2,545 research outputs found
Suzaku X-Ray Imaging and Spectroscopy of Cassiopeia A
Suzaku X-ray observations of a young supernova remnant, Cassiopeia A, were
carried out. K-shell transition lines from highly ionized ions of various
elements were detected, including Chromium (Cr-Kalpha at 5.61 keV). The X-ray
continuum spectra were modeled in the 3.4--40 keV band, summed over the entire
remnant, and were fitted with a simplest combination of the thermal
bremsstrahlung and the non-thermal cut-off power-law models. The spectral fits
with this assumption indicate that the continuum emission is likely to be
dominated by the non-thermal emission with a cut-off energy at > 1 keV. The
thermal-to-nonthermal fraction of the continuum flux in the 4-10 keV band is
best estimated as ~0.1. Non-thermal-dominated continuum images in the 4--14 keV
band were made. The peak of the non-thermal X-rays appears at the western part.
The peak position of the TeV gamma-rays measured with HEGRA and MAGIC is also
shifted at the western part with the 1-sigma confidence. Since the location of
the X-ray continuum emission was known to be presumably identified with the
reverse shock region, the possible keV-TeV correlations give a hint that the
accelerated multi-TeV hadrons in Cassiopeia A are dominated by heavy elements
in the reverse shock region.Comment: Publ. Astron. Soc. Japan 61, pp.1217-1228 (2009
Staggered Fermions and Gauge Field Topology
Based on a large number of smearing steps, we classify SU(3) gauge field
configurations in different topological sectors. For each sector we compare the
exact analytical predictions for the microscopic Dirac operator spectrum of
quenched staggered fermions. In all sectors we find perfect agreement with the
predictions for the sector of topological charge zero, showing explicitly that
the smallest Dirac operator eigenvalues of staggered fermions at presently
realistic lattice couplings are insensitive to gauge field topology. On the
smeared configurations, eigenvalues clearly separate out from the rest
on configurations of topological charge , and move towards zero in
agreement with the index theorem.Comment: LaTeX, 10 page
The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars
Aims: Projected rotational velocities (\vsini) have been estimated for 334
targets in the VLT-FLAMES Tarantula survey that do not manifest significant
radial velocity variations and are not supergiants. They have spectral types
from approximately O9.5 to B3. The estimates have been analysed to infer the
underlying rotational velocity distribution, which is critical for
understanding the evolution of massive stars.
Methods: Projected rotational velocities were deduced from the Fourier
transforms of spectral lines, with upper limits also being obtained from
profile fitting. For the narrower lined stars, metal and non-diffuse helium
lines were adopted, and for the broader lined stars, both non-diffuse and
diffuse helium lines; the estimates obtained using the different sets of lines
are in good agreement. The uncertainty in the mean estimates is typically 4%
for most targets. The iterative deconvolution procedure of Lucy has been used
to deduce the probability density distribution of the rotational velocities.
Results: Projected rotational velocities range up to approximately 450 \kms
and show a bi-modal structure. This is also present in the inferred rotational
velocity distribution with 25% of the sample having \ve100\,\kms
and the high velocity component having \ve\,\kms. There is no
evidence from the spatial and radial velocity distributions of the two
components that they represent either field and cluster populations or
different episodes of star formation. Be-type stars have also been identified.
Conclusions: The bi-modal rotational velocity distribution in our sample
resembles that found for late-B and early-A type stars. While magnetic braking
appears to be a possible mechanism for producing the low-velocity component, we
can not rule out alternative explanations.Comment: to be publisged in A&
Modified Dark Matter in Galaxies and Galaxy Clusters
Modified Dark Matter (MDM) is a phenomenological model of dark matter,
inspired by gravitational thermodynamics, that naturally accounts for the
universal acceleration constant observed in galactic rotation curve data; a
critical acceleration related to the cosmological constant, , appears
as a phenomenological manifestation of MDM. We show that the resulting mass
profiles, which are sensitve to , are consistent with observations at
the galactic and galaxy cluster scales. Our results suggest that dark matter
mass profiles contain information about the cosmological constant in a
non-trivial way.Comment: To be published in the Proceedings of the Bahamas Advanced Study
Institute and Conferences (BASIC
Small eigenvalues of the SU(3) Dirac operator on the lattice and in Random Matrix Theory
We have calculated complete spectra of the staggered Dirac operator on the
lattice in quenched SU(3) gauge theory for \beta = 5.4 and various lattice
sizes. The microscopic spectral density, the distribution of the smallest
eigenvalue, and the two-point spectral correlation function are analyzed. We
find the expected agreement of the lattice data with universal predictions of
the chiral unitary ensemble of random matrix theory up to a certain energy
scale, the Thouless energy. The deviations from the universal predictions are
determined using the disconnected scalar susceptibility. We find that the
Thouless energy scales with the lattice size as expected from theoretical
arguments making use of the Gell-Mann--Oakes--Renner relation.Comment: REVTeX, 5 pages, 4 figure
Domain wall fermion zero modes on classical topological backgrounds
The domain wall approach to lattice fermions employs an additional dimension,
in which gauge fields are merely replicated, to separate the chiral components
of a Dirac fermion. It is known that in the limit of infinite separation in
this new dimension, domain wall fermions have exact zero modes, even for gauge
fields which are not smooth. We explore the effects of finite extent in the
fifth dimension on the zero modes for both smooth and non-smooth topological
configurations and find that a fifth dimension of around ten sites is
sufficient to clearly show zero mode effects. This small value for the extent
of the fifth dimension indicates the practical utility of this technique for
numerical simulations of QCD.Comment: Updated fig. 3-7, small changes in sect. 3, added fig. 8, added more
reference
Chiral Condensate in the Deconfined Phase of Quenched Gauge Theories
We compute the low lying spectrum of the overlap Dirac operator in the
deconfined phase of finite-temperature quenched gauge theory. It suggests the
existence of a chiral condensate which we confirm with a direct stochastic
estimate. We show that the part of the spectrum responsible for the chiral
condensate can be understood as arising from a dilute gas of instantons and
anti-instantons.Comment: Revtex, 16 pages, 3 postscript figure
Harnessing nuclear spin polarization fluctuations in a semiconductor nanowire
Soon after the first measurements of nuclear magnetic resonance (NMR) in a
condensed matter system, Bloch predicted the presence of statistical
fluctuations proportional to in the polarization of an ensemble of
spins. First observed by Sleator et al., so-called "spin noise" has
recently emerged as a critical ingredient in nanometer-scale magnetic resonance
imaging (nanoMRI). This prominence is a direct result of MRI resolution
improving to better than 100 nm^3, a size-scale in which statistical spin
fluctuations begin to dominate the polarization dynamics. We demonstrate a
technique that creates spin order in nanometer-scale ensembles of nuclear spins
by harnessing these fluctuations to produce polarizations both larger and
narrower than the natural thermal distribution. We focus on ensembles
containing ~10^6 phosphorus and hydrogen spins associated with single InP and
GaP nanowires (NWs) and their hydrogen-containing adsorbate layers. We monitor,
control, and capture fluctuations in the ensemble's spin polarization in
real-time and store them for extended periods. This selective capture of large
polarization fluctuations may provide a route for enhancing the weak magnetic
signals produced by nanometer-scale volumes of nuclear spins. The scheme may
also prove useful for initializing the nuclear hyperfine field of electron spin
qubits in the solid-state.Comment: 18 pages, 5 figure
Pathologies of Quenched Lattice QCD at non--zero Density and its Effective Potential
We simulate lattice QCD at non--zero baryon density and zero temperature in
the quenched approximation, both in the scaling region and in the infinite
coupling limit. We investigate the nature of the forbidden region -- the range
of chemical potential where the simulations grow prohibitively expensive, and
the results, when available, are puzzling if not unphysical. At weak coupling
we have explored the sensitivity of these pathologies to the lattice size, and
found that using a large lattice () does not remove them. The
effective potential sheds considerable light on the problems in the
simulations, and gives a clear interpretation of the forbidden region. The
strong coupling simulations were particularly illuminating on this point.Comment: 49 pages, uu-encoded expanding to postscript;also available at
ftp://hlrz36.hlrz.kfa-juelich.de/pub/mpl/hlrz72_95.p
Quenched Approximation Artifacts: A study in 2-dimensional QED
The spectral properties of the Wilson-Dirac operator in 2-dimensional QED
responsible for the appearance of exceptional configurations in quenched
simulations are studied in detail. The mass singularity structure of the
quenched functional integral is shown to be extremely compicated, with multiple
branch points and cuts. The connection of lattice topological charge and
exactly real eigenmodes is explored using cooling techniques. The lattice
volume and spacing dependence of these modes is studied, as is the effect of
clover improvement of the action. A recently proposed modified quenched
approximation is applied to the study of meson correlators, and the results
compared with both naive quenched and full dynamical calculations of the same
quantity.Comment: 34 pages (Latex) plus 9 embedded figures; title change
- …