56 research outputs found
The radiative transfer equations for Compton scattering of polarized low frequency radiation on a hot electron gas
We deduce the equations that describe how polarized radiation is Comptonized
by a hot electron gas. Low frequencies are considered, and the equations are
expanded to second order in electron velocities. Induced scattering terms are
included and a Maxwellian velocity distribution for the electrons is assumed.
The special case of an axisymmetric radiation field is also considered, and the
corresponding radiative transfer equations are found. Our results correct
errors and misprints in previosly published transfer equations. The extension
to a moving electron gas is made, and the radiative transfer equations are
deduced to second order in gas velocity. We use the equations to study
polarization in the Sunyaev-Zeldovich effect.Comment: 9 pages, 2 figuers, MNRAS-LaTeX-style Submitted to the Monthly
Notices of the Royal astronomical Societ
Directional Variations of the Non-Gaussianity Parameter f_NL
We investigate local variations of the primordial non-Gaussianity parameter
f_NL in the WMAP data, looking for possible influence of foreground
contamination in the full-sky estimate of f_NL. We first improve the needlet
bispectrum estimate in (Rudjord et al. 2009) on the full-sky to f_NL= 73 +/- 31
using the KQ75 mask on the co-added V+W channel. We find no particular values
of f_NL estimates close to the galactic plane and conclude that foregrounds are
unlikely to affect the estimate of f_NL in the V and W bands even for the
smaller KQ85 mask. In the Q band however, we find unexpectedly high values of
f_NL in local estimates close to the galactic mask, as well as significant
discrepancies between Q band estimates and V/W band estimates. We therefore
conclude that the Q band is too contaminated to be used for non-Gaussianity
studies even with the larger KQ75 mask. We further noted that the local f_NL
estimates on the V+W channel are positive on all equatorial bands from the
north to the south pole. The probability for this to happen in a universe with
f_NL = 0 is less than one percent.Comment: 6 pages, 2 figures, included section on non-Gaussian maps, references
adde
Detection of new point sources in WMAP 7 year data using internal templates and needlets
We have developed a new needlet based method to detect point sources in
cosmic microwave background (CMB) maps and have applied it to the WMAP 7 year
data. We use both the individual frequency channels as well as internal
templates, the difference between pairs of frequency channels, with the
advantage that the CMB component is eliminated. Using the area of the sky
outside the Kq85 galactic mask, we detect a total of 2102 point sources at the
level in either the frequency maps or the internal templates. Of
these, 1116 are detected either at directly in the frequency channels
or at in the internal templates and at the
corresponding position in the frequency channels. Of the 1116 sources, 603 are
detections which have not been reported so far in WMAP data. We have made a
catalogue of these sources available with position and flux estimated in the
WMAP channels where they are seen. In total, we identified 1029 of the 1116
sources with counterparts at 5GHz and 69 at other frequencies.Comment: Accepted version, added integral number counts, 29 pages (+16 pages
for the catalogue),10 figure
Temperature and Polarization CMB Maps from Primordial non-Gaussianities of the Local Type
The forthcoming Planck experiment will provide high sensitivity polarization
measurements that will allow us to further tighten the f_NL bounds from the
temperature data. Monte Carlo simulations of non-Gaussian CMB maps have been
used as a fundamental tool to characterize non-Gaussian signatures in the data,
as they allow us to calibrate any statistical estimators and understand the
effect of systematics, foregrounds and other contaminants. We describe an
algorithm to generate high-angular resolution simulations of non-Gaussian CMB
maps in temperature and polarization. We consider non-Gaussianities of the
local type, for which the level of non-Gaussianity is defined by the
dimensionless parameter, f_NL. We then apply the temperature and polarization
fast cubic statistics recently developed by Yadav et al. to a set of
non-Gaussian temperature and polarization simulations. We compare our results
to theoretical expectations based on a Fisher matrix analysis, test the
unbiasedness of the estimator, and study the dependence of the error bars on
f_NL. All our results are in very good agreement with theoretical predictions,
thus confirming the reliability of both the simulation algorithm and the fast
cubic temperature and polarization estimator.Comment: 14 pages, 9 figures, revised version accepted by PRD, minor changes
and acknowledgements adde
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
Gabor Transforms on the Sphere with Applications to CMB Power Spectrum Estimation
The Fourier transform of a dataset apodised with a window function is known
as the Gabor transform. In this paper we extend the Gabor transform formalism
to the sphere with the intention of applying it to CMB data analysis. The Gabor
coefficients on the sphere known as the pseudo power spectrum is studied for
windows of different size. By assuming that the pseudo power spectrum
coefficients are Gaussian distributed, we formulate a likelihood ansatz using
these as input parameters to estimate the full sky power spectrum from a patch
on the sky. Since this likelihood can be calculated quickly without having to
invert huge matrices, this allows for fast power spectrum estimation. By using
the pseudo power spectrum from several patches on the sky together, the full
sky power spectrum can be estimated from full-sky or nearly full-sky
observations.Comment: 37 pages, 31 figures, accepted for publication in MNRA
Asymmetries in the local curvature of the WMAP data
We use the local curvature to investigate the possible existence of
non-Gaussianity/asymmetry in the WMAP data. Considering the full sky we find
results which are consistent with the Gaussian assumption. However, strong
non-Gaussian features emerge when considering the northern and southern
galactic hemisphere separately, particularly on scales between 1 and 5 degrees.
Quite interestingly, the maximum non-Gaussianity is found for hemispheres
centered near the ecliptic poles, which might suggest the presence of some
systematic effect. The direction of the asymmetry seems consistent with the
findings by Eriksen et al. 2004.Comment: Submitted to ApJ Letter
A possible common explanation for several cosmic microwave background (CMB) anomalies: A strong impact of nearby galaxies on observed large-scale CMB fluctuations
In Luparello et al. 2023, a new and hitherto unknown CMB foreground was
detected. A systematic decrease in Cosmic Microwave Background (CMB)
temperatures around nearby large spiral galaxies points to an unknown
interaction with CMB photons in a sphere up to several projected Mpc around
these galaxies. We investigate to which extent this foreground may impact the
CMB fluctuations map and create the so-called CMB anomalies. Using the observed
temperature decrements around the galaxies, and making some general assumptions
about the unknown interaction, we propose a common radial temperature profile.
By assigning this profile to nearby galaxies in the redshift range
we create a foreground map model. We find a remarkable
resemblance between this temperature model map based on nearby galaxies and the
Planck CMB map. Out of 1000 simulated maps, none of them show such a strong
correlation with the foreground map over both large and small angular scales.
In particular, the quadrupole, octopole, as well as and modes
correlate with the foreground map to high significance. Furthermore, one of the
most prominent temperature decrements in the foreground map coincides with the
position of the CMB cold spot. The largest scales of the CMB and thereby the
cosmological parameters, may have important changes after proper corrections of
this foreground component. However, a reliable corrected CMB map can only be
derived when suitable physical mechanisms are proposed and tested.Comment: 5 pages, 4 figures, revised version which is accepted for publication
in A&A Letter
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