4,673 research outputs found
Relativistic Conic Beams and Spatial Distribution of Gamma-Ray Bursts
We study the statistics of gamma-ray bursts, assuming that gamma-ray bursts
are cosmological and they are beamed in the form of a conical jet with a large
bulk Lorentz factor . In such a conic beam, the relativistic ejecta
may have a spatial variation in the bulk Lorentz factor and the density
distribution of gamma-ray emitting jet material. An apparent luminosity
function arises because the axis of the cone is randomly oriented with respect
to the observer's line of sight. The width and the shape of the luminosity
function are determined by the ratio of the beam opening angle of the conical
jet to the inverse of the bulk Lorentz factor, when the bulk Lorentz factor and
the jet material density is uniform on the photon emitting jet surface. We
calculate effects of spatial variation of the Lorentz factor and the spatial
density fluctuations within the cone on the luminosity function and the
statistics of gamma-ray bursts. In particular, we focus on the redshift
distribution of the observed gamma-ray bursts. The maximum distance to and the
average redshift of the gamma-ray bursts are strongly affected by the
beaming-induced luminosity function. The bursts with the angle-dependent
Lorentz factor which peaks at the center of the cone have substantially higher
average gamma-ray burst redshifts. When both the jet material density and the
Lorentz factor are inhomogeneous in the conical beam, the average redshift of
the bursts could be 5 times higher than that of the case in which relativistic
jet is completely homogeneous and structureless. Even the simplest models for
the gamma-ray burst jets and their apparent luminosity distributions have a
significant effect on the redshift distribution of the gamma-ray bursts.Comment: 15 pages, 4 figures, submitted to ApJ
The cost effectiveness of integrated care for people living with HIV including antiretroviral treatment in a primary health care centre in Bujumbura, Burundi
The incremental cost effectiveness of an integrated care package (i.e., medical care including antiretroviral therapy (ART) and other services such as psychological and social support) for people living with HIV/AIDS was calculated in a not-for-profit primary health care centre in Bujumbura run by Society of Women against AIDS-Burundi (SWAA-Burundi), an African non-governmental organisation (NGO). Results are expressed as cost-effectiveness ratio 2007, constant US per DALY averted. The package of care provided by SWAA-Burundi is therefore a very cost-effective intervention in comparison with other interventions against HIV/AIDS that include ART. It is however, less cost effective than other types of interventions against HIV/AIDS, such as preventive activities
Asymmetric Franck-Condon factors in suspended carbon nanotube quantum dots
Electronic states and vibrons in carbon nanotube quantum dots have in general
different location and size. As a consequence, the conventional
Anderson-Holstein model, coupling vibrons to the dot total charge only, may no
longer be appropriated in general. Here we explicitly address the role of the
spatial fluctuations of the electronic density, yielding space-dependent
Franck-Condon factors. We discuss the consequent marked effects on transport
which are compatible with recent measurements. This picture can be relevant for
tunneling experiments in generic nano-electromechanical systems.Comment: 4+ pages, 3 figures (2 color, 1 BW
South-West extension of the hard X-ray emission from the Coma cluster
We explore the morphology of hard (18-30 keV) X-ray emission from the Coma
cluster of galaxies. We analyze a deep (1.1 Ms) observation of the Coma cluster
with the ISGRI imager on board the \emph{INTEGRAL} satellite. We show that the
source extension in the North-East to South-West (SW) direction ()
significantly exceeds the size of the point spread function of ISGRI, and that
the centroid of the image of the source in the 18-30 keV band is displaced in
the SW direction compared to the centroid in the 1-10 keV band. To test the
nature of the SW extension we fit the data assuming different models of source
morphology. The best fit is achieved with a diffuse source of elliptical shape,
although an acceptable fit can be achieved assuming an additional point source
SW of the cluster core. In the case of an elliptical source, the direction of
extension of the source coincides with the direction toward the subcluster
falling onto the Coma cluster. If the SW excess is due to the presence of a
point source with a hard spectrum, we show that there is no obvious X-ray
counterpart for this additional source, and that the closest X-ray source is
the quasar EXO 1256+281, which is located from the centroid of the
excess. The observed morphology of the hard X-ray emission clarifies the nature
of the hard X-ray "excess" emission from the Coma cluster, which is due to the
presence of an extended hard X-ray source SW of the cluster core.Comment: 7pages, 10 figure
DLI-CVD of TiO2âCu antibacterial thin films: Growth and characterization
TiO2âCu nanocomposite films were grown by pulsed direct liquid injection chemical vapor deposition (DLICVD) on stainless steel, silicon and glass substrates with the goal to produce bactericidal surfaces. Copper bis (2,2,6,6-tetramethyl-3,5-heptadionate), Cu(TMHD)2, and titanium tetra-iso-propoxide, TTIP, were used as metalorganic precursors. Liquid solutions of these compounds in xylene were injected in a flash vaporization chamber connected to a cold wall MOCVD reactor. The deposition temperature was typically 683 K and the total pressure was 800 Pa. The copper content of the layers was controlled by the mole fraction of Cu (TMHD)2 which was adjusted by the injection parameters (injection frequency and concentration of the starting solution). The chemical, structural and physical characteristics of the films were investigated by XRD, XPS, FEG-SEM and TEM. Copper is incorporated as metal particles with a relatively large size distribution ranging from 20 to 400 nm (with a large majority in 20â100 nm) depending on the copper content of the films. The influence of the growth conditions on the structural features and the antibacterial properties of the thin films are reported and discussed
Relativistic Winds from Compact Gamma-ray Sources: I. Radiative Acceleration in the Klein-Nishina Regime
We consider the radiative acceleration to relativistic bulk velocities of a
cold, optically thin plasma which is exposed to an external source of
gamma-rays. The flow is driven by radiative momentum input to the gas, the
accelerating force being due to Compton scattering in the relativistic
Klein-Nishina limit. The bulk Lorentz factor of the plasma, Gamma, derived as a
function of distance from the radiating source, is compared with the
corresponding result in the Thomson limit. Depending on the geometry and
spectrum of the radiation field, we find that particles are accelerated to the
asymptotic Lorentz factor at infinity much more rapidly in the relativistic
regime; and the radiation drag is reduced as blueshifted, aberrated photons
experience a decreased relativistic cross section and scatter preferentially in
the forward direction. The random energy imparted to the plasma by gamma-rays
can be converted into bulk motion if the hot particles execute many Larmor
orbits before cooling. This `Compton afterburn' may be a supplementary source
of momentum if energetic leptons are injected by pair creation, but can be
neglected in the case of pure Klein-Nishina scattering. Compton drag by
side-scattered radiation is shown to be more important in limiting the bulk
Lorentz factor than the finite inertia of the accelerating medium. The
processes discussed here may be relevant to a variety of astrophysical
situations where luminous compact sources of hard X- and gamma-ray photons are
observed, including active galactic nuclei, galactic black hole candidates, and
gamma-ray bursts.Comment: LateX, 20 pages, 5 figures, revised version accepted for publication
in the Ap
Highlighting boundary condition effects for granular matter flows with numerical simulations
International audienceGranular matter flows naturally occur on small or large bodies due to gravity. Their simulation allows a better understanding of the dynamics of these bodies. However many numerical simulations operate with periodic boundary conditions for convenience , or with static grain boundaries that do not reproduce the rolling and friction effects expected at the interface. This work not only shows that boundary conditions have a long-range effect within the flow , but that dissipative effects induced by flat walls cannot be neglected compared to using static grain boundaries
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