278 research outputs found
On the Absorption of High Energy Gamma-Rays by Intergalactic Infrared Radiation
We present a new calculation of the intergalactic -ray
pair-production absorption coefficient as a function of both energy and
redshift up to the redshift of 3C279, z = 0.54. In reexamining this problem, we
make use of new observational data on the intergalactic infrared radiation
field (IIRF), together with recent theoretical models of the galactic spectral
energy distributions of the IIRF from stars and dust reradiation and estimates
of the IIRF from galaxy counts and {\it COBE} results. We present our results
for two fairly well defined IIRF spectral energy distributions, one of which is
within of our previous estimate of the IIRF at m.
We then apply our results to the -ray spectrum of Mrk 421, and obtain
good agreement with the observational data, including the recent results of the
{\it HEGRA} group.Comment: Plain TeX file with text in abs96.tex, 3 postscript figures (Fig. 1 -
ircobe.eps, Fig. 2 - taupl.eps, Fig. 3 - mki.eps), stylefile epsf.sty
included, accepted for publication in the Astrophysical Journa
Gamma rays and neutrinos from the Crab Nebula produced by pulsar accelerated nuclei
We investigate the consequences of the acceleration of heavy nuclei (e.g.
iron nuclei) by the Crab pulsar. Accelerated nuclei can photodisintegrate in
collisions with soft photons produced in the pulsar's outer gap, injecting
energetic neutrons which decay either inside or outside the Crab Nebula. The
protons from neutron decay inside the nebula are trapped by the Crab Nebula
magnetic field, and accumulate inside the nebula producing gamma-rays and
neutrinos in collisions with the matter in the nebula. Neutrons decaying
outside the Crab Nebula contribute to the Galactic cosmic rays. We compute the
expected fluxes of gamma-rays and neutrinos, and find that our model could
account for the observed emission at high energies and may be tested by
searching for high energy neutrinos with future neutrino telescopes currently
in the design stage.Comment: 8 pages, 4 figures, LaTeX uses revtex.sty, submitted to Phys. Rev.
Let
Selective laser sintering of hydroxyapatite reinforced polyethylene composites for bioactive implants and tissue scaffold development
Selective laser sintering (SLS) has been investigated for the production of bioactive implants and tissue scaffolds using composites of high-density polyethylene (HDPE) reinforced with hydroxyapatite (HA) with the aim of achieving the rapid manufacturing of customized implants. Single-layer and multilayer block specimens made of HA-HDPE composites with 30 and 40 vol % HA were sintered successfully using a CO2 laser sintering system. Laser power and scanning speed had a significant effect on the sintering behaviour. The degree of particle fusion and porosity were influenced by the laser processing parameters, hence control can be attained by varying these parameters. Moreover, the SLS processing allowed exposure of HA particles on the surface of the composites and thereby should provide bioactive products. Pores existed in the SLS-fabricated composite parts and at certain processing parameters a significant fraction of the pores were within the optimal sizes for tissue regeneration. The results indicate that the SLS technique has the potential not only to fabricate HA-HDPE composite products but also to produce appropriate features for their application as bioactive implants and tissue scaffolds
The CAT Imaging Telescope for Very-High-Energy Gamma-Ray Astronomy
The CAT (Cherenkov Array at Themis) imaging telescope, equipped with a
very-high-definition camera (546 fast phototubes with 0.12 degrees spacing
surrounded by 54 larger tubes in two guard rings) started operation in Autumn
1996 on the site of the former solar plant Themis (France). Using the
atmospheric Cherenkov technique, it detects and identifies very high energy
gamma-rays in the range 250 GeV to a few tens of TeV. The instrument, which has
detected three sources (Crab nebula, Mrk 421 and Mrk 501), is described in
detail.Comment: 24 pages, 15 figures. submitted to Elsevier Preprin
Observations of TeV photons at the Whipple Observatory
The Whipple Observatory 10 m gammaâray telescope has been used to search for TeV gammaâray emission from a number of objects. This paper reports observations of six galactic and three extragalactic objects using the Cherenkov image technique. With the introduction of a highâresolution camera (1/4° pixel) in 1988, the Crab Nebula was detected at a significance level of 20 Ï in 30 hours of onâsource observation. Upper limits at a fraction of the Crab flux are set for most of the other objects, based on the absence of any significant dc excess or periodic effect when an a priori Monte Carlo determined imaging selection criterion (the ââazwidth cutââ) is employed. There are weak indications that one source, Hercules Xâ1, may be an episodic emitter. The Whipple detection system will be improved shortly with the addition of a second reflector 11 m in diameter (GRANITE) for stereoscopic viewing of showers. The combination of the twoâreflector system should have a signalâtoânoise advantage of 103 over a simple nonimaging Cherenkov receiver.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87437/2/47_1.pd
BaFe12O19 single-particle-chain nanofibers : preparation, characterization, formation principle, and magnetization reversal mechanism
BaFe12O19 single-particle-chain
nanofibers have been successfully prepared by
an electrospinning method and calcination
process, and their morphology, chemistry,
and crystal structure have been characterized
at the nanoscale. It is found that individual
BaFe12O19 nanofibers consist of single nanoparticles which are found to stack along the
nanofiber axis. The chemical analysis shows that the atomic ratio of Ba/Fe is 1:12, suggesting a
BaFe12O19 composition. The crystal structure of the BaFe12O19 single-particle-chain nanofibers
is proved to be M-type hexagonal. The single crystallites on each BaFe12O19 single-particlechain
nanofibers have random orientations. A formation mechanism is proposed based on
thermogravimetry/differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and transmission
electron microscopy (TEM) at six temperatures, 250, 400, 500, 600, 650, and 800 ïżœC.
The magnetic measurement of the BaFe12O19 single-particle-chain nanofibers reveals that the
coercivity reaches a maximum of 5943 Oe and the saturated magnetization is 71.5 emu/g at
room temperature. Theoretical analysis at the micromagnetism level is adapted to describe the
magnetic behavior of the BaFe12O19 single-particle-chain nanofibers
BeppoSAX Observations of Unprecedented Synchrotron Activity in the BL Lac Object Mkn 501
The BL Lac object Mkn 501, one of the only three extragalactic sources (with
Mkn 421 and 1ES 2344+514) so far detected at TeV energies, was observed with
the BeppoSAX satellite on 7, 11, and 16 April 1997 during a phase of high
activity at TeV energies, as monitored with the Whipple, HEGRA and CAT
Cherenkov telescopes. Over the whole 0.1-200 keV range the spectrum was
exceptionally hard (alpha =< 1, with F_nu ~ nu^{-alpha}) indicating that the
X-ray power output peaked at (or above) ~100 keV. This represents a shift of at
least two orders of magnitude with respect to previous observations of Mkn 501,
a behavior never seen before in this or any other blazar. The overall X-ray
spectrum hardens with increasing intensity and, at each epoch, it is softer at
larger energies. The correlated variability from soft X-rays to the TeV band
points to models in which the same population of relativistic electrons
produces the X-ray continuum via synchrotron radiation and the TeV emission by
inverse Compton scattering of the synchrotron photons or other seed photons.
For the first time in any blazar the synchrotron power is observed to peak at
hard X-ray energies. The large shift of the synchrotron peak frequency with
respect to previous observations of Mkn 501 implies that intrinsic changes in
the relativistic electron spectrum caused the increase in emitted power. Due to
the very high electron energies, the inverse Compton process is limited by the
Klein-Nishina regime. This implies a quasi-linear (as opposed to quadratic)
relation of the variability amplitude in the TeV and hard X-ray ranges (for the
SSC model) and an increase of the inverse Compton peak frequency smaller than
that of the synchrotron peak frequency.Comment: 11 pages, Latex, 4 Postscript figures, to appear in The Astrophysical
Journal Letter
Control of intestinal stem cell function and proliferation by mitochondrial pyruvate metabolism.
Most differentiated cells convert glucose to pyruvate in the cytosol through glycolysis, followed by pyruvate oxidation in the mitochondria. These processes are linked by the mitochondrial pyruvate carrier (MPC), which is required for efficient mitochondrial pyruvate uptake. In contrast, proliferative cells, including many cancer and stem cells, perform glycolysis robustly but limit fractional mitochondrial pyruvate oxidation. We sought to understand the role this transition from glycolysis to pyruvate oxidation plays in stem cell maintenance and differentiation. Loss of the MPC in Lgr5-EGFP-positive stem cells, or treatment of intestinal organoids with an MPC inhibitor, increases proliferation and expands the stem cell compartment. Similarly, genetic deletion of the MPC in Drosophila intestinal stem cells also increases proliferation, whereas MPC overexpression suppresses stem cell proliferation. These data demonstrate that limiting mitochondrial pyruvate metabolism is necessary and sufficient to maintain the proliferation of intestinal stem cells
Geometric measure of quantum discord and total quantum correlations in a N-partite quantum state
Quantum discord, as introduced by Olliver and Zurek [Phys. Rev. Lett.
\textbf{88}, 017901 (2001)], is a measure of the discrepancy between quantum
versions of two classically equivalent expressions for mutual information and
is found to be useful in quantification and application of quantum correlations
in mixed states. It is viewed as a key resource present in certain quantum
communication tasks and quantum computational models without containing much
entanglement. An early step toward the quantification of quantum discord in a
quantum state was by Dakic, Vedral, and Brukner [Phys. Rev. Lett. 105,190502
(2010)] who introduced a geometric measure of quantum discord and derived an
explicit formula for any two-qubit state. Recently, Luo and Fu [Phys. Rev. A
\textbf{82}, 034302 (2010)] introduced a generic form of the geometric measure
of quantum discord for a bipartite quantum state. We extend these results and
find generic forms of the geometric measure of quantum discord and total
quantum correlations in a general N-partite quantum state. Further, we obtain
computable exact formulas for the geometric measure of quantum discord and
total quantum correlations in a N-qubit quantum state. The exact formulas for
the -qubit quantum state are experimentally implementable.Comment: 18 pages, 3 figure
The First VERITAS Telescope
The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic
Radiation Imaging Telescope Array System) has been in operation since February
2005. We present here a technical description of the instrument and a summary
of its performance. The calibration methods are described, along with the
results of Monte Carlo simulations of the telescope and comparisons between
real and simulated data. The analysis of TeV -ray observations of the
Crab Nebula, including the reconstructed energy spectrum, is shown to give
results consistent with earlier measurements. The telescope is operating as
expected and has met or exceeded all design specifications.Comment: Accepted by Astroparticle Physic
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