441 research outputs found

    Emergence and prevalence of/Mactamase producing Haemophilus influenzae in Finland and susceptibility of 102 respiratory isolates to eight antibiotics

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    A survey of 102 consecutive clinical isolates of Haemophilus influenzae mainly from otolaryngological patients revealed 13 ampicillin resistant ones, while 2 years earlier none were found. All the 13 strains which were resistant according to the broth dilution minimal inhibitory concentration (MIC) could be shown to produce /Mactamase using the chromogenic cephalosporin 87/312. Routine disc diffusion susceptibility testing had under-estimated the resistance and five of the /Mactamase producing strains had been reported as ampicillin susceptible. Amoxycillin and azidocillin were equally active and slightly less active than ampicillin against /Mactamase negative strains. The 13 ampicillin resistant strains were also resistant to amoxycillin and azidocillin, and also the MICs of cephalothin and cephalexin were significantly increased by /Mactamase production, while cefuroxime, erythromycin and chloramphenicol were not affected. A marked inoculum effect was demonstrated with the /Mactam antibiotics, and only a narrow (about tenfold) range of inoculum concentrations distinguished reliably between susceptible and resistant strains. The MICs of the penicillins against /Mactamase positive H. influenzae increased markedly with incubation time

    8. The Ionic Charge Composition of Anomalous Cosmic Rays

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    Turbulence and particle acceleration in collisionless supernovae remnant shocks: II- Cosmic-ray transport

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    Supernovae remnant shock waves could be at the origin of cosmic rays up to energies in excess of the knee (E31015E\simeq3\cdot 10^{15} eV) if the magnetic field is efficiently amplified by the streaming of accelerated particles in the shock precursor. This paper follows up on a previous paper \citep{pell05} which derived the properties of the MHD turbulence so generated, in particular its anisotropic character, its amplitude and its spectrum. In the present paper, we calculate the diffusion coefficients, also accounting for compression through the shock, and show that the predicted three-dimensional turbulence spectrum kS3d(k,k)k1kαk_\perp S_{3\rm d}(k_\parallel,k_\perp)\propto k_\parallel^{-1}k_\perp^{-\alpha} (with kk_\parallel and kk_\perp the wavenumber components along and perpendicular to the shock normal) generally leads to Bohm diffusion in the parallel direction. However, if the anisotropy is constrained by a relation of the form kk2/3k_\parallel \propto k_\perp^{2/3}, which arises when the turbulent energy cascade occurs at a constant rate independent of scale, then the diffusion coefficient loses its Bohm scaling and scales as in isotropic Kolmogorov turbulence. We show that these diffusion coefficients allow to account for X-ray observations of supernova remnants. This paper also calculates the modification of the Fermi cycle due to the energy lost by cosmic rays in generating upstream turbulence and the concomittant steepening of the energy spectrum. Finally we confirm that cosmic rays can produced an amplified turbulence in young SNr during their free expansion phase such that the maximal energy is close to the knee and the spectral index is close to 2.3 in the warm phase of the interstellar mediumComment: 13 pages, 4 figures, accepted for publication in Astronomy & Astrophysics main journa

    The AGASA/SUGAR Anisotropies and TeV Gamma Rays from the Galactic Center: A Possible Signature of Extremely High-energy Neutrons

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    Recent analysis of data sets from two extensive air shower cosmic ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays towards the Galactic Center (GC) that ``turns on'' around 101810^{18} eV. We demonstrate that the anisotropy could be due to neutrons created at the Galactic Center through charge-exchange in proton-proton collisions, where the incident, high energy protons obey an E2\sim E^{-2} power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746-2851 -- ascribed to pp induced neutral pion decay at GeV energies -- together with a very reasonable spectral index of 2.2, predicts a neutron flux at 1018\sim 10^{18} eV fully consistent with the extremely high energy cosmic ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air-Cerenkov telescope at \~TeV energies is almost equally-well compatible with the 1018\sim 10^{18} eV cosmic ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We consider the implications of this discrepancy. We discuss why the Galactic Center environment can allow diffusive shock acceleration at strong shocks up to energies approaching the ankle in the cosmic ray spectrum. Finally, we argue that the shock acceleration may be occuring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic Center. If this connection between the anisotropy and Sagittarius A East could be firmly established it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near the ankle.Comment: 57 pages, 2 figure

    An implementation plan for priorities in solar-system space physics

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    The scientific objectives and implementation plans and priorities of the Space Science Board in areas of solar physics, heliospheric physics, magnetospheric physics, upper atmosphere physics, solar-terrestrial coupling, and comparative planetary studies are discussed and recommended programs are summarized. Accomplishments of Skylab, Solar Maximum Mission, Nimbus-7, and 11 other programs are highlighted. Detailed mission plans in areas of solar and heliospheric physics, plasma physics, and upper atmospheric physics are also described

    Nonlinear Diffusive Shock Acceleration with Magnetic Field Amplification

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    We introduce a Monte Carlo model of nonlinear diffusive shock acceleration allowing for the generation of large-amplitude magnetic turbulence. The model is the first to include strong wave generation, efficient particle acceleration to relativistic energies in nonrelativistic shocks, and thermal particle injection in an internally self-consistent manner. We find that the upstream magnetic field can be amplified by large factors and show that this amplification depends strongly on the ambient Alfven Mach number. We also show that in the nonlinear model large increases in the magnetic field do not necessarily translate into a large increase in the maximum particle momentum a particular shock can produce, a consequence of high momentum particles diffusing in the shock precursor where the large amplified field converges to the low ambient value. To deal with the field growth rate in the regime of strong fluctuations, we extend to strong turbulence a parameterization that is consistent with the resonant quasi-linear growth rate in the weak turbulence limit. We believe our parameterization spans the maximum and minimum range of the fluctuation growth and, within these limits, we show that the nonlinear shock structure, acceleration efficiency, and thermal particle injection rates depend strongly on the yet to be determined details of wave growth in strongly turbulent fields. The most direct application of our results will be to estimate magnetic fields amplified by strong cosmic-ray modified shocks in supernova remnants.Comment: Accepted in ApJ July 2006, typos corrected in this versio

    The Modified Weighted Slab Technique: Models and Results

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    In an attempt to understand the source and propagation of galactic cosmic rays we have employed the Modified Weighted Slab technique along with recent values of the relevant cross sections to compute primary to secondary ratios including B/C and Sub-Fe/Fe for different galactic propagation models. The models that we have considered are the disk-halo diffusion model, the dynamical halo wind model, the turbulent diffusion model and a model with minimal reacceleration. The modified weighted slab technique will be briefly discussed and a more detailed description of the models will be given. We will also discuss the impact that the various models have on the problem of anisotropy at high energy and discuss what properties of a particular model bear on this issue.Comment: LaTeX - AASTEX format, Submitted to ApJ, 8 figures, 20 page

    On the Estimation of Solar Energetic Particle Injection Timing from Onset Times near Earth

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    We examine the accuracy of a common technique for estimating the start time of solar energetic particle injection based on a linear fit to the observed onset time versus 1/(particle velocity). This is based on a concept that the first arriving particles move directly along the magnetic field with no scattering. We check this by performing numerical simulations of the transport of solar protons between 2 and 2000 MeV from the Sun to the Earth, for several assumptions regarding interplanetary scattering and the duration of particle injection, and analyzing the results using the inverse velocity fit. We find that in most cases, the onset times align close to a straight line as a function of inverse velocity. Despite this, the estimated injection time can be in error by several minutes. Also, the estimated path length can deviate greatly from the actual path length along the interplanetary magnetic field. The major difference between the estimated and actual path lengths implies that the first arriving particles cannot be viewed as moving directly along the interplanetary magnetic field.Comment: 19 pages, 3 Postscript figures. Astrophys. J., in pres

    Propagation of cosmic-ray nucleons in the Galaxy

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    We describe a method for the numerical computation of the propagation of primary and secondary nucleons, primary electrons, and secondary positrons and electrons. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. The models are adjusted to agree with the observed cosmic-ray B/C and 10Be/9Be ratios. Models with diffusion and convection do not account well for the observed energy dependence of B/C, while models with reacceleration reproduce this easily. The height of the halo propagation region is determined, using recent 10Be/9Be measurements, as >4 kpc for diffusion/convection models and 4-12 kpc for reacceleration models. For convection models we set an upper limit on the velocity gradient of dV/dz < 7 km/s/kpc. The radial distribution of cosmic-ray sources required is broader than current estimates of the SNR distribution for all halo sizes. Full details of the numerical method used to solve the cosmic-ray propagation equation are given.Comment: 15 pages including 23 ps-figures and 3 tables, latex2e, uses emulateapj.sty (ver. of 11 May 1998, enclosed), apjfonts.sty, timesfonts.sty. To be published in ApJ 1998, v.509 (December 10 issue). More details can be found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.html Some references are correcte

    Transport of Cosmic Rays in Chaotic Magnetic Fields

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    The transport of charged particles in disorganised magnetic fields is an important issue which concerns the propagation of cosmic rays of all energies in a variety of astrophysical environments, such as the interplanetary, interstellar and even extra-galactic media, as well as the efficiency of Fermi acceleration processes. We have performed detailed numerical experiments using Monte-Carlo simulations of particle propagation in stochastic magnetic fields in order to measure the parallel and transverse spatial diffusion coefficients and the pitch angle scattering time as a function of rigidity and strength of the turbulent magnetic component. We confirm the extrapolation to high turbulence levels of the scaling predicted by the quasi-linear approximation for the scattering frequency and parallel diffusion coefficient at low rigidity. We show that the widely used Bohm diffusion coefficient does not provide a satisfactory approximation to diffusion even in the extreme case where the mean field vanishes. We find that diffusion also takes place for particles with Larmor radii larger than the coherence length of the turbulence. We argue that transverse diffusion is much more effective than predicted by the quasi-linear approximation, and appears compatible with chaotic magnetic diffusion of the field lines. We provide numerical estimates of the Kolmogorov length and magnetic line diffusion coefficient as a function of the level of turbulence. Finally we comment on applications of our results to astrophysical turbulence and the acceleration of high energy cosmic rays in supernovae remnants, in super-bubbles, and in jets and hot spots of powerful radio-galaxies.Comment: To be published in Physical Review D, 20 pages 9 figure
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