125 research outputs found
A System of Interaction and Structure
This paper introduces a logical system, called BV, which extends
multiplicative linear logic by a non-commutative self-dual logical operator.
This extension is particularly challenging for the sequent calculus, and so far
it is not achieved therein. It becomes very natural in a new formalism, called
the calculus of structures, which is the main contribution of this work.
Structures are formulae submitted to certain equational laws typical of
sequents. The calculus of structures is obtained by generalising the sequent
calculus in such a way that a new top-down symmetry of derivations is observed,
and it employs inference rules that rewrite inside structures at any depth.
These properties, in addition to allow the design of BV, yield a modular proof
of cut elimination.Comment: This is the authoritative version of the article, with readable
pictures, in colour, also available at
. (The published version contains
errors introduced by the editorial processing.) Web site for Deep Inference
and the Calculus of Structures at <http://alessio.guglielmi.name/res/cos
Model-Independent Reionization Observables in the CMB
We represent the reionization history of the universe as a free function in
redshift and study the potential for its extraction from CMB polarization
spectra. From a principal component analysis, we show that the ionization
history information is contained in 5 modes, resembling low-order Fourier modes
in redshift space. The amplitude of these modes represent a compact description
of the observable properties of reionization in the CMB, easily predicted given
a model for the ionization fraction. Measurement of these modes can ultimately
constrain the total optical depth, or equivalently the initial amplitude of
fluctuations to the 1% level regardless of the true model for reionization.Comment: 4 pages, 5 figures, submitted to PRD (rapid communications
The power-law behaviours of angular spectra of polarized Galactic synchrotron
We study the angular power spectra of polarized Galactic synchrotron in the
range 10<l<800, at several frequencies between 0.4 and 2.7 GHz and at several
Galactic latitudes up to near the North Galactic Pole. Electric- and
magnetic-parity polarization spectra are found to have slopes around alpha
_{E,B} = 1.4 - 1.5 in the Parkes and Effelsberg Galactic-Plane surveys, but
strong local fluctuations of alpha_{E,B} are found at | b | ~ 10 degree from
the 1.4 GHz Effelsberg survey. The C_{PIl} spectrum, which is insensitive to
the polarization direction, is somewhat steeper, being alpha_{PI} = 1.6 - 1.8
for the same surveys. The low-resolution multifrequency survey of Brouw and
Spoelstra (1976) shows some flattening of the spectra below 1 GHz, more intense
for C_{E,Bl} than for C_{PIl}. In no case we find evidence for really steep
spectra. The extrapolation to the cosmological window shows that at 90 GHz the
detection of E-mode harmonics in the cosmic background radiation should not be
disturbed by synchrotron, even around l~10 for a reionization optical depth tau
_{ri}>~0.05.Comment: 20 pages, 9 figures, accepted to New Astronom
An iterative destriping technique for diffuse background polarization data
We describe a simple but effective iterative procedure specifically designed
to destripe Q and U Stokes parameter data as those collected by the SPOrt
experiment onboard the International Space Station (ISS). The method is general
enough to be useful for other experiments, both in polarization and total
intensity. The only requirement for the algorithm to work properly is that the
receiver knee frequency must be lower than the signal modulation frequency,
corresponding in our case to the ISS orbit period. Detailed performances of the
technique are presented in the context of the SPOrt experiment, both in terms
of added rms noise and residual correlated noise.Comment: Accepted for publication by A&A (8 pages, 6 figures
1.4 GHz polarimetric observations of the two fields imaged by the DASI experiment
We present results of polarization observations at 1.4 GHz of the two fields
imaged by the DASI experiment (, and , ,
respectively). Data were taken with the Australia Telescope Compact Array with
3.4 arcmin resolution and mJy beam sensitivity. The emission
is dominated by point sources and we do not find evidence for diffuse
synchrotron radiation even after source subtraction. This allows to estimate an
upper limit of the diffuse polarized emission. The extrapolation to 30 GHz
suggests that the synchrotron radiation is lower than the polarized signal
measured by the DASI experiment by at least 2 orders of magnitude. This further
supports the conclusions drawn by the DASI team itself about the negligible
Galactic foreground contamination in their data set, improving by a factor
the upper limit estimated by Leitch et al. (2005).
The dominant point source emission allows us to estimate the contamination of
the CMB by extragalactic foregrounds. We computed the power spectrum of their
contribution and its extrapolation to 30 GHz provides a framework where the CMB
signal should dominate. However, our results do not match the conclusions of
the DASI team about the negligibility of point source contamination, suggesting
to take into account a source subtraction from the DASI data.Comment: 7 pages, six figures, submitted to MNRA
A polarized synchrotron template for CMBP experiments after WMAP data
We build template maps for the polarized Galactic--synchrotron emission on
large angular scales (FWHM =~7), in the 20-90 GHz microwave range, by
using WMAP data. The method, presented in a recent work, requires a synchrotron
total intensity survey and the {\it polarization horizon} to model the
polarized intensity and a starlight polarization map to model polarization
angles. The basic template is obtained directly at 23 GHz with about 94%
sky--coverage by using the synchrotron map released by the WMAP team.
Extrapolations to 32, 60 and 90 GHz are performed by computing a synchrotron
spectral index map, which strongly reduces previous uncertainties in passing
from low (1.4 GHz) to microwave frequencies. Differing from low frequency data,
none of our templates presents relevant structures out of the Galactic Plane.
Our map at 90 GHz suggests that the synchrotron emission at high Galactic
latitudes is low enough to allow a robust detection of the --mode component
of the cosmological signal on large--scale, even in models with
low--reionization (). Detection of the weaker --mode on the
largest scales () might be jeopardized unless the value found by WMAP is confirmed, and . For lower levels of the
gravitational--wave background the --mode seems to be accessible only at the
peak and in selected low--synchrotron emission areas.Comment: 13 pages, 14 figures, accepted for pubblications by MNRAS. For a
version with full resolution color figures see
http://sp0rt.bo.iasf.cnr.it:8080/Docs/Public/papers.ph
High Galactic latitude polarized emission at 1.4 GHz and implications for cosmic microwave background observations
We analyse the polarized emission at 1.4 GHz in a 3x3 deg^2 area at high
Galactic latitude (b ~ -40deg). The region, centred in (RA=5h, Dec=-49deg), was
observed with the Australia Telescope Compact Array radio-interferometer, whose
3-30 arcmin angular sensitivity range allows the study of scales appropriate
for CMB Polarization (CMBP) investigations. The angular behavior of the diffuse
emission is analysed through the E- and B-mode power spectra. These follow a
power law with slopes \beta_E = -1.97 \pm 0.08 and
\beta_B = -1.98 \pm 0.07. The emission is found to be about a factor 25 fainter
than in Galactic plane regions. The comparison of the power spectra with other
surveys indicates that this area is intermediate between strong and negligible
Faraday rotation effects. A similar conclusion can be reached by analysing both
the frequency and Galactic latitude behaviors of the diffuse Galactic emission
of the 408-1411 MHz Leiden survey data. We present an analysis of the Faraday
rotation effects on the polarized power spectra, and find that the observed
power spectra can be enhanced by a transfer of power from large to small
angular scales. The extrapolation of the spectra to 32 and 90GHz of the CMB
window suggests that Galactic synchrotron emission leaves the CMBP E-mode
uncontaminated at 32GHz. The level of the contamination at 90GHz is expected to
be more than 4 orders of magnitude below the CMBP spectrum. Extrapolating to
the relevant angular scales, this region also appears adequate for
investigation of the CMBP B-modes for models with tensor/scalar fluctuation
power ratio T/S>0.01. We also identify polarized point sources in the field,
providing a 9 object list which is complete down to the polarized flux limit of
S^p_lim = 2 mJy.Comment: 13 pages, 11 figures, accepted for publication in MNRA
Cosmological Reionization Around the First Stars: Monte Carlo Radiative Transfer
We study the evolution of ionization fronts around the first proto-galaxies
by using high resolution numerical cosmological (Lambda+CDM model) simulations
and Monte Carlo radiative transfer methods. We present the numerical scheme in
detail and show the results of test runs from which we conclude that the scheme
is both fast and accurate. As an example of interesting cosmological
application, we study the reionization produced by a stellar source of total
mass M=2 10^8 M_\odot turning on at z=12, located at a node of the cosmic web.
The study includes a Spectral Energy Distribution of a zero-metallicity stellar
population, and two Initial Mass Functions (Salpeter/Larson). The expansion of
the I-front is followed as it breaks out from the galaxy and it is channeled by
the filaments into the voids, assuming, in a 2D representation, a
characteristic butterfly shape. The ionization evolution is very well tracked
by our scheme, as realized by the correct treatment of the channeling and
shadowing effects due to overdensities. We confirm previous claims that both
the shape of the IMF and the ionizing power metallicity dependence are
important to correctly determine the reionization of the universe.Comment: 8 pages, 8 figures. Revised version, accepted for publication by
MNRA
Cosmic Microwave Background Polarization and reionization: constraining models with a double reionization
Neutral hydrogen around high-z QSO and an optical depth tau ~ 0.17 can be
reconciled if reionization is more complex than a single transition at z ~ 6-8.
Tracing its details could shed a new light on the first sources of radiation.
Here we discuss how far such details can be inspected through planned
experiments on CMB large-scale anisotropy and polarization, by simulating an
actual data analysis. By considering a set of double reionization histories of
Cen (2003) type, a relevant class of models not yet considered by previous
works, we confirm that large angle experiments rival high resolution ones in
reconstructing the reionization history. We also confirm that reionization
histories, studied with the prior of a single and sharp reionization, yield a
biased tau, showing that this bias is generic. We further find a monotonic
trend in the bias for the models that we consider, and propose an explanation
of the trend, as well as the overall bias. We also show that in long-lived
experiments such a trend can be used to discriminate between single and double
reionization patterns.Comment: 8 pages, 11 figures. Substantial rewriting, replaced with accepted
version. To be published in A&
Foregrounds for observations of the cosmological 21 cm line: I. First Westerbork measurements of Galactic emission at 150 MHz in a low latitude field
We present the first results from a series of observations conducted with the
Westerbork telescope in the 140--160 MHz range with a 2 arcmin resolution aimed
at characterizing the properties of the foregrounds for epoch of reionization
experiments. For the first time we have detected fluctuations in the Galactic
diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low
Galactic latitude area known as Fan region. Those fluctuations have an of
14 K. The total intensity power spectrum shows a power--law behaviour down to
with slope . The detection of
diffuse emission at smaller angular scales is limited by residual point
sources. We measured an confusion noise of 3 mJy beam.
Diffuse polarized emission was also detected for the first time at this
frequency. The polarized signal shows complex structure both spatially and
along the line of sight. The polarization power spectrum shows a power--law
behaviour down to with slope .
The of polarization fluctuations is 7.2 K on 4 arcmin scales. By
extrapolating the measured spectrum of total intensity emission, we find a
contamination on the cosmological signal of K on 5 arcmin scales and a corresponding value
of 18.3 K at the same angular scale. The level of the polarization power
spectrum is K on 5 arcmin scales. Given its exceptionally
bright polarized signal, the Fan region is likely to represent an upper limit
on the sky brightness at moderate and high Galactic latitude.Comment: Minor corrections made to match the final version printed on A&A. A
version with high resolution figures is available at
http://www.astro.rug.nl/~bernardi/FAN/fan.pd
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