6,538 research outputs found
Fourier Analysis of MAC Polarization
A problem of the polar code construction for multiple access channels (MACs) is that they do not always achieve the whole capacity region. Although polar codes achieve the sum-capacity of symmetric MACs, polarization may induce a loss in the capacity region which prevents polar codes from achieving the whole capacity region. This paper provides a single letter necessary and sufficient condition which characterizes all the MACs that do not lose any part of their capacity region by polarization
Measurement of the Crab nebula polarization at 90 GHz as a calibrator for CMB experiments
CMB experiments aiming at a precise measurement of the CMB polarization, such
as the Planck satellite, need a strong polarized absolute calibrator on the sky
to accurately set the detectors polarization angle and the cross-polarization
leakage. As the most intense polarized source in the microwave sky at angular
scales of few arcminutes, the Crab nebula will be used for this purpose. Our
goal was to measure the Crab nebula polarization characteristics at 90 GHz with
unprecedented precision. The observations were carried out with the IRAM 30m
telescope employing the correlation polarimeter XPOL and using two orthogonally
polarized receivers. We processed the Stokes I, Q, and U maps from our
observations in order to compute the polarization angle and linear polarization
fraction. The first is almost constant in the region of maximum emission in
polarization with a mean value of alpha_Sky=152.1+/-0.3 deg in equatorial
coordinates, and the second is found to reach a maximum of Pi=30% for the most
polarized pixels. We find that a CMB experiment having a 5 arcmin circular beam
will see a mean polarization angle of alpha_Sky=149.9+/-0.2 deg and a mean
polarization fraction of Pi=8.8+/-0.2%.Comment: Accepted for publication in A&A, 9 pages, 4 figure
POKER: Estimating the power spectrum of diffuse emission with complex masks and at high angular resolution
We describe the implementation of an angular power spectrum estimator in the
flat sky approximation. POKER (P. Of k EstimatoR) is based on the MASTER
algorithm developped by Hivon and collaborators in the context of CMB
anisotropy. It works entirely in discrete space and can be applied to arbitrary
high angular resolution maps. It is therefore particularly suitable for current
and future infrared to sub-mm observations of diffuse emission, whether
Galactic or cosmological.Comment: Astronomy and Astrophysics, in pres
Model of the polarized foreground diffuse Galactic emissions from 33 to 353 GHz
We present 3D models of the Galactic magnetic field including regular and
turbulent components, and of the distribution of matter in the Galaxy including
relativistic electrons and dust grains. By integrating along the line of sight,
we construct maps of the polarized Galactic synchrotron and thermal dust
emissions for each of these models. We perform a likelihood analysis to compare
the maps of the Ka, Q, V and W bands of the Wilkinson Microwave Anisotropy
Probe (Wmap) and the 353 GHz Archeops data to the models obtained by varying
the pitch angle of the regular magnetic field, the relative amplitude of the
turbulent magnetic field and the extrapolation spectral indices of the
synchrotron and thermal dust emissions. The best-fit parameters obtained for
the different frequency bands are very similar and globally the data seem to
favor a negligible isotropic turbulent magnetic field component at large
angular scales (an anisotropic line-of-sight ordered component can not be
studied using these data). From this study, we conclude that we are able to
propose a consistent model of the polarized diffuse Galac- tic synchrotron and
thermal dust emissions in the frequency range from 33 to 353 GHz, where most of
the CMB studies are performed and where we expect a mixture of these two main
foreground emissions. This model can be very helpful to estimate the
contamination by foregrounds of the polarized CMB anisotropies, for experiments
like the Planck satellite.Comment: 22 pages, 4 figure
Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models
We use 3D numerical MHD simulations to follow the evolution of cold,
turbulent, gaseous systems with parameters representing GMC conditions. We
study three cloud simulations with varying mean magnetic fields, but identical
initial velocity fields. We show that turbulent energy is reduced by a factor
two after 0.4-0.8 flow crossing times (2-4 Myr), and that the magnetically
supercritical cloud models collapse after ~6 Myr, while the subcritical cloud
does not collapse. We compare density, velocity, and magnetic field structure
in three sets of snapshots with matched Mach numbers. The volume and column
densities are both log-normally distributed, with mean volume density a factor
3-6 times the unperturbed value, but mean column density only a factor 1.1-1.4
times the unperturbed value. We use a binning algorithm to investigate the
dependence of kinetic quantities on spatial scale for regions of column density
contrast (ROCs). The average velocity dispersion for the ROCs is only weakly
correlated with scale, similar to the mean size-linewidth relation for clumps
within GMCs. ROCs are often superpositions of spatially unconnected regions
that cannot easily be separated using velocity information; the same difficulty
may affect observed GMC clumps. We analyze magnetic field structure, and show
that in the high density regime, total magnetic field strengths increase with
density with logarithmic slope 1/3 -2/3. Mean line-of-sight magnetic field
strengths vary widely across a projected cloud, and do not correlate with
column density. We compute simulated interstellar polarization maps at varying
orientations, and determine that the Chandrasekhar-Fermi formula multiplied by
a factor ~0.5 yields a good estimate of the plane-of sky magnetic field
strength provided the dispersion in polarization angles is < 25 degrees.Comment: 56 pages, 25 figures; Ap.J., accepte
Joint 3D modelling of the polarized Galactic synchrotron and thermal dust foreground diffuse emission
We present for the first time a coherent model of the polarized Galactic
synchrotron and thermal dust emissions which are the main diffuse foreground
for the measurement of the polarized power spectra of the CMB fluctuations with
the Planck satellite mission. We produce 3D models of the Galactic magnetic
field including regular and turbulent components, and of the distribution of
matter in the Galaxy, relativistic electrons and dust grains. By integrating
along the line of sight we construct maps of the polarized Galactic synchrotron
and thermal dust emission for each of these models and compare them to
currently available data. We consider the 408 MHz all-sky continuum survey, the
23 GHz band of the Wilkinson Microwave Anisotropy Probe and the 353 GHz
Archeops data.}{The best-fit parameters obtained are consistent with previous
estimates in the literature based only on synchrotron emission and pulsar
rotation measurements. They allows us to reproduce the large scale structures
observed on the data. Poorly understood local Galactic structures and
turbulence make difficult an accurate reconstruction of the observations in the
Galactic plane. Finally, using the best-fit model we are able to estimate the
expected polarized foreground contamination at the Planck frequency bands. For
the CMB bands, 70, 100, 143 and 217 GHz, at high Galactic latitudes although
the CMB signal dominates in general, a significant foreground contribution is
expected at large angular scales. In particular, this contribution will
dominate the CMB signal for the B modes expected from realistic models of a
background of primordial gravitational waves
Freeware solutions for spectropolarimetric data reduction
Most of the solar physicists use very expensive software for data reduction
and visualization. We present hereafter a reliable freeware solution based on
the Python language. This is made possible by the association of the latter
with a small set of additional libraries developed in the scientific community.
It provides then a very powerful and economical alternative to other
interactive data languages. Although it can also be used for any kind of
post-processing of data, we demonstrate the capabities of such a set of
freeware tools using THeMIS observations of the second solar spectrum.Comment: 4 pages, 2 figures (to appear in the Procs. of Solar Polarization
Workshop #5, eds. Berdyugina, Nagendra and Ramelli
Topological wave functions and the 4D-5D lift
We revisit the holomorphic anomaly equations satisfied by the topological
string amplitude from the perspective of the 4D-5D lift, in the context of
''magic'' N=2 supergravity theories. In particular, we interpret the
Gopakumar-Vafa relation between 5D black hole degeneracies and the topological
string amplitude as the result of a canonical transformation from 4D to 5D
charges. Moreover we use the known Bekenstein-Hawking entropy of 5D black holes
to constrain the asymptotic behavior of the topological wave function at finite
topological coupling but large K\"ahler classes. In the process, some
subtleties in the relation between 5D black hole degeneracies and the
topological string amplitude are uncovered, but not resolved. Finally we extend
these considerations to the putative one-parameter generalization of the
topological string amplitude, and identify the canonical transformation as a
Weyl reflection inside the 3D duality group.Comment: minor corrections, version to appear in JHE
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