse width at half maximum. Still, we deconvolved 1342 pulse profiles from 347 pulsars assuming a Kolmogorov spectrum of the interstellar medium turbulence. For a subset of 21 pulsars the scattering-boarding tails were suficiently long to be estimated at the lowest frequencies. Since the scatter-broadening times were only determined distinctly for the subset of pulsars at the lowest observed frequency, we were restricted to utilizing upper limits on scatter-broadening times at higher frequencies for the assessment of the scatter-broadening-time frequency dependence. We include three new direct scatter-broadening time measurements at low frequencies and they are consistent with previous studies which were scaled from higher frequencies. Our findings are consistent with a relationship between the DM and scatter-broadening time which can range over more than two orders of magnitude in DM. One of the potential reasons that we did not detect FRBs is that transients may be highly scatter-broadened at low frequencies for high DM values.
Ph. D.

Astrophysical phenomena such as exploding primordial black holes (PBHs), gamma-ray bursts (GRBs), compact object mergers, and supernovae, are expected to produce a single pulse of electromagnetic radiation detectable at the low-frequency end of the radio spectrum. Detection of any of these pulses would be significant for the study of the objects themselves, their host environments, and the interstellar/intergalactic medium. Furthermore, a detection of a radio transient from an exploding PBH could be a signature of an extra spatial dimension, which would drastically alter our perception of spacetime. However, even upper limits on the existence of PBHs, from transient searches, would be important to discussions of cosmology. We describe a method to carry out an agnostic single-dispersed-pulse search, and apply it to data collected with the Eight-meter-wavelength Transient Array (ETA). ETA is a radio telescope dedicated to searching for transient pulses. It consists of 12 crossed-dipole antenna stands with Galactic-noise-limited performance from 29-47 MHz. There is a vast amount of data collected from an ETA observation. It is therefore greatly beneficial to use a computer cluster, which works in parallel on different parts of a data set, in order to carry out a single-pulse search quickly and efficiently. Each spectrogram in a data set needs to be analyzed individually, without reference to the rest, in order to utilize a computer cluster's capabilities. The data reduction software has been developed for single-dispersed-pulse searches, and is described in this thesis. The data reduction involves sweeping through the collected data with a dedispersion routine assuming a range of dispersion measures. The resulting time series are searched with multiple matched filters for signals above a signal-to-noise threshold. Applying the single pulse search to ~ 30 hours of ETA data did not yield a compelling detection of an astrophysical signal. However, from ≈ 5 hours of interference-free data we find an observational upper limit to the rate of exploding PBHs of r ≈ 4.8 × 10⁻⁷ pc⁻³ y⁻¹ for a PBH with a fireball Lorentz-factor f = 10^4.3. This limit is applicable to PBHs in the halo of the Galaxy to distances ≲ 2 kpc, and dispersion measures ≲ 80 pc cm⁻³ . We also find a source-agnostic rate limit ≲ 0.25 events y⁻¹ deg⁻² for pulses of duration < 3 s, and having apparent energy densities ≳ 2.6 × 10⁻²³ J m⁻² Hz⁻¹ at 38 MHz.
Ph. D.

Le recherché et l’étude de sources radio transitoires, en particulier à basses fréquences, est un des projets clés de la nouvelle génération de radiotélescopes comme SKA et ses précurseurs, parmi lesquels LOFAR, NenuFAR et GURT. Ces sources incluent les étoiles à neutrons – produisant un signal périodique, détectées comme pulsars -, d’autres objets très magnétisés compacts ou des événements très dynamiques ou explosifs – produisant des transitoires non périodiques -, et des exoplanètes – dont certaines sont, comme Jupiter, susceptibles d’émettre des sursauts radio décamétriques. Les « survey » observations systématiques à basses fréquences peuvent révéler des pulsars proches, « manqués » par les surveys à plus hautes fréquances en raison de l’orientation « défavorable » de leurs cônes d’émission, qui s’élargissent aux basses fréquences. Ils devraient également permettre de mieux caractériser le spectre des pulsars détectés à la fois à hautes et basses fréquences. Les surveys de transitoires non périodiques peuvent révéler des phénomènes astrophysiques uniques, tels que les mystérieux « sursauts radio rapides » d’origine extragalactique ou leurs analogues galactiques. L’existence de sursauts radio produits par les magnétosphères exoplanétaires ou les interactions plasma exoplanète- étoile a été prédite par la théorie, en particulier aux basses fréquences (comme l’émission décamétrique de Jupiter). Mais aucun n’a été détecté jusqu’à présent. La première détection, et les études qui la suivront, ouvriront à une meilleure compréhension des processus d’émission radio et à des mesures uniques des champs magnétiques des exoplanètes, avec des répercussions sur leur habitabilité. L’objectif de cette thèse était (i) effectuer un survey de sources d’émissions radio pulsées et transitoires, et (ii) tenter de détecter ‘émission de radio d’une exoplanète connue (découverte par mesures optiques), en utilisant le radiotélescope décamétrique UTR-2 en Ukraine, en mode de réseau phasé. Les deux sujets impliquent le traitement des grands volumes de données, avec notamment des étapes d’élimination des RFI (parasites radio), d’étalonnage du signal, et de détection de sursauts faible. J’ai développé des outils logiciels génériques à partir desquels j’ai construit un pipeline de traitement automatisé des données d’UTR-2, qui peut être facilement adapté à d’autres radiotélescopes observant en mode réseau phasé (LOFAR, NenuFAR et GURT)
The search and study of transient sources of radio emission, especially at low frequencies, is one of the key projects for the new generation of radio telescopes such as SKA and its precursors among which LOFAR, NenuFAR and GRUT. These sources includes neutrons stars – producing a rotation modulated signal and detected as pulsars -, other compact highly magnetized objects or very dynamic or explosive events – producing non-periodic transients -, and exoplanets – some of which are expected to emit Jupiter -like decameter- wave radio bursts. Low-frequency surveys are expected to reveal nearby pulsars “missed” by high frequency surveys, due to the “unfavorable” orientation of their emission cones, which broaden at low frequencies, and to better characterize the spectrum of pulsars detected at both high and low frequencies. Surveys of non-periodic transients may reveal unique astrophysical phenomena, such as the mysterious extragalactic “fast radio bursts” or their galactic analogs. Radio bursts from the magnetospheres of exoplanets or from exoplanet-star plasma interaction has been theoretically predicted to exist preferably at low frequencies (as Jupiter’s decameter-wave radio emission), although none has been detected so far; first detection and subsequent study will result in better understanding of emission processes and unique measurements of exoplanets’ magnetic fields, that have implications on their habitability. The aim of this work was (i) to carry out a survey for sources of transient and pulsed radio emission, and (ii) to attempt to detect radio emission from known exoplanets (discovered from optical measurements), by using the decameter radio telescope UTR-2 in Ukraine in beamformed (phased array) mode. Both topics imply to process large volumes of data, including RFI (Radio frequency Interference) mitigation, calibration, and detection of low intensity bursts. Thus, I developed generic software tools from which I built and automated processing pipeline for UTR-2 data, that can be easily adapted to other radio telescopes observing in beamformed mde (LOFAR, NenuFAR and GRUT)

Thesis (Ph. D.)--University of Oregon, 2006.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 174-183). Also available for download via the World Wide Web; free to University of Oregon users.

Pulsars, as rotating magnetised neutron stars got much attention during the last 40 years since their discovery. Observations revealed them to be gamma-ray emitters with energies continuing up to the sub 100 GeV region. Better observation of this upper energy cut-off region will serve to enhance our theoretical understanding of pulsars and neutron stars. The H-test has been used the most extensively in the latest periodicity searches, whereas other tests have limited applications and are unsuited for pulsar searches. If the probability distribution of a test statistic is not accurately known, it is possible that, after searching through many trials, a probability for uniformity can be given, which is much smaller than the real value, possibly leading to false detections. The problem with the H-test is that one must obtain the distribution by simulation and cannot do so analytically. For such simulations, random numbers are needed and are usually obtained by utilising so-called pseudo-random number generators, which are not truly random. This immediately renders such generators as useless for the simulation of the distribution of the H-test. Alternatively there exists hardware random number generators, but such devices, apart from always being slow, are also expensive, large and most still don't exhibit the true random nature required. This was the motivation behind the development of a hardware random number generator which provides truly random U(0,l) numbers at very high speed and at low cost The development of and results obtained by such a generator are discussed. The device delivered statistically truly random numbers and was already used in a small simulation of the H-test distribution.
Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.

The development of a surface texture evaluation technique for the study of roughnesses of the order of a few microns using the signal processing of ultrasonic pulse-echo signals is described. The technique of extracting surface information by means of deconvolution is introduced. Strictly, a solution to the deconvolution problem normally does not exist or is not unique. The chosen method of approaching a solution is by the nonlinear Maximum Entropy Method (MEM), which offers superior image quality over many other filters. The algorithm is described and translated into a standalone computer programme-the development of this software is described in detail. The performance of the algorithm in the field of ultrasonics is assessed by means of the study of simulations involving images similar to those obtainable in a real application. Comparison with the linear Wiener-Hopf filter is provided particularly in instances where the comparison shows weaknesses of either technique. Also examined is the frequency restoration property of the algorithm (not shown by the Wiener-Hopf filter)-potential applications of this property are also described. The final part of the study of the MEM is an examination of the effect on performance of some of the algorithm's parameters and on computer system dependencies. A brief overview of some of the surface metrology techniques currently used is given. The aim is an introduction to surface metrology and an assessment of where the technique described here fits into the general surface metrology field. The experimental system, which of course is essential to practical applications, is considered in some detail. Also considered is a wide range of ultrasonic transducers available for the research. These show a considerable variety of characteristics. Some assessment is carried out using the Maximum Entropy Method with simulated and real data to try and establish the properties of a transducer best suited to the application intended. Finally, results from grating-type test surfaces and more general rough surfaces are presented. The former are intended as a means of establishing the potential performance of the technique; the latter build on the grating results to analyse real surfaces as made by a variety of engineering techniques. Results are compared with those obtained by a stylus instrument. Generally good agreement is found, with roughnesses of around 2 microns being accurately assessed. With the accuracy of these results being less than a micron, it is concluded that this technique has a valuable contribution to the surface metrology field.

Astrophysical phenomena such as self-annihilation of primordial black holes (PBHs), gamma ray bursts (GRBs), and supernovae are expected to produce single dispersed pulses detectable in the low end of the radio spectrum. Analysis of these pulses could provide valuable information about the sources, and the surrounding and intervening medium. The Eight-meter-wavelength Transient Array (ETA) is a radio telescope dedicated to the search for these pulses in an 18 MHz bandwidth centered at 38 MHz. ETA consists of 10 dual-polarized active dipoles providing an all-sky field of view. This thesis describes the results of a search campaign using ETA, namely, a Crab giant pulse (CGP) search. CGPs are already known to exist, and thus provide an excellent diagnostic for system performance. We found 11 CGP candidates in 14 hours of data. Although there has not been a single compelling detection (signal-to-noise ratio > 6), our analysis shows that at least a few of these candidates may be CGPs. We also explain the analysis preparation for PBH and GRB searches. Additionally, we describe the instrument and a software "toolchain" developed for the analysis of data that includes calibration, radio frequency interference (RFI) mitigation, and incoherent dedispersion. A dispersed pulse simulation code was developed and used to test the toolchain. Finally, improvements are suggested.
Master of Science

An investigation was conducted into the effects of double-pulsed laser excitation on the spectra emitted by steel targets in air at atmosphere pressure. Double-pulsed excitation was found to produce an average factor of seven increase in spectral line intensity over single-pulsed excitation of the same total laser energy. The emitted spectral line intensity and plasma electron temperature were strong functions of the laser pulse energy ratio, with both values maximised when the metal was excited by a low-energy first pulse, followed by a high-energy second pulse. The variation of laser pulse separation over the range 7.6s to 30s was found to produce no measurable variation in electron temperature. A model is proposed to explain the effects of double-pulsed laser excitation, based on the re-generation of the primary plasma by UV radiation, energetic ions and shock wave produced by the secondary plasma within the atomised remnants of the primary plasma. The resolution of chromium in a range of steels using single-pulsed and double-pulsed Laser-Induced Breakdown Spectroscopy (LIBS) was investigated. Optical and laser parameters were optimised for each excitation regime in order to maximise the electron temperature of the laser-induced plasma. The slope of the calibration curve produced by double-pulsed LIBS was in the order of 0.55 (depending on line-pair combination), compared with approximately 0.75 for single-pulsed LIBS. The improvement in the correlation coefficient of the calibration curves from an average of 0.984 for single-pulse to 0.996 for double-pulse was attributed to the increase in S/N ratio produced by double-pulsed excitation. Sorting of the spectra into electron temperature-defined subsets resulted in a minimum average standard deviation in measured intensity ratio for narrowest temperature band. The correlation coefficients of the calibration curves were not improved by this process, through the resultant severe reduction in sample size.

This thesis presents a design of a ruby laser system which would form the heart of an underwater holographic camera. The design principally emphasised upon simplicity, compactness and lightweight construction, in addition to the achievement of the necessary coherence length. The research was mainly focused on Q-switching and longitudinal mode selection which were identified as the key elements in achieving our goal. A fresh insight into dye Q-switching was made with the view of its implementation in the laser. Two major experiments were carried out on a dye Q-switch consisting of cryptocyanine dye dissolved in methanol. The first trial explored the lifetime performance of the Q-switch with regards to the number of laser pulses fired and the elapsed time. It gave an indication of the suitability of this Q-switch for the application. The second experiment concentrated on evaluating the operational characteristics of a dye Q-switched ruby laser with regards to pump energy, dye absorbance, output energy and timing of the pulse emergence. The results of this experiment clearly defined the effects of variation of one parameter on another. Consequently, these results displayed in a number of graphical plots could form the basis for the selection of optimum parameters for such lasers. In addition to its primary role of creating a giant pulse, the performance of a Q-switch could be optimised or modified to enhance the longitudinal mode selection property of a laser. The mode selection properties of Pockels cell and dye Q-switched ruby lasers were theoretically developed before experimentally confirming the derived predictions to a good degree.

Experiments with polarized "ultracold neutrons" (UCN) offer a new way to measure the decay correlations of neutron beta decay; these correlations can be used to test the completeness of the Standard Model and predict physics beyond it. Ultracold neutrons are very low energy neutrons that can be trapped inside of material and magnetic bottles. The decay correlations in combination with the neutron and muon lifetimes experimentally find the first element (Vud) of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. The CKM matrix is a unitary transform between the mass and weak eigenstates of the d, s and b quarks; if the matrix is not unitary this would imply that the Standard Model is not complete. Currently the first row of the CKM matrix is over 2 sigma from unitarity and Vud is the largest component of the row. The UCNA experiment looks at the correlation between the polarization of the neutron and the momentum of the electron resulting from the beta decay of the neutron (the A-correlation). The keys to making a high precision measurement of A-correlation are a near 100% polarization of the neutrons that decay, low"backscatter electron detectors, and small, well characterized backgrounds. UCN can be 100% polarized by passing them through a seven Telsa magnetic field. The key to the UCNA experiment is keeping them polarized until they decay or are lost. This dissertation covers the development of guides that are minimally depolarizing and efficient transporters of UCN and their use in the UCNA experiment. The entire guide development process is covered from conception to manufacturing and testing. This process includes development of a pulsed laser deposition, diamond-like carbon coating system and materials studies of the resulting coatings. After the initial studies of the guide coating, meter"long sections of guide are tested with UCN to determine their depolarization and transport properties. The guide technology developed in this dissertation has been used in the entire UCNA experiment. Also, this technology is currently the state of the art for polarized and non-polarized UCN guide systems and it is being implemented in several new UCN experiments.
Ph. D.

Diese Arbeit berichtet über die Suche nach gepulster und ungepulster hochenergetischer (VHE) Gammastrahlung mit dem High Energy Stereoscopic System (H.E.S.S.) im Energiebereich von 100 GeV bis 100 TeV von drei Pulsaren. Gepulste VHE Gammastrahlung wurde bisher nur fuer den jungen Krebspulsar gefunden. Eine besondere Gruppe von Pulsarwindnebeln (PWN) sind die zusammengesetzten Supernovaüberreste (SNR), bei denen sich ein PWN im Zentrum einer expandierenden SNR Schale befindet. Die Resultate der Suche nach gepulster und ungepulster VHE Gammastrahlung von zwei Millisekundenpulsaren, PSR J0437-4715 und PSR J1824-2452, werden im ersten Teil dieser Arbeit vorgestellt. Teile der Beobachtungen wurden in einer speziellen Triggerkonfiguration (dem Topologischen Trigger mit konvergenter Ausrichtung) durchgeführt, um die Energieschwelle des Instruments zu senken. Kein Hinweis auf gepulste oder ungepulste Emission wurde gefunden und obere Grenzen auf den gepulsten und ungepulsten Fluss wurden bestimmt. Die oberen Grenzen auf den gepulsten Fluss werden mit bestehenden Modellvorhersagen verglichen und erlauben für PSR J1824-2452 den Bereich möglicher Geometrien in einigen Modellen einzuschränken. Die Resultate der Suche nach gepulster und ungepulster VHE Gammastrahlung aus der Richtung des zusammengesetzten SNR Kes 75 werden im zweiten Teil dieser Arbeit präsentiert. Der PWN im Zentrum von Kes 75 wird von einem sehr jungen und energiereichen Pulsar, PSR J1846-0258, angetrieben, der ein aussergewöhnlich starkes Magnetfeld besitzt. Während kein Hinweis auf gepulste Strahlung gefunden wurde, konnte ungepulste Emission von VHE Gammastrahlung von einer Punktquelle mit einer statistischen Signifikanz von 10 sigma nachgewiesen werden. Die VHE Gammastrahlung ist räumlich koinzident mit dem PWN und mit der SNR Schale. Beide werden als mögliche Quelle für die beobachtete Emission diskutiert. Der Pulsar von Kes 75 wäre der jüngste bisher bekannte Pulsar, der einen Pulsarwindnebel antreibt.
This work reports on the search for pulsed and steady very-high energy (VHE) gamma-ray emission in the energy range extending from 100 GeV up to 100 TeV from the direction of three pulsars with the High Energy Stereoscopic System (H.E.S.S.). Pulsed gamma-ray radiation from pulsars with energies beyond 100 GeV was found thus far only for the young and energetic Crab pulsar. A special class of pulsar wind nebulae (PWNe) is associated with composite supernova remnants (SNRs) where the PWN is centered in an expanding SNR shell. In the first part of this thesis, the results on the search for pulsed and steady VHE gamma-ray emission from the two millisecond pulsars, PSR J0437-4715 and PSR J1824-2452, are presented. Parts of the observations were conducted in a special trigger setup (the topological trigger with convergent pointing) to reduce the energy threshold of the instrument. No signal of pulsed or steady emission is found and upper limits on the pulsed and steady gamma-ray flux are derived. The upper limits on the pulsed gamma-ray flux are compared to existing model predictions and, in the case of PSR J1824-2452, allow the range of possible viewing geometries in some models to be constrained. In the second part of this work, results on the search for pulsed and steady VHE gamma-ray emission from the direction of the composite SNR Kes 75 are presented. The PWN in the center of Kes 75 is powered by a very young and powerful pulsar, PSR J1846-0258, that has an exceptionally high magnetic field. While no hint for pulsed emission is found, steady VHE gamma-ray emission is detected with a statistical significance of 10 sigma from a point-like source. The VHE gamma-ray emission is spatially coincident with the PWN and the SNR shell. Both are discussed as a possible origin for the observed emission. The pulsar of Kes 75 would be the youngest pulsar known to date to power a VHE PWN.

Radio emission from pulsars is believed to originate from charged particles streaming along the open magnetic field lines, radiating within a narrow cone at each of the two magnetic poles. In each rotation of the star, the emission beam sweeping across the observer’s line of sight, is seen as a pulse of radio emission. Average pulse profiles integrated over several hundreds of individual pulses, along with polarization information, reveal the viewing geometry and various emission properties(e.g., emission in multiple cones, frequency dependence of the emission altitude, notches in the average profiles, etc.), and provide some clues about the possible emission mechanisms. The sequence of individual pulses generally exhibit richer details, e.g., pulse-nulling, variety of subpulse drifting, polarization mode-changing, micro-structure and giant pulse emission, etc., and seem to be more crucial and promising in probing the underlying physical processes. The physical understanding of many of the above properties and phenomena is still far from complete. In first two parts of this thesis, we address a few of these aspects, and probe related details by mapping the pulsar polar emission patterns, while in the last part, we present our searches for dispersed signals(periodic as well as transient) at very low frequencies. More specifically, Part-I makes use of the present understanding of drifting subpulses phenomenon to reconstruct the emission patterns in nearly complete polar cap region of the pulsar B1237+25, and addresses the origin of emission in multiple cones using these reconstructed emission maps. In Part-II, we discuss a need for new instrumentation primarily motivated by the need for tomographic studies of pulsar polar emission regions. We report the consequent design and development of a novel, self-contained multi-band receiver (MBR)system, intended for use with a single large aperture to facilitate sensitive and high time-resolution observations simultaneously in 10 discrete frequency bands sampling a wide spectral span(100–1500MHz) in a nearly log-periodic fashion. Part-III presents our deep searches designed to detect radio transient as well as periodic signals from the (so far) “radio-quiet” gamma-ray pulsars — a population of radio silent pulsars recently discovered using the Large Area Telescope on the Fermi-satellite. Brief descriptions of the issues addressed in the three parts of the thesis, along with a summary of respective results, is as follows. 1. Origin of Radio Emission in Multiple Cones Many pulsars exhibit systematic variations in position and intensity of their subpulses, a phenomenon now well known as “subpulse drifting”. Ruderman & Sutherland(1975) suggested this regular modulation to be a manifestation of a carousel of “spark” discharges in the acceleration zone of the star, circulating around the magnetic axis because of the E×B drift. In the qualitative framework of the above carousel model, the coherent modulation in a subpulse sequence can be mapped back to the underlying pattern of sub-beams/emission-columns (see, for example, Deshpande & Rankin, 1999). However, the completeness with which the underlying configuration of sub-beams can be sampled depends on how close our line of sight approaches the magnetic axis. The bright pulsar B1237+25 has a special viewing geometry where the sightline traverses almost through the magnetic axis, thus providing an excellent opportunity to map and study the underlying patterns across the full transverse slice of its polar emission region. However, the rich variety in pulse-to-pulse fluctuations in this pulsar makes this task challenging. In Chapter 2, we present our analysis of a number of pulse-sequences from this star observed with the Arecibo telescope, wherein we search for, and use, coherent modulation in sub-sequences, to map the underlying emission patterns. The reconstructed maps provide a convenient way to study the details in multiple emission cones, and any inter-relationship between them. More specifically, we have utilized these maps to explore whether the multiple cones of this pulsar originate from a common seed pattern or not. A summary of results The results obtained from our study of B1237+25 are summarized below: 1 The underlying carousel of sparks for this pulsar appears to lack stability over long durations. The circulation period, deduced using smaller length sub-sequences, appears to vary over a large range(about18 to34 times the rotation period). 2. The emission patterns corresponding to the outer and the inner cones are found to be significantly correlated with each other, implying that the emission in the two cones share a common seed pattern of sparks. This main result is consistent with the same radio frequency emission in the two cones, originating from a common seed pattern of sparks at two different altitudes. 3 The emission patterns corresponding to the outer and the inner cones are found to be offset from each other, consistently across various sub-sequences, by about 10◦ in magnetic azimuth. This large offset indicates certainly a twist in the emission columns, and most likely in the magnetic field geometry, between the two different emission altitudes. 4. The core component also seems to share its origin with the conal counterparts. Presence of a compact, diffuse and further-in carousel of sub-beams is consistent with the observed modulation in the core component of this pulsar. The featureless spectrum observed for many core-single pulsars can be explained readily when the diffuse pattern approaches uniformity. 2.Tomography of the Pulsar Magnetosphere: Development of a Multi-band Receiver Although drifting subpulses are now routinely interpreted in the qualitative framework of the carousel model, estimation of circulation time associated with the system of emission columns has been possible so far in only a handful of pulsars, and the important details determining such configurations, their evolution across the magnetosphere, and the pattern circulation are yet to be understood. Radius-to-frequency mapping in pulsars suggests that the lower frequency emission originates farther away from the surface of the star than the higher frequency emission. Hence, the sub-beam configuration mapped at a particular frequency provides a view of a single slice of the polar emission region at the corresponding emission altitude. Mapping of the underlying emission patterns simultaneously at a number of frequencies would amount to viewing a “tomograph” of the pulsar magnetosphere. Such tomographic studies would reveal not only the evolution of sub-beams across the magnetosphere but can also provide much needed clues about the generation of the sub-beam patterns, and their possible connection with the profile/polarization mode changes observed in various pulsars. Simultaneous multi-frequency observations, which are required for many other interesting astronomical studies as well, are usually carried out by using several telescope, each observing at different frequency. Such an approach has inherent complexity in coordinating various telescopes, in addition to numerous other difficulties which limit the desired advantages of such observations. Some of these difficulties, which we faced in our attempt of carrying out simultaneous multi-frequency observations using five different telescopes, are discussed in Chapter 3. We suggest an optimum approach to carry out simultaneous multi-frequency observations, using a single large aperture. In Chapter 4, we present the design of a novel, “self-contained” multi-band receiver(MBR) system developed for this purpose. The MBR system includes a suitable feed, broadband front-end, parallel analog and digital receiver pipelines, along with appropriate monitoring, synchronization and data recording systems. When used with a large aperture, the MBR facilitates high time-resolution observations simultaneouslyin10discretefrequencybandssampling a wide spectral span(100–1500MHz) in a nearly log-periodic fashion. The raw voltage time sequences corresponding to each of the two linear polarization channels for each of the 10 spectral bands are simultaneously recorded, each sampling a bandwidth of 16 MHz at the Nyquist rate. The dual-polarization multi-band feed, a key component of the MBR, is designed to have good responses only overthe10discretebandspre-selected as relatively RFI-free, and hence provides preliminary immunity against RFI. The MBR also offers significant tunability of the center frequencies of each of the 16-MHz sub-bands separately, within the spectral spans of respective bands. Similarity of the 10 sub-band receiver chains provides desired compatibility, in addition to an easy inter-changeability of these units, if required, and an overall modularity to the system. The MBR was used with the 110 meter Green Bank Telescope to conduct test observations on a few bright continuum sources, and about 20 hours of observations on a number of bright pulsars. Using these observations, we have constructed a preliminary tomograph of the polar emission region of B0809+74, and studied the spectral evolution of emission altitudes and flux density ofB0329+54(Chapter5). Although the MBR system design is optimized for tomographic studies of pulsar polar emission regions, the simultaneous multi-frequency observations with such a system offer particular advantages in fast transient searches. The MBR is also suitable for several other astronomical investigations, e.g., studying the spectral evolution of average properties of pulsars and propagation effects, single-dish continuum studies and surveys/studies of recombination lines. 3. Searches for Decameter-wavelength Counterparts of Radio-quiet Gamma-ray Pulsars Before the launch of the Fermi gamma-ray space telescope, the “radio-quiet” gamma-ray pulsar population consisted of only one pulsar ,i.e., Geminga (for example, see Bignami& Caraveo,1996; Abdo etal.,2009). High sensitivity of the Large Area Telescope(LAT) on the Fermi-satellite made it possible, for the first time, to perform blind searches for pulsars in γ-rays. Since the Fermi-operation started, the number of pulsars known to emit in γ-rays has seen an extraordinary increase — from less than 10 to 117 pulsars. About one-third of these pulsars have been discovered in blind searches of the LAT data. Despite deep radio searches, only 4 of these LAT-discovered pulsars could be detected, suggesting the rest of these to be “radio-quiet” gamma-ray pulsars. One of the possible explanations for the apparent absence of radio emission from these pulsars is that their narrow radio beams miss the line of sight towards earth (Brazier & Johnston, 1999), and hence appear as “radio-quiet”. The radius-to-frequency mapping in radio pulsars suggests that the emission beam becomes wider at low frequencies, increasing the probability of our line of sight passing through the beam. However, all of the deep searches mentioned above were carried out at higher radio frequencies(∼1GHz and above, and some at300MHz,Ray etal.,2011;Pletsch etal.,2012),and the lower frequency domain(<≈100 MHz) has remained relatively unexplored. Given the expected widening of emission beam, follow-up searches of the radio-quiet pulsars at low radiofrequencies could also be revealing. With this view, we searched the archival data of the pulsar/transient survey at 34.5 MHz, carried out using the Gauribidanur telescope during 2002-2006,for any periodic or transient dispersed signal along the direction of many of the LAT-discovered pulsars. Motivated by an intriguing possible detection of the pulsar J1732−3131 from the above search, we carried out further extensive follow-up observations and deep searches for pulsed(periodic as well as transient) radio emission from a selected sample of radio-quiet pulsars. Chapters 6 and 7 present details of our observations, detection strategies and methodologies, and interesting results obtained in a few of the target directions. The results obtained from these searches include: 1 A possible detection of periodic radio pulses from J1732−3131 was made, using the archival data, at a dispersion measure(DM) of15.44 ±0.32 pc/cc. We also detected 10 individual bright pulses in the same observing session, although marginally above the detection threshold, at a DM consistent with that associated with the periodic signal. The apparent brightness of these single pulses, and similarity of their apparent distribution in pulse-longitude with that of giant pulses in J0218+4232, suggest that these might be giant pulses. Our DM-based distance estimate, using Cordes & Lazio electron density model(2002),matches well with earlier estimates based on gamma-ray emission efficiency. 2 In our follow-up deep searches, we could not detect any readily apparent pulsed radio signal(neither periodic nor single pulses) from J1732−3131, i.e., above a detection threshold of 8σ. However, when we time-aligned and co-added data from observing sessions at 21different epochs, and dedispersed using the DM estimated from the candidate detection, the average profile shape is found to be completely consistent with that from the candidate detection. Finding the same profile shape after 10 years of the original detection suggests that the signal is unlikely to be due to RFI or a mere manifestation of random noise. 3.In a couple of the observing sessions towards the telescope pointing direction of RA=06:34:30, DEC=10◦ , we detected a few ultra-bright pulses at two different DMs of about2pc/cc and3.3 pc/cc, respectively. However, when dedispersed at the DMs suggested by the bright single pulses, no significant signal was found at the expected periodicities of our targetpulsarsJ0633+0632 andJ0633+1746,which would be in the telescope beam centered at above coordinates. Energies of these strong pulses in the two observing sessions are comparable to typical energies of giant pulses from the Crab pulsar at decameter wavelengths. 4. No significant pulsed signal(periodic or transient), above a detection threshold of 8σ,was found towards the directions of other selected radio-quiet gamma-ray pulsars. Time-aligning and combining of observations at different epochs allowed us to carry out deep searches for signals at the expected periodicities of these pulsars. Despite the large background sky-temperature at decameter wavelengths, the minimum detectable flux density in our deep searches are comparable with those from previous searches at higher frequencies, when scaled using a spectral index of −2.0 and assuming no turn-over in the spectrum.

Millisecond pulsars (MSP) are old neutron stars that have been spun up to high spin frequencies (as fast as 716 Hz) through the accretion of matter from a companion star. The extreme steller densities in the core of globular clusters creates numerous accreting neutron star systems through exchange interactions: this leads to the formation of MSPs in larger numbers than the galactic disk. Over the course of this project, we have collected over 17 TB of data on the 3 globular clusters M28 NGC6440 and NGC6441 plus 2 observations on NGC6522 and NGC6624 as part of the recently begun S-band survey using the Green Bank telescope. I have analyzed and conducted acceleration searches on 70% of the data and discovered 7 of the 23 new millisecond pulsars reported in this work. One year of timing observations of the pulsars in M28 and NGC6440 has led to the phase connected solution for 12 of the 15 new pulsars in those two clusters, 7 of which are in binaries. We have measured the rate of advance of periastron for two highly eccentric binaries and assuming this is purely due to general relativity, this leads to total system masses of (1.616 ± 0.014)M [circle with central dot] and 2.2 ± 0.8)M [circle with central dot] for M28C and NGC6440B respectively. The small mass function combined with this information imply that the most likely neutron star mass of NGC6440B is either very large or else there could be significant contribution to the advance of periastron from a nonzero quadrupole moment due to tidal interaction with the companion. Measurements of the period derivatives for many of the pulsars show that they are dominated by the dynamical effect of the gravitational field of the clusters. Finally, we have discovered the potential presence of a Mars-mass planet orbiting the pulsar NGC6440C with a period of ~21 days. A dedicated timing campaign will be necessary to confirm the presence of such an object.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

We report the results of four different pulsar searches, covering radio, X-ray, and gamma-ray wavelengths. These searches targeted pulsars in virtually all of their guises: young and old, long-period and short-period, accretion-powered and rotation-powered. Ten new pulsars were discovered.

There are very few known gamma-ray pulsars, all of which were found by folding gamma-ray data with a pulse period known from other wavelengths. Some emission models indicate that there may be a large number of gamma-ray pulsars that are undetectable at lower energies. We searched several of the brightest unidentified gamma-ray sources for pulsations. This was the first attempt to identify gamma-ray pulsars by a direct search of gamma-ray data. No new identifications resulted and we report upper limits.

Even more rare than gamma-ray pulsars are accreting millisecond pulsars. We searched for coherent pulsations from Aql X-1, a low-mass X-ray binary suspected of harboring such an object. No pulsations were detected, and we argue that the quiescent emission of this system has a thermal origin (i.e., it is not due to low-level accretion).

The two radio searches included here were both designed to detect millisecond pulsars. First, we report the results of a large area survey from Arecibo. Five new slow pulsars were discovered, including an apparent orthogonal rotator and an extremely unusual bursting radio pulsar. No short-period pulsars were discovered and we place some of the first useful observational constraints on the limiting spin period of a neutron star.

We also performed pointed searches of several globular clusters using the new Green Bank Telescope. Three new binary millisecond pulsars were found in M62. These were the first new objects found with the GBT, and they bring the total pulsar population in M62 to six. We also discovered two isolated pulsars, one each in NGC 6544 and NGC 6624.

Many of the methods we developed will be relevant to future searches. Perhaps the most significant contribution is a dynamic power spectrum-based technique that finally allows sensitive searches for binary pulsars whose orbital periods are of the same order as the observation time.

Radio pulsars are rapidly spinning, highly magnetized neutron stars which emit beams of radio waves and are observed to pulse when the beam crosses the Earth. They represent the end-point in the evolution of massive stars, and are excellent laboratories for the study of the bulk properties of matter at nuclear densities and beyond. Millisecond pulsars are old pulsars reborn through accretion of matter from a companion star, spinning so fast that the surface velocities approach the speed of light. We describe several high-sensitivity searches for radio pulsars, both for very recently born pulsars in supernova remnants and for ancient millisecond pulsars born early in the history of the Galaxy. We have conducted a survey of 18 supernova remnants for young pulsars using the 305-m radio telescope in Arecibo, Puerto Rico at 430 MHz and 1400 MHz. No pulsars were discovered in this survey which was sensitive to pulsars as faint as 0.2 mJy. The selection effects making pulsars difficult to find in supernova remnants, including high background temperatures of the remnants and high birth velocities of pulsars, are discussed. We conclude that deeper and more extensive surveys are required to constrain the pulsar population in supernova remnants. We have also performed several very large area surveys with excellent sensitivity to pulsars as fast as 1 millisecond, also employing the Arecibo 305-m dish. These surveys will help place limits on the population of low-luminosity pulsars in the Galaxy. A total of 12 non-recycled pulsars were discovered with periods ranging from 96 ms to 2.06 seconds. The primary motivation for these surveys was the discovery of new millisecond pulsars. One 5.9 ms pulsar was discovered and initial timing observations show that it is in a binary system with orbital period 56.2 d and semi-major axis 20.1 lt-s. The implied companion mass is at least 0.2 solar masses. This pulsar, as well as a number of others discovered in recent surveys, are providing excellent laboratories for studies of the formation and evolution of millisecond pulsars, as well as measurements of general relativistic parameters, constraints on the cosmological background of gravitational waves, a pulsar based time standard and dynamical-optical frame ties.

by Fronefield Crawford, III.
Supervised by Victoria M. Kaspi.
Also issued as Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2000.
Includes bibliographical references (p. 193-199).

abstract: Seeking an upper limit of the Neutron Electric Dipole Moment (nEDM) is a test of charge-parity (CP) violation beyond the Standard Model. The present experimentally tested nEDM upper limit is 3x10^(26) e cm. An experiment to be performed at the Oak Ridge National Lab Spallation Neutron Source (SNS) facility seeks to reach the 3x10^(28) e cm limit. The experiment is designed to probe for a dependence of the neutron's Larmor precession frequency on an applied electric eld. The experiment will use polarized helium-3 (3He) as a comagnetometer, polarization analyzer, and detector. Systematic influences on the nEDM measurement investigated in this thesis include (a) room temperature measurements on polarized 3He in a measurement cell made from the same materials as the nEDM experiment, (b) research and development of the Superconducting QUantum Interference Devices (SQUID) which will be used in the nEDM experiment, (c) design contributions for an experiment with nearly all the same conditions as will be present in the nEDM experiment, and (d) scintillation studies in superfluid helium II generated from alpha particles which are fundamentally similar to the nEDM scintillation process. The result of this work are steps toward achievement of a new upper limit for the nEDM experiment at the SNS facility.
Dissertation/Thesis
Doctoral Dissertation Physics 2019