A Search for Gravitational Radiation from PSR 1937+214

A search for gravitational radiation from the "millisecond pulsar", PSR 1937+214, using a 40 meter baseline laser interferometric detector is described. Four days of observation yielded 1.2 x 105 seconds of data. Throughout the experiment, the pulsar phase was synthesized to an accuracy of better than one tenth of the pulsar period. A trigger generated from this signal synchronized the data averaging. Narrow band amplitude spectra centered at the pulsar's fundamental electromagnetic pulsation frequency (~642 Hz) and its first harmonic were obtained. The spectra, one for each combination of polarization and center frequency, place 99.7% confidence level limits on the emitted gravitational radiation. In dimensionless strain, h, the rms limits are: 642 Hz "plus" polarization 1.6 x 10-17 " "cross" " 3.1 x 10-17 1294 Hz "plus" polarization 1.1 x 10-17 " "cross" " 1.5 x 10-17 Over the four day observing period, the performance of the detector varied with changing temperature. During the stable night hours, the two optical cavities remained locked to reflection minima for 20 to 80 minutes before momentarily losing lock. Temperature changes of 1° to 2°C in the morning and evening necessitated compensating adjustments to the optics to maintain good fringe visibility. The interferometer senses changes in the separations between three test masses. The test masses hang like pendulums so that they are free to move in response to gravitational radiation. The suspension system is designed to provide passive isolation from seismic and environmental vibration noise. The orientation of each test mass is stabilized with a feedback loop. The design of the test masses, their suspension systems, and the servo system which controls their orientation is described.</p

Hierarchical multithreading: programming model and system software

This paper addresses the underlying sources of performance degradation (e.g. latency, overhead, and starvation) and the difficulties of programmer productivity (e.g. explicit locality management and scheduling, performance tuning, fragmented memory, and synchronous global barriers) to dramatically enhance the broad effectiveness of parallel processing for high end computing. We are developing a hierarchical threaded virtual machine (HTVM) that defines a dynamic, multithreaded execution model and programming model, providing an architecture abstraction for HEC system software and tools development. We are working on a prototype language, LITL-X (pronounced "little-X") for latency intrinsic-tolerant language, which provides the application programmers with a powerful set of semantic constructs to organize parallel computations in a way that hides/manages latency and limits the effects of overhead. This is quite different from locality management, although the intent of both strategies is to minimize the effect of latency on the efficiency of computation. We work on a dynamic compilation and runtime model to achieve efficient LITL-X program execution. Several adaptive optimizations were studied. A methodology of incorporating domain-specific knowledge in program optimization was studied. Finally, we plan to implement our method in an experimental testbed for a HEC architecture and perform a qualitative and quantitative evaluation on selected applications

The star formation histories of low surface brightness galaxies

We have performed deep imaging of a diverse sample of 26 low surface brightness galaxies (LSBGs) in the optical and the near-infrared. Using stellar population synthesis models, we find that it is possible to place constraints on the ratio of young to old stars (which we parametrize in terms of the average age of the galaxy), as well as the metallicity of the galaxy, using optical and near-infrared colours. LSBGs have a wide range of morphologies and stellar populations, ranging from older, high-metallicity earlier types to much younger and lower-metallicity late-type galaxies. Despite this wide range of star formation histories, we find that colour gradients are common in LSBGs. These are most naturally interpreted as gradients in mean stellar age, with the outer regions of LSBGs having lower ages than their inner regions. In an attempt to understand what drives the differences in LSBG stellar populations, we compare LSBG average ages and metallicities with their physical parameters. Strong correlations are seen between an LSBG's star formation history and its K-band surface brightness, K-band absolute magnitude and gas fraction. These correlations are consistent with a scenario in which the star formation history of an LSBG primarily correlates with its surface density and its metallicity correlates with both its mass and its surface densit

The Stellar Populations of Low Surface Brightness Galaxies

Near-infrared (NIR) K' images of a sample of five low surface brightness disc galaxies (LSBGs) were combined with optical data, with the aim of constraining their star formation histories. Both red and blue LSBGs were imaged to enable comparison of their stellar populations. For both types of galaxy strong colour gradients were found, consistent with mean stellar age gradients. Very low stellar metallicities were ruled out on the basis of metallicity-sensitive optical-NIR colours. These five galaxies suggest that red and blue LSBGs have very different star formation histories and represent two independent routes to low B band surface brightness. Blue LSBGs are well described by models with low, roughly constant star formation rates, whereas red LSBGs are better described by a `faded disc' scenario.Comment: 5 pages LaTeX; 2 embedded figures; MNRAS Letters, Accepte

Large-Scale Modeling of Epileptic Seizures: Scaling Properties of Two Parallel Neuronal Network Simulation Algorithms

Our limited understanding of the relationship between the behavior of individual neurons and large neuronal networks is an important limitation in current epilepsy research and may be one of the main causes of our inadequate ability to treat it. Addressing this problem directly via experiments is impossibly complex; thus, we have been developing and studying medium-large-scale simulations of detailed neuronal networks to guide us. Flexibility in the connection schemas and a complete description of the cortical tissue seem necessary for this purpose. In this paper we examine some of the basic issues encountered in these multiscale simulations. We have determined the detailed behavior of two such simulators on parallel computer systems. The observed memory and computation-time scaling behavior for a distributed memory implementation were very good over the range studied, both in terms of network sizes (2,000 to 400,000 neurons) and processor pool sizes (1 to 256 processors). Our simulations required between a few megabytes and about 150 gigabytes of RAM and lasted between a few minutes and about a week, well within the capability of most multinode clusters. Therefore, simulations of epileptic seizures on networks with millions of cells should be feasible on current supercomputers