5,272 research outputs found
Dynamical locality of the nonminimally coupled scalar field and enlarged algebra of Wick polynomials
We discuss dynamical locality in two locally covariant quantum field
theories, the nonminimally coupled scalar field and the enlarged algebra of
Wick polynomials. We calculate the relative Cauchy evolution of the enlarged
algebra, before demonstrating that dynamical locality holds in the nonminimally
coupled scalar field theory. We also establish dynamical locality in the
enlarged algebra for the minimally coupled massive case and the conformally
coupled massive case.Comment: 39p
Radio Lobes of Pictor A: an X-ray spatially resolved Study
A new XMM observation has made possible a detailed study of both lobes of the
radio galaxy Pictor A. Their X-ray emission is of non thermal origin and due to
Inverse Compton scattering of the microwave background photons by relativistic
electrons in the lobes, as previously found. In both lobes, the equipartition
magnetic field (Beq) is bigger than the Inverse Compton value (Bic), calculated
from the radio and X-ray flux ratio. The Beq/Bic ratio never gets below 2, in
spite of the large number of reasonable assumptions tested to calculate Beq,
suggesting a lobe energetic dominated by particles. The X-ray data quality is
good enough to allow a spatially resolved analysis. Our study shows that Bic
varies through the lobes. It appears to increase behind the hot spots. On the
contrary, a rather uniform distribution of the particles is observed. As a
consequence, the radio flux density variation along the lobes appears to be
mainly driven by magnetic field changes.Comment: 15 pages, 3 figures, ApJ accepte
Unveiling radio halos in galaxy clusters in the LOFAR era
Giant radio halos are mega-parsec scale synchrotron sources detected in a
fraction of massive and merging galaxy clusters. Radio halos provide one of the
most important pieces of evidence for non-thermal components in large scale
structure. Statistics of their properties can be used to discriminate among
various models for their origin. Therefore, theoretical predictions of the
occurrence of radio halos are important as several new radio telescopes are
about to begin to survey the sky at low frequencies with unprecedented
sensitivity. In this paper we carry out Monte Carlo simulations to model the
formation and evolution of radio halos in a cosmological framework. We extend
previous works on the statistical properties of radio halos in the context of
the turbulent re-acceleration model. First we compute the fraction of galaxy
clusters that show radio halos and derive the luminosity function of radio
halos. Then, we derive differential and integrated number count distributions
of radio halos at low radio frequencies with the main goal to explore the
potential of the upcoming LOFAR surveys. By restricting to the case of clusters
at redshifts <0.6, we find that the planned LOFAR all sky survey at 120 MHz is
expected to detect about 350 giant radio halos. About half of these halos have
spectral indices larger than 1.9 and substantially brighten at lower
frequencies. If detected they will allow for a confirmation that turbulence
accelerates the emitting particles. We expect that also commissioning surveys,
such as MSSS, have the potential to detect about 60 radio halos in clusters of
the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will
allow us to constrain how the rate of formation of radio halos in these
clusters depends on cluster mass.Comment: 12 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Radio Halos From Simulations And Hadronic Models II: The Scaling Relations of Radio Halos
We use results from a constrained, cosmological MHD simulation of the Local
Universe to predict radio halos and their evolution for a volume limited set of
galaxy clusters and compare to current observations. The simulated magnetic
field inside the clusters is a result of turbulent amplification within them,
with the magnetic seed originating from star-burst driven, galactic outflows.
We evaluate three models, where we choose different normalizations for the
Cosmic Ray proton population within clusters. Similar to our previous analysis
of the Coma cluster (Donnert et al. 2010), the radial profile and the
morphological properties of observed radio halos can not be reproduced, even
with a radially increasing energy fraction within the cosmic ray proton
population. Scaling relations between X-ray luminosity and radio power can be
reproduced by all models, however all models fail in the prediction of clusters
with no radio emission. Also the evolutionary tracks of our largest clusters in
all models fail to reproduce the observed bi-modality in radio luminosity. This
provides additional evidence that the framework of hadronic, secondary models
is disfavored to reproduce the large scale diffuse radio emission of galaxy
clusters. We also provide predictions for the unavoidable emission of
-rays from the hadronic models for the full cluster set. None of such
secondary models is yet excluded by the observed limits in -ray
emission, emphasizing that large scale diffuse radio emission is a powerful
tool to constrain the amount of cosmic ray protons in galaxy clusters
A Novel Broadband Measurement Method for the Magnetoimpedance of Ribbons and Thin Films
A novel broad-band measurement method of the MI in thin films and ribbons is
presented. It is based on the automated measurement of the reflection
coefficient of a cell loaded with the sample. Illustrative results obtained
with a permalloy multilayer thin film are presented and discussed.Comment: Paper submitted to International Conference on Magnetism (Rome 2003
Structure and Turbulence in Simulated Galaxy Clusters and the Implications for the Formation of Radio Halos
We track the histories of massive clusters of galaxies formed within a
cosmological hydrodynamic simulation. Specifically, we track the time evolution
of the energy in random bulk motions of the intracluster medium and X-ray
measures of cluster structure and their relationship to cluster mergers. We aim
to assess the viability of the turbulent re-acceleration model for the
generation of giant radio halos by comparing the level of turbulent kinetic
energy in simulated clusters with the observed properties of radio halo
clusters, giving particular attention to the association of radio halos to
clusters with disturbedX-ray structures. The evolution of X-ray cluster
structure and turbulence kinetic energy, k, in simulations can then inform us
about the expected lifetime of radio halos and the fraction of clusters as a
function of redshift expected to host them. We find strong statistical
correlation of disturbed structure measures and the presence of enhancements in
k. Specifically, quantitatively "disturbed", radio halo-like X-ray morphology
in our sample indicates a 92% chance of the cluster in question having k
elevated to more than twice its minimum value over the cluster's life. The
typical lifetime of episodes of elevated turbulence is on the order of 1 Gyr,
though these periods can last 5 Gyrs or more. This variation reflects the wide
range of cluster histories; while some clusters undergo complex and repeated
mergers spending a majority of their time in elevated k states, other clusters
are relaxed over nearly their entire history. We do not find a bimodal
relationship between cluster X-ray luminosity and the total energy in
turbulence that might account directly for a bimodal L_X-P_{1.4 GHz} relation.
However, our result may be consistent with the observed bimodality, as here we
are not including a full treatment of cosmic rays sources and magnetic fields.Comment: 15 pages, 12 figures, MNRAS Submitte
Is the Sunyaev-Zeldovich effect responsible for the observed steepening in the spectrum of the Coma radio halo ?
The spectrum of the radio halo in the Coma cluster is measured over almost
two decades in frequency. The current radio data show a steepening of the
spectrum at higher frequencies, which has implications for models of the radio
halo origin. There is an on-going debate on the possibility that the observed
steepening is not intrinsic to the emitted radiation, but is instead caused by
the SZ effect. Recently, the Planck satellite measured the SZ signal and its
spatial distribution in the Coma cluster allowing to test this hypothesis.
Using the Planck results, we calculated the modification of the radio halo
spectrum by the SZ effect in three different ways. With the first two methods
we measured the SZ-decrement within the aperture radii used for flux
measurements of the halo at the different frequencies. First we adopted the
global compilation of data from Thierbach et al. and a reference aperture
radius consistent with those used by the various authors. Second we used the
available brightness profiles of the halo at different frequencies to derive
the spectrum within two fixed apertures, and derived the SZ-decrement using
these apertures. As a third method we used the quasi-linear correlation between
the y and the radio-halo brightness at 330 MHz discovered by Planck to derive
the modification of the radio spectrum by the SZ-decrement in a way that is
almost independent of the adopted aperture radius. We found that the spectral
modification induced by the SZ-decrement is 4-5 times smaller than that
necessary to explain the observed steepening. Consequently a break or cut-off
in the spectrum of the emitting electrons is necessary to explain current data.
We also show that, if a steepening is absent from the emitted spectrum, future
deep observations at 5 GHz with single dishes are expected to measure a halo
flux in a 40 arcmin radius that would be 7-8 times higher than currently seen.Comment: 8 pages, 6 figures, accepted in Astronomy and Astrophysics (date of
acceptance 19/08/2013
A high resolution view of the jet termination shock in a hot spot of the nearby radio galaxy Pictor A: implications for X-ray models of radio galaxy hot spots
Images made with the VLBA have resolved the region in a nearby radio galaxy,
Pictor A, where the relativistic jet that originates at the nucleus terminates
in an interaction with the intergalactic medium, a so-called radio galaxy hot
spot. This image provides the highest spatial resolution view of such an object
to date (16 pc), more than three times better than previous VLBI observations
of similar objects. The north-west Pictor A hot spot is resolved into a complex
set of compact components, seen to coincide with the bright part of the hot
spot imaged at arcsecond-scale resolution with the VLA. In addition to a
comparison with VLA data, we compare our VLBA results with data from the HST
and Chandra telescopes, as well as new Spitzer data. The presence of pc-scale
components in the hot spot, identifying regions containing strong shocks in the
fluid flow, leads us to explore the suggestion that they represent sites of
synchrotron X-ray production, contributing to the integrated X-ray flux of the
hot spot, along with X-rays from synchrotron self-Compton scattering. This
scenario provides a natural explanation for the radio morphology of the hot
spot and its integrated X-ray emission, leading to very different predictions
for the higher energy X-ray spectrum compared to previous studies. From the
sizes of the individual pc-scale components and their angular spread, we
estimate that the jet width at the hot spot is in the range 70 - 700 pc, which
is comparable to similar estimates in PKS 2153-69, 3C 205, and 4C 41.17. The
lower limit in this range arises from the suggestion that the jet may dither in
its direction as it passes through hot spot backflow material close to the jet
termination point, creating a "dentist drill" effect on the inside of a cavity
700 pc in diameter.Comment: Accepted by the Astronomical Journal. 35 pages, 6 figure
Endomorphism Semigroups and Lightlike Translations
Certain criteria are demonstrated for a spatial derivation of a von Neumann
algebra to generate a one-parameter semigroup of endomorphisms of that algebra.
These are then used to establish a converse to recent results of Borchers and
of Wiesbrock on certain one-parameter semigroups of endomorphisms of von
Neumann algebras (specifically, Type III_1 factors) that appear as lightlike
translations in the theory of algebras of local observables.Comment: 9 pages, Late
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