441 research outputs found
Emergence and prevalence of/Mactamase producing Haemophilus influenzae in Finland and susceptibility of 102 respiratory isolates to eight antibiotics
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
Turbulence and particle acceleration in collisionless supernovae remnant shocks: II- Cosmic-ray transport
Supernovae remnant shock waves could be at the origin of cosmic rays up to
energies in excess of the knee (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
(with and 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 ,
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
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 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 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 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 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
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
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
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
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
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
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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