7,097 research outputs found
Supernova-Remnant Origin of Cosmic Rays?
It is thought that Galactic cosmic ray (CR) nuclei are gradually accelerated
to high energies (up to ~300 TeV/nucleon, where 1TeV=10^12eV) in the expanding
shock-waves connected with the remnants of powerful supernova explosions.
However, this conjecture has eluded direct observational confirmation^1,2 since
it was first proposed in 1953 (ref. 3). Enomoto et al.^4 claim to have finally
found definitive evidence that corroborates this model, proposing that the
very-high-energy, TeV-range, gamma-rays from the supernova remnant (SNR) RX
J1713.7-3946 are due to the interactions of energetic nuclei in this region.
Here we argue that their claim is not supported by the existing multiwavelength
spectrum of this source. The search for the origin(s) of Galactic cosmic ray
nuclei may be closing in on the long-suspected supernova-remnant sources, but
it is not yet over.Comment: 4 pages, 1 Figur
Evidence for moving breathers in a layered crystal insulator at 300K
We report the ejection of atoms at a crystal surface caused by energetic
breathers which have travelled more than 10^7 unit cells in atomic chain
directions. The breathers were created by bombardment of a crystal face with
heavy ions. This effect was observed at 300K in the layered crystal muscovite,
which has linear chains of atoms for which the surrounding lattice has C_2
symmetry. The experimental techniques described could be used to study
breathers in other materials and configurations.Comment: 7 pages, 3 figure
Chandra Observations of the Gamma-ray Binary LSI+61303: Extended X-ray Structure?
We present a 50 ks observation of the gamma-ray binary LSI+61303 carried out
with the ACIS-I array aboard the Chandra X-ray Observatory. This is the highest
resolution X-ray observation of the source conducted so far. Possible evidence
of an extended structure at a distance between 5 and 12 arcsec towards the
North of LSI+61303 have been found at a significance level of 3.2 sigma. The
asymmetry of the extended emission excludes an interpretation in the context of
a dust-scattered halo, suggesting an intrinsic nature. On the other hand, while
the obtained source flux, of F_{0.3-10 keV}=7.1^{+1.8}_{-1.4} x 10^{-12}
ergs/cm^2/s, and hydrogen column density, N_{H}=0.70+/-0.06 x 10^{22} cm^{-2},
are compatible with previous results, the photon index Gamma=1.25+/-0.09 is the
hardest ever found. In light of these new results, we briefly discuss the
physics behind the X-ray emission, the location of the emitter, and the
possible origin of the extended emission ~0.1 pc away from LSI+61303.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter
Studying Flow Close to an Interface by Total Internal Reflection Fluorescence Cross Correlation Spectroscopy: Quantitative Data Analysis
Total Internal Reflection Fluorescence Cross Correlation Spectroscopy
(TIR-FCCS) has recently (S. Yordanov et al., Optics Express 17, 21149 (2009))
been established as an experimental method to probe hydrodynamic flows near
surfaces, on length scales of tens of nanometers. Its main advantage is that
fluorescence only occurs for tracer particles close to the surface, thus
resulting in high sensitivity. However, the measured correlation functions only
provide rather indirect information about the flow parameters of interest, such
as the shear rate and the slip length. In the present paper, we show how to
combine detailed and fairly realistic theoretical modeling of the phenomena by
Brownian Dynamics simulations with accurate measurements of the correlation
functions, in order to establish a quantitative method to retrieve the flow
properties from the experiments. Firstly, Brownian Dynamics is used to sample
highly accurate correlation functions for a fixed set of model parameters.
Secondly, these parameters are varied systematically by means of an
importance-sampling Monte Carlo procedure in order to fit the experiments. This
provides the optimum parameter values together with their statistical error
bars. The approach is well suited for massively parallel computers, which
allows us to do the data analysis within moderate computing times. The method
is applied to flow near a hydrophilic surface, where the slip length is
observed to be smaller than 10nm, and, within the limitations of the
experiments and the model, indistinguishable from zero.Comment: 18 pages, 12 figure
Pharmacology: The Pharmacodynamics of Nutrients and Nutrient Interactions in Biological Functions
Jet-Induced Nucleosynthesis in Misaligned Microquasars
The jet axes and the orbital planes of microquasar systems are usually
assumed to be approximately perpendicular, eventhough this is not currently an
observational requirement. On the contrary, in one of the few systems where the
relative orientations are well-constrained, V4641Sgr, the jet axis is known to
lie not more than ~36 degrees from the binary plane. Such a jet, lying close to
the binary plane, and traveling at a significant fraction of the speed of light
may periodically impact the secondary star initiating nuclear reactions on its
surface. The integrated yield of such nuclear reactions over the age of the
binary system (less the radiative mass loss) will detectably alter the
elemental abundances of the companion star. This scenario may explain the
anomalously high Li enhancements (roughly ~20-200 times the sun's photospheric
value; or, equivalently, 0.1-1 times the average solar system value) seen in
the companions of some black-hole X-ray binary systems. (Such enhancements are
puzzling since Li nuclei are exceedingly fragile - being easily destroyed in
the interiors of stars - and Li would be expected to be depleted rather than
enhanced there.) Gamma-ray line signatures of the proposed process could
include the 2.22 MeV neutron capture line as well as the 0.478 MeV 7Li*
de-excitation line, both of which may be discernable with the INTEGRAL
satellite if produced in an optically thin region during a large outburst. For
very energetic jets, a relatively narrow neutral pion gamma-decay signature at
67.5 MeV could also be measurable with the GLAST satellite. We argue that about
10-20% of all microquasar systems ought to be sufficiently misaligned as to be
undergoing the proposed jet-secondary impacts.Comment: ApJ, accepted. Includes referee's suggestions and some minor
clarifications over previous versio
Concentrating Membrane Proteins Using Asymmetric Traps and AC Electric Fields
Membrane proteins are key components of the plasma membrane and are responsible for control of chemical ionic gradients, metabolite and nutrient transfer, and signal transduction between the interior of cells and the external environment. Of the genes in the human genome, 30% code for membrane proteins (Krogh et al. J. Mol. Biol.2001, 305, 567). Furthermore, many FDA-approved drugs target such proteins (Overington et al. Nat. Rev. Drug Discovery2006, 5, 993). However, the structure-function relationships of these are notably sparse because of difficulties in their purification and handling outside of their membranous environment. Methods that permit the manipulation of membrane components while they are still in the membrane would find widespread application in separation, purification, and eventual structure-function determination of these species (Poo et al. Nature1977, 265, 602). Here we show that asymmetrically patterned supported lipid bilayers in combination with AC electric fields can lead to efficient manipulation of charged components. We demonstrate the concentration and trapping of such components through the use of a “nested trap” and show that this method is capable of yielding an approximately 30-fold increase in the average protein concentration. Upon removal of the field, the material remains trapped for several hours as a result of topographically restricted diffusion. Our results indicate that this method can be used for concentrating and trapping charged membrane components while they are still within their membranous environment. We anticipate that our approach could find widespread application in the manipulation and study of membrane proteins
- …