691 research outputs found
Brane Induced Gravity: From a No-Go to a No-Ghost Theorem
Numerous claims in the literature suggest that gravity induced on a higher
co-dimensional surface violates unitarity in the weak coupling regime. However,
it remained unclear, why a conserved source localized on this surface and
giving rise to an induced gravity term at low energies would absorb and emit
the associated ghost, given a consistent source-free theory. In this article it
is shown that the appearance of the induced Einstein Hilbert term does not
threaten the unitarity of the theory. The physics arguments behind this
statement are presented in a semi-covariant language, but the detailed proof is
given using Dirac's constraint analysis. It is shown that the would-be ghost
highlighted in previous works is non-dynamical and therefore not associated
with a state in the Hilbert space. As a result of these investigations, brane
induced gravity goes without a ghost, opening an exciting window of opportunity
for consistent deformations of gravity at the largest observable distances.Comment: 13 pages, v2: matches version published in Physical Review
Very-High Energy Gamma-Ray Flux Limits for Nearby Active Galactic Nuclei
Combining the results of targeted observations, H.E.S.S. has accumulated a
large amount of extra-galactic exposure at TeV energies. Due to its large field
of view a considerable part of the sky (0.6 sr) has been observed with high
sensitivity outside the targeted observation positions. Since this exposure
region contains little inherent bias, it is well suited for studies of
extra-galactic source populations. Given the correlation between ultra-high
energy cosmic rays and nearby extra-galactic objects recently claimed by the
Auger collaboration, this unbiased sky sample by H.E.S.S. is of interest since
it includes (besides the targeted sources) 63 AGN within 100 Mpc, for which
very-high energy gamma-ray flux limits are derived.Comment: 4 pages, 5 figures, Submitted to Proceedings of "4th International
Symposium on High Energy Gamma-Ray Astronomy 2008
Orientation-dependent indentation response of helium-implanted tungsten
A literature review of studies investigating the topography of nano-indents
in ion-implanted materials reveals seemingly inconsistent observations, with
report of both pile-up and sink-in. This may be due to the crystallographic
orientation of the measured sample point, which is often not considered when
evaluating implantation-induced changes in the deformation response. Here we
explore the orientation dependence of spherical nano-indentation in pure and
helium-implanted tungsten, considering grains with , and
out-of-plane orientations. Atomic force microscopy (AFM) of indents in
unimplanted tungsten shows little orientation dependence. However, in the
implanted material a much larger, more localised pile-up is observed for
grains than for and orientations. Based on the observations for
grains, we hypothesise that a large initial hardening due to
helium-induced defects is followed by localised defect removal and subsequent
strain softening. A crystal plasticity finite element model of the indentation
process, formulated based on this hypothesis, accurately reproduces the
experimentally-observed orientation-dependence of indent morphology. The
results suggest that the mechanism governing the interaction of helium-induced
defects with glide dislocations is orientation independent. Rather, differences
in pile-up morphology are due to the relative orientations of the crystal slip
systems, sample surface and spherical indenter. This highlights the importance
of accounting for crystallographic orientation when probing the deformation
behaviour of ion-implanted materials using nano-indentation
Hardening and Strain Localisation in Helium-Ion-Implanted Tungsten
Tungsten is the main candidate material for plasma-facing armour components
in future fusion reactors. In-service, fusion neutron irradiation creates
lattice defects through collision cascades. Helium, injected from plasma,
aggravates damage by increasing defect retention. Both can be mimicked using
helium-ion-implantation. In a recent study on 3000 appm helium-implanted
tungsten (W-3000He), we hypothesized helium-induced irradiation hardening,
followed by softening during deformation. The hypothesis was founded on
observations of large increase in hardness, substantial pile-up and slip-step
formation around nano-indents and Laue diffraction measurements of localised
deformation underlying indents. Here we test this hypothesis by implementing it
in a crystal plasticity finite element (CPFE) formulation, simulating
nano-indentation in W-3000He at 300 K. The model considers thermally-activated
dislocation glide through helium-defect obstacles, whose barrier strength is
derived as a function of defect concentration and morphology. Only one fitting
parameter is used for the simulated helium-implanted tungsten; defect removal
rate. The simulation captures the localised large pile-up remarkably well and
predicts confined fields of lattice distortions and geometrically necessary
dislocation underlying indents which agree quantitatively with previous Laue
measurements. Strain localisation is further confirmed through high resolution
electron backscatter diffraction and transmission electron microscopy
measurements on cross-section lift-outs from centre of nano-indents in
W-3000He
Implications of the VHE Gamma-Ray Detection of the Quasar 3C279
The MAGIC collaboration recently reported the detection of the quasar 3C279
at > 100 GeV gamma-ray energies. Here we present simultaneous optical (BVRI)
and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the
implications of the snap-shot spectral energy distribution for jet models of
blazars. A one-zone synchrotron-self-Compton origin of the entire SED,
including the VHE gamma-ray emission can be ruled out. The VHE emission could,
in principle, be interpreted as Compton upscattering of external radiation
(e.g., from the broad-line regions). However, such an interpretation would
require either an unusually low magnetic field of B ~ 0.03 G or an
unrealistically high Doppler factor of Gamma ~ 140. In addition, such a model
fails to reproduce the observed X-ray flux. This as well as the lack of
correlated variability in the optical with the VHE gamma-ray emission and the
substantial gamma-gamma opacity of the BLR radiation field to VHE gamma-rays
suggests a multi-zone model. In particular, an SSC model with an emission
region far outside the BLR reproduces the simultaneous X-ray -- VHE gamma-ray
spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing
the observed SED of 3C279 reasonably well. However, the hadronic model requires
a rather extreme jet power of L_j ~ 10^{49} erg s^{-1}, compared to a
requirement of L_j ~ 2 X 10^{47} erg s^{-1} for a multi-zone leptonic model.Comment: Accepted for pulication. Several clarifications and additions to the
manuscript to match the accepted versio
Multiwavelength Signatures of Cosmic Ray Acceleration by Young Supernova Remnants
An overview is given of multiwavelength observations of young supernova
remnants, with a focus on the observational signatures of efficient cosmic ray
acceleration. Some of the effects that may be attributed to efficient cosmic
ray acceleration are the radial magnetic fields in young supernova remnants,
magnetic field amplification as determined with X-ray imaging spectroscopy,
evidence for large post-shock compression factors, and low plasma temperatures,
as measured with high resolution optical/UV/X-ray spectroscopy. Special
emphasis is given to spectroscopy of post-shock plasma's, which offers an
opportunity to directly measure the post-shock temperature. In the presence of
efficient cosmic ray acceleration the post-shock temperatures are expected to
be lower than according to standard equations for a strong shock. For a number
of supernova remnants this seems indeed to be the case.Comment: Invited review, to appear in the proceedings of "4th Heidelberg
International Symposium on High Energy Gamma-Ray Astronomy 2008
Hadronic Production of Gamma Rays and Starburst Galaxies
The Milky Way has been estabished to emit gamma rays. These gamma rays are
presumably dominated by decays of neutral pions, although inverse Compton
scatterings and bremsstrahlung also contribute. It is plausible that other
galaxies can be diffuse sources of gamma rays in a similar manner. Starburst
galaxies are particularly interesting to study as they are expected to have
much higher cosmic-ray fluxes and interstellar matter densities. The neutral
pions are created in cosmic-ray interactions with interstellar matter.
Presented here is an overview of the recent work by Karlsson and co-workers on
proton-proton interactions and the resulting secondary particle inclusive cross
sections and angular distributions. This model can be used to calculated the
component of the gamma-ray yield and spectrum from a starburst
galaxy. The yield is expected to increase significantly (30% to 50%) and the
spectrum to be harder than the incident proton spectrum.Comment: 4 pages, 4 figures, submitted for the Proceedings of the 4th
Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, July
7-11, 2008, in Heidelberg, German
Galactic and Extragalactic Magnetic Fields
The strength of the total magnetic field in our Milky Way from radio Zeeman
and synchrotron measurements is about 6 muG near the Sun and several mG in
dense clouds, pulsar wind nebulae, and filaments near the Galactic Center.
Diffuse polarized radio emission and Faraday rotation of the polarized emission
from pulsars and background sources show many small-scale magnetic features,
but the overall field structure in our Galaxy is still under debate. -- Radio
synchrotron observations of nearby galaxies reveal dynamically important
magnetic fields of 10-30 muG total strength in the spiral arms. Fields with
random orientations are concentrated in spiral arms, while ordered fields
(observed in radio polarization) are strongest in interarm regions and follow
the orientation of the adjacent gas spiral arms. Faraday rotation of the
diffuse polarized radio emission from the disks of spiral galaxies sometimes
reveals large-scale patterns which are signatures of coherent fields generated
by dynamos, but in most galaxies the field structure is more complicated. --
Strong magnetic fields are also observed in radio halos around edge-on
galaxies, out to large distances from the plane. The synchrotron scaleheight of
radio halos allows to measure the mean outflow velocity of the cosmic-ray
electrons. The ordered halo fields mostly form an X-shaped pattern, but no
large-scale pattern is seen in the Faraday rotation data. Diffuse polarized
radio emission in the outer disks and halos is an excellent tracer of galaxy
interactions and ram pressure by the intergalactic medium. -- Intracluster gas
can also be significantly magnetized and highly polarized due to shocks or
cluster mergers.Comment: 14 pages, 14 figures. To be published in "High Energy Gamma-Ray
Astronomy", eds. F.A. Aharonian, W. Hofmann, and F.M. Rieger, AIP Conf. Proc.
Updated and added references 28/11/2008; typo corrected and references
updated 07/01/2009; typos corrected 12/01/200
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