Very high energy gamma rays from isolated pulsars and non-pulsating objects

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

The challenges of mobile devices for human computer interaction

Current mobile computing devices such as palmtop computers, personal digital assistants (PDAs) and mobile phones, and future devices such as Bluetooth and GSM enabled cameras, and music players have many implications for the design of the user interface. These devices share a common problem: attempting to give users access to powerful computing services and resources through small interfaces, which typically have tiny visual displays, poor audio interaction facilities and limited input techniques. They also introduce new challenges such as designing for intermittent and expensive network access, and design for position awareness and context sensitivity. No longer can designers base computing designs around the traditional model of a single user working with a personal computer at his/her workplace. In addition to mobility and size requirements, mobile devices will also typically be used by a larger population spread than traditional PCs and without any training or support networks, whether formal or informal. Furthermore, unlike early computers which had many users per computer, and PCs with usually one computer per user, a single user is likely to own many mobiles devices [1] which they interact with indifferent ways and for different tasks

Human computer interaction with mobile devices (editorial for special edition)

The second international workshop on human-computer interaction with mobile devices took place on 30th August,1999 as part of the IFIP INTERACT '99 conference held in Edinburgh, UK. We had over 60 participants with an almost equal mix between academic and industrial attendees from within Europe, North America and Asia.The first workshop had been held in Glasgow the year before and was one of the first to bring together researchers interested in how to design usable interfaces for mobile computers. It was such a success that we decided to run another- this was obviously an area where there were many problems and many people looking for solutions. The growth of the mobile computing market is rapid. The take-up of mobile telephones and personal digital assistants has been dramatic - huge numbers of people now own a mobile device of some kind. But there are still big problems with usability - it is hard to design interfaces and interactions with devices that have small or no screens and limited computing resources. This is becoming worse as more and more complexity is being integrated into these small devices

Dynamic scaling of I-V data for the neutral 2D Coulomb gas

The value of the dynamic critical exponent z has been studied for experimental two-dimensional superconducting and Josephson Junction array systems in zero magnetic field via the Fisher-Fisher-Huse dynamic scaling analysis. We found z~5.6, a relatively large value indicative of non-diffusive dynamics. We extend this work here to simulational I-V curves that are also found to be characterized by the same large value of z.Comment: 2 pages, 2 embedded figures, LT22 proceedings, Physica

Asymptotics of lattice walks via analytic combinatorics in several variables

We consider the enumeration of walks on the two dimensional non-negative integer lattice with short steps. Up to isomorphism there are 79 unique two dimensional models to consider, and previous work in this area has used the kernel method, along with a rigorous computer algebra approach, to show that 23 of the 79 models admit D-finite generating functions. In 2009, Bostan and Kauers used Pad\'e-Hermite approximants to guess differential equations which these 23 generating functions satisfy, in the process guessing asymptotics of their coefficient sequences. In this article we provide, for the first time, a complete rigorous verification of these guesses. Our technique is to use the kernel method to express 19 of the 23 generating functions as diagonals of tri-variate rational functions and apply the methods of analytic combinatorics in several variables (the remaining 4 models have algebraic generating functions and can thus be handled by univariate techniques). This approach also shows the link between combinatorial properties of the models and features of its asymptotics such as asymptotic and polynomial growth factors. In addition, we give expressions for the number of walks returning to the x-axis, the y-axis, and the origin, proving recently conjectured asymptotics of Bostan, Chyzak, van Hoeij, Kauers, and Pech.Comment: 10 pages, 3 tables, as accepted to proceedings of FPSAC 2016 (without conference formatting

The laws, regulations, and industry practices that protect consumers who use electronic payment systems: ACH E-checks & prepaid cards

This is the second in a series of three papers that examines the protections available to users of various electronic payment vehicles who fall victim to fraud, discover an error on their statement, or have a dispute with a merchant after making a purchase. Specifically, it examines the federal and state laws that protect consumers in the three situations described above as well as the relevant association, network, and bank policies that may apply. The protection information included in this paper is derived from a wide range of public and non-public sources, including federal and state statutes, issuer-consumer contracts, and interviews with scores of payments industry experts. This second paper focuses on two increasingly popular electronic payment methods: ACH electronic check applications (e-checks) and branded prepaid cards. The first paper in the series examined the two most widely used electronic payment vehicles: credit and debit cards. The third paper will discuss broader industry and policy implications of the authors’ findings.Consumer protection ; Stored-value cards