181 research outputs found
A needlet ILC analysis of WMAP 9-year polarisation data: CMB polarisation power spectra
We estimate Cosmic Microwave Background (CMB) polarisation power spectra, and
temperature-polarisation cross-spectra, from the 9-year data of the Wilkinson
Microwave Anisotropy Probe (WMAP). Foreground cleaning is implemented using
minimum variance linear combinations of the coefficients of needlet
decompositions of sky maps for all WMAP channels, to produce maps for CMB
temperature anisotropies (T-mode) and polarisation (E-mode and B-mode), for 9
different years of observation. The final power spectra are computed from
averages of all possible cross-year power spectra obtained using
foreground-cleaned maps for the different years. Our analysis technique yields
a measurement of the EE spectrum that is in excellent agreement with
theoretical expectations from the current cosmological model. By comparison,
the publicly available WMAP EE power spectrum is higher on average (and
significantly higher than the predicted EE spectrum from the current best fit)
at scales larger than about a degree, an excess that is not confirmed by our
analysis.Comment: 13 pages, 7 figures, Significantly changed version accepted for
publication in MNRA
Testing the isotropy of high energy cosmic rays using spherical needlets
For many decades, ultrahigh energy charged particles of unknown origin that
can be observed from the ground have been a puzzle for particle physicists and
astrophysicists. As an attempt to discriminate among several possible
production scenarios, astrophysicists try to test the statistical isotropy of
the directions of arrival of these cosmic rays. At the highest energies, they
are supposed to point toward their sources with good accuracy. However, the
observations are so rare that testing the distribution of such samples of
directional data on the sphere is nontrivial. In this paper, we choose a
nonparametric framework that makes weak hypotheses on the alternative
distributions and allows in turn to detect various and possibly unexpected
forms of anisotropy. We explore two particular procedures. Both are derived
from fitting the empirical distribution with wavelet expansions of densities.
We use the wavelet frame introduced by [SIAM J. Math. Anal. 38 (2006b) 574-594
(electronic)], the so-called needlets. The expansions are truncated at scale
indices no larger than some , and the distances between
those estimates and the null density are computed. One family of tests (called
Multiple) is based on the idea of testing the distance from the null for each
choice of , whereas the so-called PlugIn approach is
based on the single full expansion, but with thresholded wavelet
coefficients. We describe the practical implementation of these two procedures
and compare them to other methods in the literature. As alternatives to
isotropy, we consider both very simple toy models and more realistic
nonisotropic models based on Physics-inspired simulations. The Monte Carlo
study shows good performance of the Multiple test, even at moderate sample
size, for a wide sample of alternative hypotheses and for different choices of
the parameter . On the 69 most energetic events published by the
Pierre Auger Collaboration, the needlet-based procedures suggest statistical
evidence for anisotropy. Using several values for the parameters of the
methods, our procedures yield -values below 1%, but with uncontrolled
multiplicity issues. The flexibility of this method and the possibility to
modify it to take into account a large variety of extensions of the problem
make it an interesting option for future investigation of the origin of
ultrahigh energy cosmic rays.Comment: Published in at http://dx.doi.org/10.1214/12-AOAS619 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Some sources of systematic errors on CMB polarized measurements with bolometers
Some sources of systematic errors, specific to polarized CMB measurements
using bolometers, are examined. Although the evaluations we show have been made
in the context of the Planck mission (and more specifically the Planck HFI),
many of our conclusions are valid for other experiments as well.Comment: Contribution to the International workshop "Background Polarized
Emission from Radio to Microwave Wavelengths" October 9-12, 2001, Bologna,
Ital
Foreground component separation with generalised ILC
The 'Internal Linear Combination' (ILC) component separation method has been
extensively used to extract a single component, the CMB, from the WMAP
multifrequency data. We generalise the ILC approach for separating other
millimetre astrophysical emissions. We construct in particular a
multidimensional ILC filter, which can be used, for instance, to estimate the
diffuse emission of a complex component originating from multiple correlated
emissions, such as the total emission of the Galactic interstellar medium. The
performance of such generalised ILC methods, implemented on a needlet frame, is
tested on simulations of Planck mission observations, for which we successfully
reconstruct a low noise estimate of emission from astrophysical foregrounds
with vanishing CMB and SZ contamination.Comment: 11 pages, 6 figures (2 figures added), 1 reference added,
introduction expanded, V2: version accepted by MNRA
Optimal scan strategies for future CMB satellite experiments
The B-mode polarisation power spectrum in the Cosmic Microwave Background
(CMB) is about four orders of magnitude fainter than the CMB temperature power
spectrum. Any instrumental imperfections that couple temperature fluctuations
to B-mode polarisation must therefore be carefully controlled and/or removed.
We investigate the role that a scan strategy can have in mitigating certain
common systematics by averaging systematic errors down with many crossing
angles. We present approximate analytic forms for the error on the recovered
B-mode power spectrum that would result from differential gain, differential
pointing and differential ellipticity for the case where two detector pairs are
used in a polarisation experiment. We use these analytic predictions to search
the parameter space of common satellite scan strategies in order to identify
those features of a scan strategy that have most impact in mitigating
systematic effects. As an example we go on to identify a scan strategy suitable
for the CMB satellite proposed for the ESA M5 call. considering the practical
considerations of fuel requirement, data rate and the relative orientation of
the telescope to the earth. Having chosen a scan strategy we then go on to
investigate the suitability of the scan strategy.Comment: 21 pages, 11 figures, Comments welcom
CMB and SZ effect separation with Constrained Internal Linear Combinations
The `Internal Linear Combination' (ILC) component separation method has been
extensively used on the data of the WMAP space mission, to extract a single
component, the CMB, from the WMAP multifrequency data. We extend the ILC
approach for reconstructing millimeter astrophysical emissions beyond the CMB
alone. In particular, we construct a Constrained ILC to extract clean maps of
both the CMB or the thermal Sunyaev Zeldovich (SZ) effect, with vanishing
contamination from the other. The performance of the Constrained ILC is tested
on simulations of Planck mission observations, for which we successfully
reconstruct independent estimates of the CMB and of the thermal SZ.Comment: 7 pages, 3 figures, submitted to MNRA
Issues and methods for CMB anisotropy data reduction
Major issues and existing methods for the reduction of CMB anisotropy data
are reviewed. An emphasis is put on the proper modelling of the data. It is
suggested that the robustness of methods could be improved by taking into
account the uncertainty of the model for finding optimal solutions.Comment: 12 pages, 4 figures, submitted to New Astronomy Review
Impact of calibration errors on CMB component separation using FastICA and ILC
The separation of emissions from different astrophysical processes is an
important step towards the understanding of observational data. This topic of
component separation is of particular importance in the observation of the
relic Cosmic Microwave Background Radiation, as performed by the WMAP satellite
and the more recent Planck mission, launched May 14th, 2009 from Kourou and
currently taking data. When performing any sort of component separation, some
assumptions about the components must be used. One assumption that many
techniques typically use is knowledge of the frequency scaling of one or more
components. This assumption may be broken in the presence of calibration
errors. Here we compare, in the context of imperfect calibration, the recovery
of a clean map of emission of the Cosmic Microwave Background from
observational data with two methods: FastICA (which makes no assumption of the
frequency scaling of the components), and an `Internal Linear Combination'
(ILC), which explicitly extracts a component with a given frequency scaling. We
find that even in the presence of small calibration errors with a Planck-style
mission, the ILC method can lead to inaccurate CMB reconstruction in the high
signal-to-noise regime, because of partial cancellation of the CMB emission in
the recovered map. While there is no indication that the failure of the ILC
will translate to other foreground cleaning or component separation techniques,
we propose that all methods which assume knowledge of the frequency scaling of
one or more components be careful to estimate the effects of calibration
errors.Comment: 13 pages, 5 figure
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