1,542 research outputs found
The two-and three-point correlation functions of the polarized five-year WMAP sky maps
We present the two- and three-point real space correlation functions of the
five-year WMAP sky maps, and compare the observed functions to simulated LCDM
concordance model ensembles. In agreement with previously published results, we
find that the temperature correlation functions are consistent with
expectations. However, the pure polarization correlation functions are
acceptable only for the 33GHz band map; the 41, 61, and 94 GHz band correlation
functions all exhibit significant large-scale excess structures. Further, these
excess structures very closely match the correlation functions of the two
(synchrotron and dust) foreground templates used to correct the WMAP data for
galactic contamination, with a cross-correlation statistically significant at
the 2sigma-3sigma confidence level. The correlation is slightly stronger with
respect to the thermal dust template than with the synchrotron template.Comment: 10 pages, 5 figures, published in ApJ. v2: New title, minor changes
to appendix, and fixed some typos. v3: Matches version published in Ap
CMB likelihood approximation by a Gaussianized Blackwell-Rao estimator
We introduce a new CMB temperature likelihood approximation called the
Gaussianized Blackwell-Rao (GBR) estimator. This estimator is derived by
transforming the observed marginal power spectrum distributions obtained by the
CMB Gibbs sampler into standard univariate Gaussians, and then approximate
their joint transformed distribution by a multivariate Gaussian. The method is
exact for full-sky coverage and uniform noise, and an excellent approximation
for sky cuts and scanning patterns relevant for modern satellite experiments
such as WMAP and Planck. A single evaluation of this estimator between l=2 and
200 takes ~0.2 CPU milliseconds, while for comparison, a single pixel space
likelihood evaluation between l=2 and 30 for a map with ~2500 pixels requires
~20 seconds. We apply this tool to the 5-year WMAP temperature data, and
re-estimate the angular temperature power spectrum, , and likelihood,
L(C_l), for l<=200, and derive new cosmological parameters for the standard
six-parameter LambdaCDM model. Our spectrum is in excellent agreement with the
official WMAP spectrum, but we find slight differences in the derived
cosmological parameters. Most importantly, the spectral index of scalar
perturbations is n_s=0.973 +/- 0.014, 1.9 sigma away from unity and 0.6 sigma
higher than the official WMAP result, n_s = 0.965 +/- 0.014. This suggests that
an exact likelihood treatment is required to higher l's than previously
believed, reinforcing and extending our conclusions from the 3-year WMAP
analysis. In that case, we found that the sub-optimal likelihood approximation
adopted between l=12 and 30 by the WMAP team biased n_s low by 0.4 sigma, while
here we find that the same approximation between l=30 and 200 introduces a bias
of 0.6 sigma in n_s.Comment: 10 pages, 7 figures, submitted to Ap
Reconstructing the shape of the correlation function
We develop an estimator for the correlation function which, in the ensemble
average, returns the shape of the correlation function, even for signals that
have significant correlations on the scale of the survey region. Our estimator
is general and works in any number of dimensions. We develop versions of the
estimator for both diffuse and discrete signals. As an application, we examine
Monte Carlo simulations of X-ray background measurements. These include a
realistic, spatially-inhomogeneous population of spurious detector events. We
discuss applying the estimator to the averaging of correlation functions
evaluated on several small fields, and to other cosmological applications.Comment: 10 pages, 5 figures, submitted to ApJS. Methods and results unchanged
but text is expanded and significantly reordered in response to refere
Search For Unresolved Sources In The COBE-DMR Two-Year Sky Maps
We have searched the temperature maps from the COBE Differential Microwave
Radiometers (DMR) first two years of data for evidence of unresolved sources.
The high-latitude sky (|b| > 30\deg) contains no sources brighter than 192 uK
thermodynamic temperature (322 Jy at 53 GHz). The cumulative count of sources
brighter than threshold T, N(> T), is consistent with a superposition of
instrument noise plus a scale-invariant spectrum of cosmic temperature
fluctuations normalized to Qrms-PS = 17 uK. We examine the temperature maps
toward nearby clusters and find no evidence for any Sunyaev-Zel'dovich effect,
\Delta y < 7.3 x 10^{-6} (95% CL) averaged over the DMR beam. We examine the
temperature maps near the brightest expected radio sources and detect no
evidence of significant emission. The lack of bright unresolved sources in the
DMR maps, taken with anisotropy measurements on smaller angular scales, places
a weak constraint on the integral number density of any unresolved
Planck-spectrum sources brighter than flux density S, n(> S) < 2 x 10^4 (S/1
Jy)^{-2} sr^{-1}.Comment: 16 pages including 2 figures, uuencoded PostScript, COBE preprint
94-0
The effect of asymmetric beams in the Wilkinson Microwave Anisotropy Probe experiment
We generate simulations of the CMB temperature field as observed by the WMAP
satellite, taking into account the detailed shape of the asymmetric beams and
scanning strategy of the experiment, and use these to re-estimate the WMAP beam
transfer functions. This method avoids the need of artificially symmetrizing
the beams, as done in the baseline WMAP approach, and instead measures the
total convolution effect by direct simulation. We find only small differences
with respect to the nominal transfer functions, typically less than 1%
everywhere, and less than 0.5% at l<400. The net effect on the CMB power
spectrum is less than 0.6%. The effect on all considered cosmological
parameters are negligible. For instance, we find that the spectral index of
scalar perturbations after taking into account the beam asymmetries is n_s =
0.964 +- 0.014, corresponding to a negative shift of -0.1 sigma compared to the
previously released WMAP results. Our CMB sky simulations are made publicly
available, and can be used for general studies of asymmetric beam effects in
the WMAP data.Comment: 14 pages, 7 figures, published versio
Non-Gaussianities in the local curvature of the 5-year WMAP data
Using the 5 year WMAP data, we re-investigate claims of non-Gaussianities and
asymmetries detected in local curvature statistics of the 1 year WMAP data. In
Hansen et al 2004, it was found that the northern ecliptic hemisphere was
non-Gaussian at the ~1% level testing the densities of hill-, lake and saddle
points based on the second derivatives of the CMB temperature map. The 5 year
WMAP data has a much lower noise level and better control of systematics. Using
these, we find that the anomalies are still present at a consistent level. Also
the direction of maximum non-Gaussianity remains. Due to limited availability
of computer resources, Hansen et al. 2004 were unable to calculate the full
covariance matrix for the chi^2 test used. Here we apply the full covariance
matrix instead of the diagonal approximation and find that the
non-Gaussianities disappear and there is no preferred non-Gaussian direction.
We compare with simulations of weak lensing to see if this may cause the
observed non-Gaussianity when using diagonal covariance matrix. We conclude
that weak lensing does not produce non-Gaussianity in the local curvature
statistics at the scales investigated in this paper. The cause of the
non-Gaussian detection in the case of a diagonal matrix remains unclear.Comment: 7 pages, 6 figures, included test on weak lensing simulation
Angular Power Spectrum of the Microwave Background Anisotropy seen by the COBE Differential Microwave Radiometer
The angular power spectrum estimator developed by Peebles (1973) and Hauser &
Peebles (1973) has been modified and applied to the 4 year maps produced by the
COBE DMR. The power spectrum of the observed sky has been compared to the power
spectra of a large number of simulated random skies produced with noise equal
to the observed noise and primordial density fluctuation power spectra of power
law form, with . The best fitting value of the spectral index
in the range of spatial scales corresponding to spherical harmonic indices is an apparent spectral index = 1.13 (+0.3)
(-0.4) which is consistent with the Harrison-Zel'dovich primordial spectral
index The best fitting amplitude for is = 18 uK.Comment: 17 pages including 3 PostScript figures. Submitted to The
Astrophysical Journal (Letters
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies?
(Abridged) A simple six-parameter LCDM model provides a successful fit to
WMAP data, both when the data are analyzed alone and in combination with other
cosmological data. Even so, it is appropriate to search for any hints of
deviations from the now standard model of cosmology, which includes inflation,
dark energy, dark matter, baryons, and neutrinos. The cosmological community
has subjected the WMAP data to extensive and varied analyses. While there is
widespread agreement as to the overall success of the six-parameter LCDM model,
various "anomalies" have been reported relative to that model. In this paper we
examine potential anomalies and present analyses and assessments of their
significance. In most cases we find that claimed anomalies depend on posterior
selection of some aspect or subset of the data. Compared with sky simulations
based on the best fit model, one can select for low probability features of the
WMAP data. Low probability features are expected, but it is not usually
straightforward to determine whether any particular low probability feature is
the result of the a posteriori selection or of non-standard cosmology. We
examine in detail the properties of the power spectrum with respect to the LCDM
model. We examine several potential or previously claimed anomalies in the sky
maps and power spectra, including cold spots, low quadrupole power,
quadropole-octupole alignment, hemispherical or dipole power asymmetry, and
quadrupole power asymmetry. We conclude that there is no compelling evidence
for deviations from the LCDM model, which is generally an acceptable
statistical fit to WMAP and other cosmological data.Comment: 19 pages, 17 figures, also available with higher-res figures on
http://lambda.gsfc.nasa.gov; accepted by ApJS; (v2) text as accepte
2-Point Correlations in the COBE DMR 4-Year Anisotropy Maps
The 2-point temperature correlation function is evaluated from the 4-year
COBE DMR microwave anisotropy maps. We examine the 2-point function, which is
the Legendre transform of the angular power spectrum, and show that the data
are statistically consistent from channel to channel and frequency to
frequency. The most likely quadrupole normalization is computed for a
scale-invariant power-law spectrum of CMB anisotropy, using a variety of data
combinations. For a given data set, the normalization inferred from the 2-point
data is consistent with that inferred by other methods. The smallest and
largest normalization deduced from any data combination are 16.4 and 19.6 uK
respectively, with a value ~18 uK generally preferred.Comment: Sumbitted to ApJ Letter
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