92 research outputs found
Particle Transport in intense small scale magnetic turbulence with a mean field
Various astrophysical studies have motivated the investigation of the
transport of high energy particles in magnetic turbulence, either in the source
or en route to the observation sites. For strong turbulence and large rigidity,
the pitch-angle scattering rate is governed by a simple law involving a mean
free path that increases proportionally to the square of the particle energy.
In this paper, we show that perpendicular diffusion deviates from this behavior
in the presence of a mean field. We propose an exact theoretical derivation of
the diffusion coefficients and show that a mean field significantly changes the
transverse diffusion even in the presence of a stronger turbulent field. In
particular, the transverse diffusion coefficient is shown to reach a finite
value at large rigidity instead of increasing proportionally to the square of
the particle energy. Our theoretical derivation is corroborated by a dedicated
Monte Carlo simulation. We briefly discuss several possible applications in
astrophysics.Comment: (9 pages, 6 figures, revised version with minor changes
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
Super Star Clusters in SBS0335-052E
As one of the lowest metallicity star forming galaxies, with a nucleus of
several super star clusters, SBS0335-052E is the subject of substantial current
study. We present new insights on this galaxy based on new and archival high
spatial resolution NICMOS and ACS images. We provide new measurements and
limits on the size of several of the SSCs. The images have sufficient
resolution to divide the star formation into compact regions and newly
discovered extended regions, indicating a bi-modal form of star formation. The
star formation regions are dated via the equivalent width of the Pa alpha
emission and we find that two of the extended regions of star formation are
less than 10 million years old. Our previous finding that stellar winds confine
the photo-ionizing flux to small regions around individual stars is consistent
with the new observations. This may allow planet formation in what would
traditionally be considered a harsh environment and has implications for the
number of planets around globular cluster stars. In addition the images
pinpoint the regions of H2 emission as located in, but not at the center of the
two star forming super star clusters, S1 and S2.Comment: Accepted by the Astrophysical Journa
An objective, markerless videosystem for staging facial palsy
Background: The clinical classification of unilateral peripheral facial palsy (UPFP) is still based on subjective conventional methodology, leaving several missing points for an exact depiction of face deformity.
Aim of the study: To propose a new objective, video recording method that relies on mathematical algorithms allowing the software to recognize numerical points on the face surface that would be indicative of facial nerve impairment, without positioning of markers on the face.
Material and methods: Patients with UPFP of different House-Brackman (HB) degrees, from II to V, have been evaluated after video recording during two selected facial movements (forehead frowning and smiling) by a software trained to recognize the face points as numbers. Numerical parameters in millimeters have been obtained as indicative values of the shifting of the face points, and the shifting ratio between the healthy (denominator) and the affected side (numerator), i.e. the asymmetry index for the two movements taken into consideration.
Results: For each HB grade, specific ranges of asymmetry index have been identified with a positive correlation for shift differences and negative correlation for asymmetry indexes.
Conclusions: The use of the present objective system enabled the identification of numerical ranges of asymmetry index between the healthy and the affected side, that were found to be consistent with the outcome from the subjective methods currently in use
Molecular and clinical studies in five index cases with novel mutations in the GLA gene
Fabry disease is a metabolic and lysosomal storage disorder caused by the functional defect of the α-galactosidase A enzyme; this defect is due to mutations in the GLA gene, that is composed of seven exons and is located on the long arm of the X-chromosome (Xq21–22).
The enzymatic deficit is responsible for the accumulation of glycosphingolipids in lysosomes of different cellular types, mainly in those ones of vascular endothelium. It consequently causes a cellular and microvascular dysfunction.
In this paper, we described five novel mutations in the GLA gene, related to absent enzymatic activity and typical manifestations of Fabry disease. We identified three mutations (c.846_847delTC, p.E341X and p.C382X) that lead to the introduction of a stop codon in positions 297, 341 and 382. Moreover we found a missense mutation (p.R227P) in the exon 5 of the GLA gene and a single point mutation (c.639 + 5 G > T) occurring five base pairs beyond the end of the exon 4. These mutations have never been found in our group of healthy control subjects > 2300.
The studied patients presented some clinical manifestations, such as cornea verticillata, hypo-anhidrosis, left ventricular hypertrophy, cerebrovascular disorders and renal failure, that, considering the null enzymatic activity, suggest that the new mutations reported here are related to the classic form of Fabry disease.
The identification of novel mutations in patients with symptomatology referable to FD increases the molecular knowledge of the GLA gene and it gives clinicians an important support for the proper diagnosis of the disease
Gamma-Rays and the Far-Infrared-Radio Continuum Correlation Reveal a Powerful Galactic Centre Wind
We consider the thermal and non-thermal emission from the inner 200 pc of the
Galaxy. The radiation from this almost star-burst-like region is ultimately
driven dominantly by on-going massive star formation. We show that this
region's radio continuum (RC) emission is in relative deficit with respect to
the expectation afforded by the Far- infrared-Radio Continuum Correlation
(FRC). Likewise we show that the region's gamma-ray emission falls short of
that expected given its star formation and resultant supernova rates. These
facts are compellingly explained by positing that a powerful (400-1200 km/s)
wind is launched from the region. This wind probably plays a number of
important roles including advecting positrons into the Galactic bulge thus
explaining the observed ~kpc extension of the 511 keV positron annihilation
signal around the GC. We also show that the large-scale GC magnetic field falls
in the range ~100-300 microG and that - in the time they remain in the region -
GC cosmic rays do not penetrate into the region's densest molecular material.Comment: Version accepted for publication in MNRAS Letters. Discussion
extended and references adde
Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: II. Comparison with Observation, Clump Properties, and Scaling to Physical Units
Ambipolar diffusion is important in redistributing magnetic flux and in
damping Alfven waves in molecular clouds. The importance of ambipolar diffusion
on a length scale is governed by the ambipolar diffusion Reynolds
number, \rad=\ell/\lad, where \lad is the characteristic length scale for
ambipolar diffusion. The logarithmic mean of the AD Reynolds number in a sample
of 15 molecular clumps with measured magnetic fields (Crutcher 1999) is 17,
comparable to the theoretically expected value. We identify several regimes of
ambipolar diffusion in a turbulent medium, depending on the ratio of the flow
time to collision times between ions and neutrals; the clumps observed by
Crutcher (1999) are all in the standard regime of ambipolar diffusion, in which
the neutrals and ions are coupled over a flow time. We have carried out
two-fluid simulations of ambipolar diffusion in isothermal, turbulent boxes for
a range of values of \rad. The mean Mach numbers were fixed at \calm=3 and
\ma=0.67; self-gravity was not included. We study the properties of
overdensities--i.e., clumps--in the simulation and show that the slope of the
higher-mass portion of the clump mass spectrum increases as \rad decreases,
which is qualitatively consistent with Padoan et al. (2007)'s finding that the
mass spectrum in hydrodynamic turbulence is significantly steeper than in ideal
MHD turbulence. For a value of \rad similar to the observed value, we find a
slope that is consistent with that of the high-mass end of the Initial Mass
Function for stars. However, the value we find for the spectral index in our
ideal MHD simulation differs from theirs, presumably because our simulations
have different initial conditions. This suggests that the mass spectrum of the
clumps in the Padoan et al. (2007) turbulent fragmentation model for the IMF
depends on the environment, which would conflict with evidence ...Comment: 33 pages, 7 figure
Understanding hadronic gamma-ray emission from supernova remnants
We aim to test the plausibility of a theoretical framework in which the
gamma-ray emission detected from supernova remnants may be of hadronic origin,
i.e., due to the decay of neutral pions produced in nuclear collisions
involving relativistic nuclei. In particular, we investigate the effects
induced by magnetic field amplification on the expected particle spectra,
outlining a phenomenological scenario consistent with both the underlying
Physics and the larger and larger amount of observational data provided by the
present generation of gamma experiments, which seem to indicate rather steep
spectra for the accelerated particles. In addition, in order to study to study
how pre-supernova winds might affect the expected emission in this class of
sources, the time-dependent gamma-ray luminosity of a remnant with a massive
progenitor is worked out. Solid points and limitations of the proposed scenario
are finally discussed in a critical way.Comment: 30 pages, 5 figures; Several comments, references and a figure added.
Some typos correcte
Very high energy gamma rays from the direction of Sagittarius A*.
We report the detection of a point-like source of very high energy (VHE) -rays coincident within 1' of Sgr A *, obtained with the HESS array of Cherenkov telescopes. The -rays exhibit a power-law energy spectrum with a spectral index of and a flux above the 165 GeV threshold of m -2 s -1. The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration
Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: III. Implications for Observations and Turbulent Enhancement
Ambipolar diffusion (AD) is believed to be a crucial process for
redistributing magnetic flux in the dense molecular gas that occurs in regions
of star formation. We carry out numerical simulations of this process in
regions of low ionization using the heavy ion approximation. The simulations
are for regions of strong field (plasma \beta=0.1) and mildly supersonic
turbulence (M=3, corresponding to an Alfven mach number of 0.67). The velocity
power spectrum of the neutral gas changes from an Iroshnikov-Kraichnan spectrum
in the case of ideal MHD to a Burgers spectrum in the case of a shock-dominated
hydrodynamic system. The magnetic power spectrum shows a similar behavior. We
use a 1D radiative transfer code to post-process our simulation results; the
simulated emission from the CS J=2-1 and H13CO+ J=1-0 lines shows that the
effects of AD are observable in principle. Linewidths of ions are observed to
be less than those of neutrals, and we confirm previous suggestions that this
is due to AD. We show that AD is unlikely to affect the Chandrasekhar-Fermi
method for inferring field strengths unless the AD is stronger than generally
observed. Finally, we present the first fully 3D study of the enhancement of AD
by turbulence, finding that AD is accelerated by factor 2-4.5 for non
self-gravitating systems with the level of turbulence we consider.Comment: 22 pages, 8 figure
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