108,776 research outputs found
Evaluating Cache Coherent Shared Virtual Memory for Heterogeneous Multicore Chips
The trend in industry is towards heterogeneous multicore processors (HMCs),
including chips with CPUs and massively-threaded throughput-oriented processors
(MTTOPs) such as GPUs. Although current homogeneous chips tightly couple the
cores with cache-coherent shared virtual memory (CCSVM), this is not the
communication paradigm used by any current HMC. In this paper, we present a
CCSVM design for a CPU/MTTOP chip, as well as an extension of the pthreads
programming model, called xthreads, for programming this HMC. Our goal is to
evaluate the potential performance benefits of tightly coupling heterogeneous
cores with CCSVM
Lensing and high-z supernova surveys
Gravitational lensing causes the distribution of observed brightnesses of
standard candles at a given redshift to be highly non-gaussian. The
distribution is strongly, and asymmetrically, peaked at a value less than the
expected value in a homogeneous Robertson-Walker universe. Therefore, given any
small sample of observations in an inhomogeneous universe, the most likely
observed luminosity is at flux values less than the Robertson-Walker value.
This paper explores the impact of this systematic error due to lensing upon
surveys predicated on measuring standard candle brightnesses. We re-analyze
recent results from the high-z supernova team (Riess et al. 1998), both when
most of the matter in the universe is in the form of compact objects
(represented by the empty-beam expression, corresponding to the maximal case of
lensing), and when the matter is continuously distributed in galaxies. We find
that the best-fit model remains unchanged (at Omega_m=0, Omega_Lambda=0.45),
but the confidence contours change size and shape, becoming larger (and thus
allowing a broader range of parameter space) and dropping towards higher values
of matter density, Omega_m (or correspondingly, lower values of the
cosmological constant, Omega_Lambda). These effects are slight when the matter
is continuously distributed. However, the effects become considerably more
important if most of the matter is in compact objects. For example, neglecting
lensing, the Omega_m=0.5, Omega_Lambda=0.5 model is more than 2 sigma away from
the best fit. In the empty-beam analysis, this cosmology is at 1 sigma.Comment: 11 pages, 3 ps figures. uses aaspp4.sty. accepted to ApJ Letters.
includes analysis of lensing due to matter continuously distributed in
galaxie
Off equilibrium dynamics of the Frustrated Ising Lattice Gas
We study by means of Monte Carlo simulations the off equilibrium properties
of a model glass, the Frustrated Ising Lattice Gas (FILG) in three dimensions.
We have computed typical two times quantities, like density-density
autocorrelations and the autocorrelation of internal degrees of freedom. We
find an aging scenario particularly interesting in the case of the density
autocorrelations in real space which is very reminiscent of spin glass
phenomenology. While this model captures the essential features of structural
glass dynamics, its analogy with spin glasses may bring the possibility of its
complete description using the tools developed in spin glass theory.Comment: Phys. Rev. E (Rapid Communication), 1999 (probably May
A Comparison of X-ray and Optical Emission in Cassiopeia A
Broadband optical and narrowband Si XIII X-ray images of the young Galactic
supernova remnant Cas A obtained over several decades are used to investigate
spatial and temporal correlations on both large and small scales. The data
consist of optical and near infrared ground-based and Hubble Space Telescope
images taken between 1951 and 2011, and X-ray images from Einstein, ROSAT, and
Chandra taken between 1979 and 2013. We find weak spatial correlations between
the remnant's emission features on large scales, but several cases of good
optical/X-ray correlations on small scales for features which have brightened
due to recent interaction with the reverse shock. We also find instances where:
(i) a time delay is observed between the appearance of a feature's optical and
X-ray emissions, (ii) displacements of several arcseconds between a feature's
X-ray and optical emission peaks and, (iii) regions showing no corresponding
X-ray or optical emissions. To explain this behavior, we propose a
inhomogeneous model for Cas A's ejecta consisting of small, dense optically
emitting knots (n ~ 10^(2-3)/cm^(3)) and a much lower density (n ~ 0.1 -
1/cm^(3)) diffuse X-ray emitting component often spatially associated with
optical emission knots. The X-ray emitting component is sometimes linked to
optical clumps through shock induced mass ablation generating trailing material
leading to spatially offset X-ray/optical emissions. A range of ejecta
densities can also explain the observed X-ray/optical time delays since the
remnant's 5000 km/s reverse shock heats dense ejecta clumps to temperatures
around 3x10^4 K relatively quickly which then become optically bright while
more diffuse ejecta become X-ray bright on longer timescales. Highly
inhomogeneous ejecta as proposed here for Cas A may help explain some of the
X-ray/opticalfeatures seen in other young core collapse SN remnants.Comment: 31 pages, 21 figures. Submitted to the Astrophysical Journal. Please
contact the corresponding author for higher resolution postscript versions of
the figures ([email protected]
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