169 research outputs found
Lithium Production in Companions of Accreting X-Ray Binaries by Neutron Spallation of C,N,O Elements
We examine the processes which could lead to the observed enhancement of Li
and possibly other light elements (Be, B) in the companions of a number of
X-ray novae. We conclude that one of the most promising mechanisms is the
spallation of CNO elements on the surface of the companion induced by the
neutron flux produced in the hot accretion flow onto the compact object. Direct
production of the observed Li and its deposition onto the dwarf companion seem
less likely, mainly because of the possibility of its destruction in the
production region itself and difficulties in its deposition associated with the
configuration of the companion's magnetic field. We discuss other potential
observables of the above scenario.Comment: 23 pages Latex, of which 5 pages of tables, to appear in the
Astrophysical Journal, Vol 512, Feb 10 issu
Formation and dynamics of self-sustained neutron haloes in disk accreting sources
It has been recognized long ago that the presence of hot plasma in the inner
accretion disks around black holes could lead to the neutron production via
dissociation of helium nuclei. We show that, for a broad range of accretion
parameters, neutrons effectively decouple from protons and pile up in the inner
disk leading to the formation of self-sustained halo. This means that new
neutrons in the halo are supplied mainly by the splitting of helium nuclei in
their collisions with existing neutrons. Once formed, such a halo can exist
even if the proton temperature is much lower than the energy threshold of
helium dissociation. We show that neutron haloes can be the natural source of
relativistic electrons and positrons, providing characteristic comptonization
spectra and hard spectral tails observed in many black hole candidates, and
also giving rise to relativistic outflows. Deuterium gamma-ray line at 2.2 MeV
resulting from neutron capture is also expected at a level detectable by future
INTEGRAL mission. Furthermore, the presence of a neutron halo strongly affects
the dynamics of accretion and leads to the rich variety of transient dynamical
regimes.Comment: 10 pages, submitted to Astronomy and Astrophysic
ANETAC: Arabic named entity transliteration and classification dataset
In this paper, we make freely accessible ANETAC, our English-Arabic named entity transliteration and classification dataset that we built from freely available parallel translation corpora. The dataset contains 79, 924 instances, each instance is a triplet (e, a, c), where e is the English named entity, a is its Arabic transliteration and c is its class that can be either a Person, a Location, or an Organization. The ANETAC dataset is mainly aimed for the researchers that are working on Arabic named entity transliteration, but it can also be used for named entity classification purposes. This dataset was developed and used as part of a previous research study done by Hadj Ameur et al. [1]
The 511 keV emission from positron annihilation in the Galaxy
The first gamma-ray line originating from outside the solar system that was
ever detected is the 511 keV emission from positron annihilation in the Galaxy.
Despite 30 years of intense theoretical and observational investigation, the
main sources of positrons have not been identified up to now. Observations in
the 1990's with OSSE/CGRO showed that the emission is strongly concentrated
towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's
INTEGRAL gamma-ray observatory allowed scientists to measure that emission
across the entire Galaxy, revealing that the bulge/disk luminosity ratio is
larger than observed in any other wavelength. This mapping prompted a number of
novel explanations, including rather "exotic ones (e.g. dark matter
annihilation). However, conventional astrophysical sources, like type Ia
supernovae, microquasars or X-ray binaries, are still plausible candidates for
a large fraction of the observed total 511 keV emission of the bulge. A closer
study of the subject reveals new layers of complexity, since positrons may
propagate far away from their production sites, making it difficult to infer
the underlying source distribution from the observed map of 511 keV emission.
However, contrary to the rather well understood propagation of high energy
(>GeV) particles of Galactic cosmic rays, understanding the propagation of low
energy (~MeV) positrons in the turbulent, magnetized interstellar medium, still
remains a formidable challenge. We review the spectral and imaging properties
of the observed 511 keV emission and we critically discuss candidate positron
sources and models of positron propagation in the Galaxy.Comment: 62 pages, 35 figures. Review paper to appear in Reviews of Modern
Physic
Control of Stand-Alone Photovoltaic System Using Fuzzy-Logic Controller
With industrial development the problem of energy shortage is
more and more aggravating. The photovoltaic (PV) systems are
rapidly expanding and have increasing in electric power
technology and regarded as the green energy of the new century
control, sizing and management of stand-alone photovoltaic
systems are based on static method and energy estimation
allowing the simulation of PV system in average condition.
Indeed these approaches provide information about the expected
performances and acceptable satisfactions rates they do not
perform an optimisation of the PV plant for the local climate
conditions. The power supplied by solar arrays depends on the
radiation, temperature and array voltage, the operation point of a
load connected at the boundaries of a photovoltaic generator
always does not coincide with that of the optimal point and it
varied according to the weather conditions, in this way it's
necessary to track the maximum power of solar arrays. The
purpose of this work is to use the fuzzy-logic controller (flc) in
order to control of maximum power point tracker (mppt) of PV
generator under variable radiation conditions. For this, we used
as solution the control of the mppt variation in order to deliver
the highest power to the load; also we implemented a controller
circuit between PC and the regulator system. The designed
system is applicable for remote areas or isolated leads and the
sizing of stand-alone PV system. Obtained result indicates that
the proposed method can successfully be used for control of
mppt for stand-alone PV system
Novel Technique of Sizing the Stand-Alone Photovoltaic Systems Using the Radial Basis Function Neural Networks: Application in Isolated Sites
The objective of this work is to investigate the Radial Basis
Function Neural Networks (RBFN) to identifying and
modeling the optimal sizing couples of stand-alone
photovoltaic (PV) system using a minimum of input data,
These optimal couples allow to the users of stand-alone PV
systems to determine the number of solar panel modules
and storage batteries necessary to satisfy a given
consumption. The advantage of this model is to estimate of
the sizing PV system in any site in Algeria particularly in
isolated sites, where the global solar radiation data is not
always available. A RBFN has been trained by using 200
known sizing couples data corresponding to 200 locations.
In this way, it was trained to accept and even handle a
number of unusual case, known sizing couples were
subsequently used to investigate the accuracy of prediction
the training of the RBFN model was performed with
adequate accuracy. Subsequently, the unknown validation
sizing couples set produced very set accurate predictions
with the correlation coefficient between the actual and the
RBFN model identified data of 98% was obtained. This
result indicates that the proposed method can be
successfully used for estimating of optimal sizing couples
of PV systems for any locations in Algeria, but the
methodology can be generalized using different locations in
the world
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Augmented compression in exchange nailing for femoral and tibial non-unions accelerates time to radiographic union.
PURPOSE: Exchange nailing is widely used for the management of aseptic femoral and tibial non-union. Compressive forces markedly reduce strain, increasing rate and incidence of union. Additional compressive forces can be applied to the non-union site by using the design features of some modern nailing systems. This study hypothesises that the use of additional compression in exchange nailing results in faster time to union. METHODS: All femoral and tibial shaft non-unions were identified over a 4-year period between 2014-2018. Intraoperative compression during exchange nailing was either applied or not applied with a dedicated active compression device through the intramedullary nail. An initial 'radiographic union score for tibia' (RUST) score was calculated from preoperative lateral and AP radiographs and compared with the postoperative radiographs at 6-8Â weeks. Healing was defined as bridging callus on at least three cortices (RUSTâ>â10). RESULTS: A total of 119 patients were identified. Following application of exclusion criteria, we analysed data for 19 patients, 10 undergoing exchange nailing with intraoperative compression and 9 without. The pre-exchange RUST score was comparable between the compressed group and standard exchange group with mean of 7.11 versus 7.5 (pâ=â0.636). At 6-8Â weeks post-op, there was a significant difference between the median RUST score in the compressed group vs standard exchange group, 11 compared to 8.39 (pâ=â0.001). CONCLUSIONS: Our study shows that time to union was accelerated when additional compression was applied to exchange nailing, resulting in reduced follow-up visits and number of radiographs required
Improving Arabic neural machine translation via n-best list re-ranking
Even though the rise of the Neural Machine Translation (NMT) paradigm has brought a great deal of improvement to the machine translation field, the current translation results are still not perfect. One of the main reasons for this imperfection is the decoding task complexity. Indeed, the problem of finding the one best translation from the space of all possible translations was and still is a challenging problem. One of the most successful ways to address it is via n-best list re-ranking which attempts to reorder the n-best decoder translations according to some defined features. In this paper, we propose a set of new re-ranking features that can be extracted directly from the parallel corpus without needing any external tools. The features set that we propose takes into account lexical, syntactic, and even semantic aspects of the n-best list translations. We also present a method for feature weights optimization that uses a Quantum-behaved Particle Swarm Optimization (QPSO) algorithm. Our system has been evaluated on multiple English-to-Arabic and Arabic-to-English machine translation test sets, and the obtained re-ranking results yield noticeable improvements over the baseline NMT systems
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