99 research outputs found
Nonlinear resonant absorption of fast magnetoacoustic waves in strongly anisotropic and dispersive plasmas
The nonlinear theory of driven magnetohydrodynamics (MHD) waves in strongly anisotropic and dispersive plasmas, developed for slow resonance by Clack and Ballai [Phys. Plasmas 15, 2310 (2008)] and Alfvén resonance by Clack et al. [Astron. Astrophys. 494, 317 (2009)] , is used to study the weakly nonlinear interaction of fast magnetoacoustic (FMA) waves in a one-dimensional planar plasma. The magnetic configuration consists of an inhomogeneous magnetic slab sandwiched between two regions of semi-infinite homogeneous magnetic plasmas. Laterally driven FMA waves penetrate the inhomogeneous slab interacting with the localized slow or Alfvén dissipative layer and are partly reflected, dissipated, and transmitted by this region. The nonlinearity parameter defined by Clack and Ballai (2008) is assumed to be small and a regular perturbation method is used to obtain analytical solutions in the slow dissipative layer. The effect of dispersion in the slow dissipative layer is to further decrease the coefficient of energy absorption, compared to its standard weakly nonlinear counterpart, and the generation of higher harmonics in the outgoing wave in addition to the fundamental one. The absorption of external drivers at the Alfvén resonance is described within the linear MHD with great accuracy
Aqueye optical observations of the Crab Nebula pulsar
We observed the Crab pulsar in October 2008 at the Copernico Telescope in
Asiago - Cima Ekar with the optical photon counter Aqueye (the Asiago Quantum
Eye) which has the best temporal resolution and accuracy ever achieved in the
optical domain (hundreds of picoseconds). Our goal was to perform a detailed
analysis of the optical period and phase drift of the main peak of the Crab
pulsar and compare it with the Jodrell Bank ephemerides. We determined the
position of the main peak using the steepest zero of the cross-correlation
function between the pulsar signal and an accurate optical template. The pulsar
rotational period and period derivative have been measured with great accuracy
using observations covering only a 2 day time interval. The error on the period
is 1.7 ps, limited only by the statistical uncertainty. Both the rotational
frequency and its first derivative are in agreement with those from the Jodrell
Bank radio ephemerides archive. We also found evidence of the optical peak
leading the radio one by ~230 microseconds. The distribution of phase-residuals
of the whole dataset is slightly wider than that of a synthetic signal
generated as a sequence of pulses distributed in time with the probability
proportional to the pulse shape, such as the average count rate and background
level are those of the Crab pulsar observed with Aqueye. The counting
statistics and quality of the data allowed us to determine the pulsar period
and period derivative with great accuracy in 2 days only. The time of arrival
of the optical peak of the Crab pulsar leads the radio one in agreement with
what recently reported in the literature. The distribution of the phase
residuals can be approximated with a Gaussian and is consistent with being
completely caused by photon noise (for the best data sets).Comment: 7 pages, 7 figures. Accepted for publication in Astronomy and
Astrophysic
Generic Tracking of Multiple Apparent Horizons with Level Flow
We report the development of the first apparent horizon locator capable of
finding multiple apparent horizons in a ``generic'' numerical black hole
spacetime. We use a level-flow method which, starting from a single arbitrary
initial trial surface, can undergo topology changes as it flows towards
disjoint apparent horizons if they are present. The level flow method has two
advantages: 1) The solution is independent of changes in the initial guess and
2) The solution can have multiple components. We illustrate our method of
locating apparent horizons by tracking horizon components in a short
Kerr-Schild binary black hole grazing collision.Comment: 13 pages including figures, submitted to Phys Rev
The relativistic shift of narrow spectral features from black-hole accretion discs
Transient spectral features have been discovered in the X-ray spectra of
Active Galactic Nuclei, mostly in the 5--7 keV energy range. Several
interpretations were proposed for the origin of these features. We examined a
model of Doppler boosted blue horns of the iron line originating from a spot in
a black hole accretion disc, taking into account different approximations of
general relativistic light rays and the resulting shift of energy of photons.
We provide a practical formula for the blue horn energy of an intrinsically
narrow line and assess its accuracy by comparing the approximation against an
exact value, predicted under the assumption of a planar accretion disc. The
most accurate approximation provides excellent agreement with the spot orbital
radius down to the marginally stable orbit of a non-rotating black hole.Comment: Accepted for publication in A&A; 8 pages, 5 figure
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Change but no climate change: discourses of climate change in corporate social responsibility reporting in the oil industry
Using corpus linguistic tools and methods, this paper investigates the discourses of climate change in corporate social responsibility (CSR) and environmental reports produced by major oil companies from 2000 to 2013. It focuses on the frequency of key references to climatic changes and examines in detail discourses surrounding the most frequently used term âclimate changeâ. The analysis points to shifting patterns in the ways in which climate change has been discursively constructed in the studied sample. Whereas in the mid-2000s, it was seen as a phenomenon that something could be done about, in recent years the corporate discourse has increasingly emphasised the notion of risk portraying climate change as an unpredictable agent. A pro-active stance signalled by the use of force metaphors is offset by a distancing strategy often indicated through the use of hedging devices and ârelocationâ of climate change to the future and other stakeholders. In doing so, the discourse obscures the sectorâs large contribution to environmental degradation and âgroomsâ the public perception to believe that the industry actively engages in climate change mitigation. At the methodological level, this study shows how a combination of quantitative corpus-linguistic and qualitative discourse-analytical techniques can offer insights into the existence of salient discursive patterns and contribute to a better understanding of the role of language in performing ideological work in corporate communications
Computational Models for Prediction of Yeast Strain Potential for Winemaking from Phenotypic Profiles
Saccharomyces cerevisiae strains from diverse natural habitats harbour a vast amount of phenotypic diversity, driven by interactions between yeast and the respective environment. In grape juice fermentations, strains are exposed to a wide array of biotic and abiotic stressors, which may lead to strain selection and generate naturally arising strain diversity. Certain phenotypes are of particular interest for the winemaking industry and could be identified by screening of large number of different strains. The objective of the present work was to use data mining approaches to identify those phenotypic tests that are most useful to predict a strain's potential for winemaking. We have constituted a S. cerevisiae collection comprising 172 strains of worldwide geographical origins or technological applications. Their phenotype was screened by considering 30 physiological traits that are important from an oenological point of view. Growth in the presence of potassium bisulphite, growth at 40 degrees C, and resistance to ethanol were mostly contributing to strain variability, as shown by the principal component analysis. In the hierarchical clustering of phenotypic profiles the strains isolated from the same wines and vineyards were scattered throughout all clusters, whereas commercial winemaking strains tended to co-cluster. Mann-Whitney test revealed significant associations between phenotypic results and strain's technological application or origin. Naive Bayesian classifier identified 3 of the 30 phenotypic tests of growth in iprodion (0.05 mg/mL), cycloheximide (0.1 mu g/mL) and potassium bisulphite (150 mg/mL) that provided most information for the assignment of a strain to the group of commercial strains. The probability of a strain to be assigned to this group was 27% using the entire phenotypic profile and increased to 95%, when only results from the three tests were considered. Results show the usefulness of computational approaches to simplify strain selection procedures.Ines Mendes and Ricardo Franco-Duarte are recipients of a fellowship from the Portuguese Science Foundation, FCT (SFRH/BD/74798/2010, SFRH/BD/48591/2008, respectively) and Joao Drumonde-Neves is recipient of a fellowship from the Azores government (M3.1.2/F/006/2008 (DRCT)). Financial support was obtained from FEDER funds through the program COMPETE and by national funds through FCT by the projects FCOMP-01-0124-008775 (PTDC/AGR-ALI/103392/2008) and PTDC/AGR-ALI/121062/2010. Lan Umek and Blaz Zupan acknowledge financial support from Slovene Research Agency (P2-0209). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.info:eu-repo/semantics/publishedVersio
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