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

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

Efficient FPGA implementation of high-throughput mixed radix multipath delay commutator FFT processor for MIMO-OFDM

This article presents and evaluates pipelined architecture designs for an improved high-frequency Fast Fourier Transform (FFT) processor implemented on Field Programmable Gate Arrays (FPGA) for Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM). The architecture presented is a Mixed-Radix Multipath Delay Commutator. The presented parallel architecture utilizes fewer hardware resources compared to Radix-2 architecture, while maintaining simple control and butterfly structures inherent to Radix-2 implementations. The high-frequency design presented allows enhancing system throughput without requiring additional parallel data paths common in other current approaches, the presented design can process two and four independent data streams in parallel and is suitable for scaling to any power of two FFT size N. FPGA implementation of the architecture demonstrated significant resource efficiency and high-throughput in comparison to relevant current approaches within literature. The proposed architecture designs were realized with Xilinx System Generator (XSG) and evaluated on both Virtex-5 and Virtex-7 FPGA devices. Post place and route results demonstrated maximum frequency values over 400 MHz and 470 MHz for Virtex-5 and Virtex-7 FPGA devices respectively

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 6-8 weeks. Healing was defined as bridging callus on at least three cortices (RUST > 10). 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 (p = 0.636). At 6-8 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 (p = 0.001). 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

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

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