The Properties of the Hot Gas in Galaxy Groups and Clusters from 1-D Hydrodynamical Simulations -- I. Cosmological Infall Models

We report the results of 1-D hydrodynamical modelling of the evolution of gas in galaxy clusters. We have incorporated many of the effects missing from earlier 1-D treatments: improved modelling of the dark matter and galaxy distributions, cosmologically realistic evolution of the cluster potential, and the effects of a multiphase cooling flow. The model utilises a fairly standard 1-D Lagrangian hydrodynamical code to calculate the evolution of the intracluster gas. This is coupled to a theoretical model for the growth of dark matter density perturbations. The main advantages of this treatment over 3-D codes are (1) improved spatial resolution within the cooling flow region, (2) much faster execution time, allowing a fuller exploration of parameter space, and (3) the inclusion of additional physics. In the present paper, we explore the development of infall models -- in which gas relaxes into a deepening potential well -- covering a wide range of cluster mass scales. We find that such simple models reproduce many of the global properties of observed clusters. Very strong cooling flows develop in these 1-D cluster models. In practice, disruption by major mergers probably reduces the cooling rate in most clusters. The models overpredict the gas fraction in low mass systems, indicating the need for additional physical processes, such as preheating or galaxy winds, which become important on small mass scales.Comment: 38 pages, 21 encapsulated postscript figures, accepted for publication in MNRA

An experimental evaluation of error seeding as a program validation technique

A previously reported experiment in error seeding as a program validation technique is summarized. The experiment was designed to test the validity of three assumptions on which the alleged effectiveness of error seeding is based. Errors were seeded into 17 functionally identical but independently programmed Pascal programs in such a way as to produce 408 programs, each with one seeded error. Using mean time to failure as a metric, results indicated that it is possible to generate seeded errors that are arbitrarily but not equally difficult to locate. Examination of indigenous errors demonstrated that these are also arbitrarily difficult to locate. These two results support the assumption that seeded and indigenous errors are approximately equally difficult to locate. However, the assumption that, for each type of error, all errors are equally difficult to locate was not borne out. Finally, since a seeded error occasionally corrected an indigenous error, the assumption that errors do not interfere with each other was proven wrong. Error seeding can be made useful by taking these results into account in modifying the underlying model

One-way quantum computation with four-dimensional photonic qudits

We consider the possibility of performing linear optical quantum computation making use of extra photonic degrees of freedom. In particular we focus on the case where we use photons as quadbits. The basic 2-quadbit cluster state is a hyper-entangled state across polarization and two spatial mode degrees of freedom. We examine the non-deterministic methods whereby such states can be created from single photons and/or Bell pairs, and then give some mechanisms for performing higher-dimensional fusion gates.Comment: 10 figures (typos are corrected

Reverse Current in Solar Flares

The theory that impulsive X ray bursts are produced by high energy electrons streaming from the corona to the chromosphere is investigated. Currents associated with these streams are so high that either the streams do not exist or their current is neutralized by a reverse current. Analysis of a simple model indicates that the primary electron stream leads to the development of an electric field in the ambient corona which decelerates the primary beam and produces a neutralizing reverse current. It appears that, in some circumstances, this electric field could prevent the primary beam from reaching the chromosphere. In any case, the electric field acts as an energy exchange mechanism, extracting kinetic energy from the primary beam and using it to heat the ambient plasma. This heating is typically so rapid that it must be expected to have important dynamical consequences

Full-vector analysis of a realistic photonic crystal fiber

We analyze the guiding problem in a realistic photonic crystal fiber using a novel full-vector modal technique, a biorthogonal modal method based on the nonselfadjoint character of the electromagnetic propagation in a fiber. Dispersion curves of guided modes for different fiber structural parameters are calculated along with the 2D transverse intensity distribution of the fundamental mode. Our results match those achieved in recent experiments, where the feasibility of this type of fiber was shown.Comment: 3 figures, submitted to Optics Letter

The implementation and use of Ada on distributed systems with reliability requirements

The issues involved in the use of the programming language Ada on distributed systems are discussed. The effects of Ada programs on hardware failures such as loss of a processor are emphasized. It is shown that many Ada language elements are not well suited to this environment. Processor failure can easily lead to difficulties on those processors which remain. As an example, the calling task in a rendezvous may be suspended forever if the processor executing the serving task fails. A mechanism for detecting failure is proposed and changes to the Ada run time support system are suggested which avoid most of the difficulties. Ada program structures are defined which allow programs to reconfigure and continue to provide service following processor failure