36,122 research outputs found
Gunrock: A High-Performance Graph Processing Library on the GPU
For large-scale graph analytics on the GPU, the irregularity of data access
and control flow, and the complexity of programming GPUs have been two
significant challenges for developing a programmable high-performance graph
library. "Gunrock", our graph-processing system designed specifically for the
GPU, uses a high-level, bulk-synchronous, data-centric abstraction focused on
operations on a vertex or edge frontier. Gunrock achieves a balance between
performance and expressiveness by coupling high performance GPU computing
primitives and optimization strategies with a high-level programming model that
allows programmers to quickly develop new graph primitives with small code size
and minimal GPU programming knowledge. We evaluate Gunrock on five key graph
primitives and show that Gunrock has on average at least an order of magnitude
speedup over Boost and PowerGraph, comparable performance to the fastest GPU
hardwired primitives, and better performance than any other GPU high-level
graph library.Comment: 14 pages, accepted by PPoPP'16 (removed the text repetition in the
previous version v5
Plausible fluorescent Ly-alpha emitters around the z=3.1 QSO0420-388
We report the results of a survey for fluorescent Ly-alpha emission carried
out in the field surrounding the z=3.1 quasar QSO0420-388 using the FORS2
instrument on the VLT. We first review the properties expected for fluorescent
Ly-alpha emitters, compared with those of other non-fluorescent Ly-alpha
emitters. Our observational search detected 13 Ly-alpha sources sparsely
sampling a volume of ~14000 comoving Mpc^3 around the quasar. The properties of
these in terms of i) the line equivalent width, ii) the line profile and iii)
the value of the surface brightness related to the distance from the quasar,
all suggest that several of these may be plausibly fluorescent. Moreover, their
number is in good agreement with the expectation from theoretical models. One
of the best candidates for fluorescence is sufficiently far behind QSO0420-388
that it would imply that the quasar has been active for (at least) ~60 Myrs.
Further studies on such objects will give information about proto-galactic
clouds and on the radiative history (and beaming) of the high-redshift quasars.Comment: 10 pages, 4 figures.Update to match the version published on ApJ 657,
135, 2007 March
The role of spatial and temporal radiation deposition in inertial fusion chambers: the case of HiPER¿
The first wall armour for the reactor chamber of HiPER will have to face short energy pulses of 5 to 20 MJ mostly in the form of x-rays and charged particles at a repetition rate of 5–10 Hz. Armour material and chamber dimensions have to be chosen to avoid/minimize damage to the chamber, ensuring the proper functioning of the facility during its planned lifetime. The maximum energy fluence that the armour can withstand without risk of failure, is determined by temporal and spatial deposition of the radiation energy inside the material. In this paper, simulations on the thermal effect of the radiation–armour interaction are carried out with an increasing definition of the temporal and spatial deposition of energy to prove their influence on the final results. These calculations will lead us to present the first values of the thermo-mechanical behaviour of the tungsten armour designed for the HiPER project under a shock ignition target of 48 MJ. The results will show that only the crossing of the plasticity limit in the first few micrometres might be a threat after thousands of shots for the survivability of the armour
A significant hardening and rising shape detected in the MeV/GeV nuFnu spectrum from the recently-discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February
We report on Fermi Large Area Telescope (LAT) and multi-wavelength results on
the recently-discovered very-high-energy (VHE, 100 GeV) blazar S4 0954+65
() during an exceptionally bright optical flare in 2015 February.
During the time period (2015 February, 13/14, or MJD 57067) when the MAGIC
telescope detected VHE -ray emission from the source, the Fermi-LAT
data indicated a significant spectral hardening at GeV energies, with a
power-law photon index of ---compared with the 3FGL value
(averaged over four years of observation) of . In contrast,
Swift/XRT data showed a softening of the X-ray spectrum, with a photon index of
(compared with averaged during the flare from
MJD 57066 to 57077), possibly indicating a modest contribution of synchrotron
photons by the highest-energy electrons superposed on the inverse Compton
component. Fitting of the quasi-simultaneous ( day) broadband spectrum with
a one-zone synchrotron plus inverse-Compton model revealed that GeV/TeV
emission could be produced by inverse-Compton scattering of external photons
from the dust torus. We emphasize that a flaring blazar showing high flux of
photons cm s ( 100 MeV) and a
hard spectral index of detected by Fermi-LAT on daily
time scales is a promising target for TeV follow-up by ground-based Cherenkov
telescopes to discover high-redshift blazars, investigate their temporal
variability and spectral features in the VHE band, and also constrain the
intensity of the extragalactic background light.Comment: 15 pages, 3 figures, 2 tables. Accepted by PAS
Theoretically Efficient Parallel Graph Algorithms Can Be Fast and Scalable
There has been significant recent interest in parallel graph processing due
to the need to quickly analyze the large graphs available today. Many graph
codes have been designed for distributed memory or external memory. However,
today even the largest publicly-available real-world graph (the Hyperlink Web
graph with over 3.5 billion vertices and 128 billion edges) can fit in the
memory of a single commodity multicore server. Nevertheless, most experimental
work in the literature report results on much smaller graphs, and the ones for
the Hyperlink graph use distributed or external memory. Therefore, it is
natural to ask whether we can efficiently solve a broad class of graph problems
on this graph in memory.
This paper shows that theoretically-efficient parallel graph algorithms can
scale to the largest publicly-available graphs using a single machine with a
terabyte of RAM, processing them in minutes. We give implementations of
theoretically-efficient parallel algorithms for 20 important graph problems. We
also present the optimizations and techniques that we used in our
implementations, which were crucial in enabling us to process these large
graphs quickly. We show that the running times of our implementations
outperform existing state-of-the-art implementations on the largest real-world
graphs. For many of the problems that we consider, this is the first time they
have been solved on graphs at this scale. We have made the implementations
developed in this work publicly-available as the Graph-Based Benchmark Suite
(GBBS).Comment: This is the full version of the paper appearing in the ACM Symposium
on Parallelism in Algorithms and Architectures (SPAA), 201
Near infrared and optical morphology of the dusty galaxy NGC972
Near infrared (NIR) and optical surface photometric analyses of the dusty
galaxy NGC972 are presented. The photometric profiles in the BVRJHK bands can
be fitted with a combination of gaussian and exponential profiles,
corresponding to a starburst nucleus and a stellar disk respectively. The
exponential scale length in the B-band is 2.8 times larger than in the K-band,
which implies a central B-band optical depth as high as 11. A bulge is absent
even in the NIR bands and hence the galaxy must be of a morphological type
later than the usually adopted Sb type. Relatively low rotational velocity and
high gas content also favor a later type, probably Sd, for the galaxy. Only one
arm can be traced in the distribution of old stars; the second arm, however,
can be traced in the distribution of dust and HII regions. Data suggest a short
NIR bar, which ends inside the nuclear ring. The slowly rising nature of the
rotation curve rules out a resonance origin of the the nuclear ring. The ring
is most likely not in the plane of the galaxy, given its circular appearance in
spite of the moderately high inclination of the galaxy. The off-planar nature
of the star forming ring, the unusually high fraction (30%) of the total mass
in molecular form, the presence of a nuclear starburst and the asymmetry of
spiral arms, are probably the result of a merger with a gas-rich companion
galaxy.Comment: Uses aas2pp4.sty and epsfig.sty, 12 pages To appear in Astronomical
Journal, October 199
Cosmological applications of a wavelet analysis on the sphere
The cosmic microwave background (CMB) is a relic radiation of the Big Bang
and as such it contains a wealth of cosmological information. Statistical
analyses of the CMB, in conjunction with other cosmological observables,
represent some of the most powerful techniques available to cosmologists for
placing strong constraints on the cosmological parameters that describe the
origin, content and evolution of the Universe. The last decade has witnessed
the introduction of wavelet analyses in cosmology and, in particular, their
application to the CMB. We review here spherical wavelet analyses of the CMB
that test the standard cosmological concordance model. The assumption that the
temperature anisotropies of the CMB are a realisation of a statistically
isotropic Gaussian random field on the sphere is questioned. Deviations from
both statistical isotropy and Gaussianity are detected in the reviewed works,
suggesting more exotic cosmological models may be required to explain our
Universe. We also review spherical wavelet analyses that independently provide
evidence for dark energy, an exotic component of our Universe of which we know
very little currently. The effectiveness of accounting correctly for the
geometry of the sphere in the wavelet analysis of full-sky CMB data is
demonstrated by the highly significant detections of physical processes and
effects that are made in these reviewed works.Comment: 17 pages, 8 figures; JFAA invited review, in pres
Ghost excitonic insulator transition in layered graphite
Some unusual properties of layered graphite, including a linear energy
dependence of the quasiparticle damping and weak ferromagnetism at low doping,
are explained as a result of the proximity of a single graphene sheet to the
excitonic insulator phase which can be further stabilized in a doped system of
many layers stacked in the staggered () configuration
Generalized Bounds on Majoron-neutrino couplings
We discuss limits on neutrino-Majoron couplings both from laboratory
experiments as well as from astrophysics. They apply to the simplest class of
Majoron models which covers a variety of possibilities where neutrinos acquire
mass either via a seesaw-type scheme or via radiative corrections. By adopting
a general framework including CP phases we generalize bounds obtained
previously. The combination of complementary bounds enables us to obtain a
highly non-trivial exclusion region in the parameter space. We find that the
future double beta project GENIUS, together with constraints based on supernova
energy release arguments, could restrict neutrino-Majoron couplings down to the
10^{-7} level.Comment: 17 pages, LateX, 7 figures, version to be published in Phys. Rev.
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