Using our new 3-D relativistic particle-in-cell (PIC) code parallelized with MPI, we investigated long-term particle acceleration associated with a relativistic electron–positron jet propagating in ...an unmagnetized ambient electron–positron plasma. The simulations were performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks. Acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value as predicted by hydrodynamic shock compression. In the jet (reverse) shock behind the bow (forward) shock the strongest electromagnetic fields are generated. These fields may lead to time dependent afterglow emission. In order to calculate radiation from first principles that goes beyond the standard synchrotron model used in astrophysical objects we have used PIC simulations. Initially we calculated radiation from electrons propagating in a uniform parallel magnetic field to verify the technique. We then used the technique to calculate emission from electrons in a small simulation system. From these simulations we obtained spectra which are consistent with those generated from electrons propagating in turbulent magnetic fields with red noise. This turbulent magnetic field is similar to the magnetic field generated at an early nonlinear stage of the Weibel instability. A fully developed shock within a larger simulation system may generate a jitter/synchrotron spectrum.
The study presents an application of two different optical measurement techniques for the detection of cracks at the surface of a realistically sized concrete beam subjected to flexural loading ...conditions. Both techniques provide displacement measurements in a discrete number of points on the surface. Based on these displacement fields, deformations are calculated by means of the Green-Lagrange strain expression. The study deals with the relationship between cracks and the concept of deformation and it examines which of the two methods presented appears to be the most suitable for crack prediction or detection. The study shows that it is possible to detect the appearance and evolution of cracks, even before the cracks become visually detectable, with both methods and reveals their complementarities.
This paper describes the identification of hardening parameters of DC05 sheet metal and contact friction coefficients using a multi-layered upsetting test (MLUT), the modified two specimen method ...(MTSM) and a finite element based inverse method. The MTSM is adopted in order to identify the friction coefficient between the tools and the stacked circular specimens which are cut from the base material by spark erosion. Next, the hardening behaviour is identified inversely by combining the results of a MLUT and finite element simulations of the test setup. Finally, the results are compared with standard tensile tests and it is shown that the MLUT is a viable alternative for the identification of the local hardening behaviour of sheet metal where standard test specimens cannot be prepared due to size limitations of the specimen.
We present a model of the multifrequency variability of the blazar Mrk 421. The model explains correlated variability observed from Very High Energy (VHE) gamma rays to radio frequencies. We assume ...that the dominant part of the stationary emission from the radio frequencies to the X-rays is generated by the synchrotron radiation of relativistic electrons ejected from the central engine. The particles move from the center of the source with relativistic velocities and form an inhomogeneous jet. We perform detailed calculations of the radiation transfer and calculate evolution of the electron energy spectrum along the jet. We explain the observed variability by the evolving synchrotron and Inverse-Compton (IC) radiation of a compact component (a blob) which travels along the jet. Two scenarios have been considered as mechanisms to generate VHE flares. The first scenario assumes that the high energy electrons, necessary for generation of the VHE flares, are injected into the jet, directly from the central engine or from an acceleration zone (e.g., a shock wave). The second scheme assumes that the high energy electrons are generated in situ by acceleration, for example by diffusive shock waves or a localized turbulence inside the jet. The particles evolve along the jet. They are cooled by the radiative processes and by the adiabatic expansion which compete with the acceleration process and the injection of high energy electrons. We present new observations we obtained in the radio domain for Mrk 421. The radio data gathered in February–April 2001 show a well defined radio outburst which corresponds to an X-ray outburst observed by RXTE-ASM and a gamma-ray flare detected by HEGRA in the TeV range. The best of our knowledge, this is the first direct observational evidence for a flare observed simultaneously in the radio range and at very high energies. Our scenario with acceleration of electrons in the middle part of the jet describes well the temporal evolution of such multispectral flare.
Objective
Robson's Ten Group Classification System (TGCS) creates clinically relevant sub‐groups for monitoring caesarean birth rates. This study assesses whether this classification can be derived ...from routine data in Europe and uses it to analyse national caesarean rates.
Design
Observational study using routine data.
Setting
Twenty‐seven EU member states plus Iceland, Norway, Switzerland and the UK.
Population
All births at ≥22 weeks of gestational age in 2015.
Methods
National statistical offices and medical birth registers derived numbers of caesarean births in TGCS groups.
Main outcome measures
Overall caesarean rate, prevalence and caesarean rates in each of the TGCS groups.
Results
Of 31 countries, 18 were able to provide data on the TGCS groups, with UK data available only from Northern Ireland. Caesarean birth rates ranged from 16.1 to 56.9%. Countries providing TGCS data had lower caesarean rates than countries without data (25.8% versus 32.9%, P = 0.04). Countries with higher caesarean rates tended to have higher rates in all TGCS groups. Substantial heterogeneity was observed, however, especially for groups 5 (previous caesarean section), 6, 7 (nulliparous/multiparous breech) and 10 (singleton cephalic preterm). The differences in percentages of abnormal lies, group 9, illustrate potential misclassification arising from unstandardised definitions.
Conclusions
Although further validation of data quality is needed, using TGCS in Europe provides valuable comparator and baseline data for benchmarking and surveillance. Higher caesarean rates in countries unable to construct the TGCS suggest that effective routine information systems may be an indicator of a country's investment in implementing evidence‐based caesarean policies.
Tweetable
Many European countries can provide Robson's Ten‐Group Classification to improve caesarean rate comparisons.
Tweetable
Many European countries can provide Robson's Ten‐Group Classification to improve caesarean rate comparisons.
Modelling rapid TeV variability of PKS 2155−304 Katarzyński, K.; Lenain, J.-P.; Zech, A. ...
Monthly notices of the Royal Astronomical Society,
10/2008, Letnik:
390, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present theoretical modelling for the very rapid TeV variability of PKS 2155−304 observed recently by the High Energy Spectroscopic System (HESS) experiment. To explain the light curve, where at ...least five flaring events were well observed, we assume five independent components of a jet that are characterized by slightly different physical parameters. An additional, significantly larger component is used to explain the emission of the source at long time-scales. This component dominates the emission in the X-ray range, whereas the other components are dominant in the TeV range. The model used for our simulation describes precisely the evolution of the particle energy spectrum inside each component and takes into account light travel time effects. We show that a relatively simple synchrotron self-Compton scenario may explain this very rapid variability. Moreover, we find that absorption of the TeV emission inside the components due to the pair creation process is negligible.
The effects of relativistic dynamics and thermodynamics in the development of Kelvin-Helmholtz instabilities in planar, relativistic jets along the early phases (namely linear and saturation phases) ...of evolution has been studied by a combination of linear stability analysis and high-resolution numerical simulations for the most unstable first reflection modes in the temporal approach. Three different values of the jet Lorentz factor (5, 10 and 20) and a few different values of specific internal energy of the jet matter (from 0.08 to 60.0c super(2)) have been considered. Figures illustrating the evolution of the perturbations are also shown. Our simulations reproduce the linear regime of evolution of the excited eigenmodes of the different models with a high accuracy. In all the cases the longitudinal velocity perturbation is the first quantity that departs from the linear growth when it reaches a value close to the speed of light in the jet reference frame. The saturation phase extends from the end of the linear phase up to the saturation of the transversal velocity perturbation (at approximately 0.5c in the jet reference frame). The saturation times for the different numerical models are explained from elementary considerations, i.e. from properties of linear modes provided by the linear stability analysis and from the limitation of the transversal perturbation velocity. The limitation of the components of the velocity perturbation at the end of the linear and saturation phases allows us to conclude that the relativistic nature of the flow appears to be responsible for the departure of the system from linear evolution. The high accuracy of our simulations in describing the early stages of evolution of the KH instability (as derived from the agreement between the computed and expected linear growth rates and the consistency of the saturation times) establishes a solid basis to study the fully nonlinear regime, to be done elsewhere. The present paper also sets the theoretical and numerical background for these further studies.
Context. We present high-spatial and medium-spectral resolution imaging and spectroscopy of the core of the Seyfert II galaxy NGC 1068, obtained in the N-band spectral range with the VLT/VISIR (the ...VLT Imager and Spectrometer in the InfraRed). Spectra were acquired in the ArIII 8.99 μm, SIV 10.51 μm and NeII 12.81 μm filters for two orientations of the long slit $(2'\times0\farcs4)$: PA = -15° and 90° oriented N to E. Aims. The spatial evolution of the ArIII+MgVII, SIV and NeII lines are investigated to constrain the gas dynamics inside the narrow line region (NLR), and to deduce the geometry and physics of the ionization cones. Methods. Spectra are extracted in 0$\farcs$381 steps along the two slit positions and we obtain line parameters from fits with Gaussian profiles. The computed line ratios are compared to photoionization models. Results. Lines are resolved and detected in emission up to ~3″ NW and ~3″ SE of the core, generally double-peaked. Features trace an outflow with radial velocities up to 103 km s-1, well reproduced by a hollow biconical structure associated with the ionization bicone of NGC 1068. It is inclined at ~11° and the maximum aperture is ~80°, in good agreement with optical spectroscopic studies. Conclusions. The velocity profiles traced in the MIR are interpreted as evidence for a gas outflow along the edges of the ionization bicones. The comparison of line ratios with standard dust-free and dusty, radiation pressure-dominated photoionization models shows that the associated gas metallicity is close to the solar value and points to a relatively weak ionization parameter of a typical value U0 = 10-2.6 at PA = -15° along the ionization bicone.
Textile reinforcements are characterised by a hierarchy of structure, which allows a general structural approach to their behaviour and their use in composite processing. The hierarchical approach ...led to the development of a software tool for describing the geometry and mechanics of textiles. This tool is used as a pre-processor for meso-mechanical and permeability models for textile composites. Together the pre-processor, permeability and meso-mechanical models constitute an integrated design tool for textile composite modelling on a meso-scale level. The permeability-related part of this tool is presented in this paper. The focus is on the strategy and numerical approach to permeability modelling of textiles. The proposed strategy allows integrating in a natural way the different processing steps of composite manufacturing, while a numerical approach based on the lattice Boltzmann method allows dealing with the complexity of flow in textiles. Comparison with model test cases and with experimental data on permeability of woven fabrics is provided.
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