161 research outputs found
Spatial variations in the spectral index of polarized synchrotron emission in the 9 yr WMAP sky maps
We estimate the spectral index, beta, of polarized synchrotron emission as
observed in the 9 yr WMAP sky maps using two methods, linear regression ("T-T
plot") and maximum likelihood. We partition the sky into 24 disjoint sky
regions, and evaluate the spectral index for all polarization angles between 0
deg and 85 deg in steps of 5. Averaging over polarization angles, we derive a
mean spectral index of beta_all-sky=-2.99+-0.01 in the frequency range of 23-33
GHz. We find that the synchrotron spectral index steepens by 0.14 from low to
high Galactic latitudes, in agreement with previous studies, with mean spectral
indices of beta_plane=-2.98+-0.01 and beta_high-lat=-3.12+-0.04. In addition,
we find a significant longitudinal variation along the Galactic plane with a
steeper spectral index toward the Galactic center and anticenter than toward
the Galactic spiral arms. This can be well modeled by an offset sinusoidal,
beta(l)=-2.85+0.17sin(2l-90). Finally, we study synchrotron emission in the
BICEP2 field, in an attempt to understand whether the claimed detection of
large-scale B-mode polarization could be explained in terms of synchrotron
contamination. Adopting a spectral index of beta=-3.12, typical for high
Galactic latitudes, we find that the most likely bias corresponds to about 2%
of the reported signal (r=0.003). The flattest index allowed by the data in
this region is beta=-2.5, and under the assumption of a straight power-law
frequency spectrum, we find that synchrotron emission can account for at most
20% of the reported BICEP2 signal.Comment: 11 pages, 9 figures, updated to match version published in Ap
The effect of systematics on polarized spectral indices
We study four particularly bright polarized compact objects (Tau A, Virgo A,
3C273 and Fornax A) in the 7-year WMAP sky maps, with the goal of understanding
potential systematics involved in estimation of foreground spectral indices. We
estimate the spectral index, the polarization angle, the polarization fraction
and apparent size and shape of these objects when smoothed to a nominal
resolution of 1 degree FWHM. Second, we compute the spectral index as a
function of polarization orientation, alpha. Because these objects are
approximately point sources with constant polarization angle, this function
should be constant in the absence of systematics. However, computing it for the
K- and Ka-band WMAP data we find strong index variations for all four sources.
For Tau A, we find a spectral index beta=-2.59+-0.03 for alpha=30 degrees, and
beta=-2.03+-0.01 for alpha=50 degrees. On the other hand, the spectral index
between Ka and Q band is found to be stable. A simple elliptical Gaussian toy
model with parameters matching those observed in Tau A reproduces the observed
signal, and shows that the spectral index is in particular sensitive to the
detector polarization angle. Based on these findings, we first conclude that
estimation of spectral indices with the WMAP K-band polarization data at 1
degree scales is not robust. Second, we note that these issues may be of
concern for ground-based and sub-orbital experiments that use the WMAP
polarization measurements of Tau A for calibration of gain and polarization
angles.Comment: 5 pages, 6 figures, submitted to ApJ; new figure and expanded
conclusio
Constraints on the spectral index of polarized synchrotron emission from WMAP and Faraday-corrected S-PASS data
We constrain the spectral index of polarized synchrotron emission, ,
by correlating the recently released 2.3 GHz S-Band Polarization All Sky Survey
(S-PASS) data with the 23 GHz 9-year Wilkinson Microwave Anisotropy Probe
(WMAP) sky maps. We sub-divide the S-PASS field, which covers the Southern
Ecliptic hemisphere, into regions, and estimate
the spectral index of polarized synchrotron emission within each region using a
simple but robust T-T plot technique. Three different versions of the S-PASS
data are considered, corresponding to either no correction for Faraday
rotation; Faraday correction based on the rotation measure model presented by
the S-PASS team; or Faraday correction based on a rotation measure model
presented by Hutschenreuter and En{\ss}lin. We find that the correlation
between S-PASS and WMAP is strongest when applying the S-PASS model. Adopting
this correction model, we find that the mean spectral index of polarized
synchrotron emission gradually steepens from at low
Galactic latitudes to at high Galactic latitudes, in good
agreement with previously published results. Finally, we consider two special
cases defined by the BICEP2 and SPIDER fields, and obtain mean estimates of
and , respectively.
Adopting the WMAP 23 GHz sky map bandpass filtered to including angular scales
only between and as a spatial template, we constrain
the root-mean-square synchrotron polarization amplitude to be less than
() at 90 GHz (150 GHz) for the BICEP2 field,
corresponding roughly to a tensor-to-scalar ratio of
(), respectively. Very similar constraints are obtained for the
SPIDER field.Comment: 14 pages, 13 Figures, to be submitted to A&
A Monte Carlo comparison between template-based and Wiener-filter CMB dipole estimators
We review and compare two different CMB dipole estimators discussed in the
literature, and assess their performances through Monte Carlo simulations. The
first method amounts to simple template regression with partial sky data, while
the second method is an optimal Wiener filter (or Gibbs sampling)
implementation. The main difference between the two methods is that the latter
approach takes into account correlations with higher-order CMB temperature
fluctuations that arise from non-orthogonal spherical harmonics on an
incomplete sky, which for recent CMB data sets (such as Planck) is the dominant
source of uncertainty. For an accepted sky fraction of 81% and an angular CMB
power spectrum corresponding to the best-fit Planck 2018 CDM model, we
find that the uncertainty on the recovered dipole amplitude is about six times
smaller for the Wiener filter approach than for the template approach,
corresponding to 0.5 and 3K, respectively. Similar relative differences
are found for the corresponding directional parameters and other sky fractions.
We note that the Wiener filter algorithm is generally applicable to any dipole
estimation problem on an incomplete sky, as long as a statistical and
computationally tractable model is available for the unmasked higher-order
fluctuations. The methodology described in this paper forms the numerical basis
for the most recent determination of the CMB solar dipole from Planck, as
summarized by arXiv:2007.04997.Comment: 8 pages, 10 figures, submitted to A&
B-mode polarization forecasts for GreenPol
We present tensor-to-scalar ratio forecasts for GreenPol, a hypothetical
ground-based B-mode experiment aiming to survey the cleanest regions of the
Northern Galactic hemisphere at five frequencies between 10 and 44 GHz. Its
primary science goal would be to measure large-scale CMB polarization
fluctuations at multipoles , and thereby constrain the
primordial tensor-to-scalar ratio. The observations for the suggested
experiment would take place at the Summit Station (72deg N, 38deg W) on
Greenland, at an altitude of 3216 meters above sea level. In this paper we
simulate various experimental setups, and derive limits on the tensor-to-scalar
ratio after CMB component separation using a Bayesian component separation
implementation called Commander. When combining the proposed experiment with
Planck HFI observations for constraining polarized thermal dust emission, we
find a projected limit of r<0.02 at 95 % confidence for the baseline
configuration. This limit is very robust with respect to a range of important
experimental parameters, including sky coverage, detector weighting, foreground
priors etc. Overall, GreenPol would have the possibility to provide deep CMB
polarization measurements of the Northern Galactic hemisphere at low
frequencies.Comment: 10 pages, 8 figures. To be submitted to A&
Cosmoglobe DR1. III. First full-sky model of polarized synchrotron emission from all WMAP and Planck LFI data
We present the first model of full-sky polarized synchrotron emission that is
derived from all WMAP and Planck LFI frequency maps. The basis of this analysis
is the set of end-to-end reprocessed Cosmoglobe Data Release 1 sky maps
presented in a companion paper, which have significantly lower instrumental
systematics than the legacy products from each experiment. We find that the
resulting polarized synchrotron amplitude map has an average noise rms of
at 30 GHz and FWHM, which is 30% lower than
the recently released BeyondPlanck model that included only LFI+WMAP Ka-V data,
and 29% lower than the WMAP K-band map alone. The mean -to- power
spectrum ratio is , with amplitudes consistent with those measured
previously by Planck and QUIJOTE. Assuming a power law model for the
synchrotron spectral energy distribution, and using the -- plot method,
we find a full-sky inverse noise-variance weighted mean of
between Cosmoglobe DR1 K-band and 30 GHz, in
good agreement with previous estimates. In summary, the novel Cosmoglobe DR1
synchrotron model is both more sensitive and systematically cleaner than
similar previous models, and it has a more complete error description that is
defined by a set of Monte Carlo posterior samples. We believe that these
products are preferable over previous Planck and WMAP products for all
synchrotron-related scientific applications, including simulation, forecasting
and component separation.Comment: 15 pages, 15 figures, submitted to A&
Cosmoglobe: Towards end-to-end CMB cosmological parameter estimation without likelihood approximations
We implement support for a cosmological parameter estimation algorithm as
proposed by Racine et al. (2016) in Commander, and quantify its computational
efficiency and cost. For a semi-realistic simulation similar to Planck LFI 70
GHz, we find that the computational cost of producing one single sample is
about 60 CPU-hours and that the typical Markov chain correlation length is
100 samples. The net effective cost per independent sample is 6 000
CPU-hours, in comparison with all low-level processing costs of 812 CPU-hours
for Planck LFI and WMAP in Cosmoglobe Data Release 1. Thus, although
technically possible to run already in its current state, future work should
aim to reduce the effective cost per independent sample by at least one order
of magnitude to avoid excessive runtimes, for instance through multi-grid
preconditioners and/or derivative-based Markov chain sampling schemes. This
work demonstrates the computational feasibility of true Bayesian cosmological
parameter estimation with end-to-end error propagation for high-precision CMB
experiments without likelihood approximations, but it also highlights the need
for additional optimizations before it is ready for full production-level
analysis.Comment: 10 pages, 8 figures. Submitted to A&
Cosmoglobe DR1 results. II. Constraints on isotropic cosmic birefringence from reprocessed WMAP and Planck LFI data
Cosmic birefringence is a parity-violating effect that might have rotated the
plane of linearly polarized light of the cosmic microwave background (CMB) by
an angle since its emission. This has recently been measured to be
non-zero at a statistical significance of in the official Planck
PR4 and 9-year WMAP data. In this work, we constrain using the
reprocessed BeyondPlanck LFI and Cosmoglobe DR1 WMAP polarization maps. These
novel maps have both lower systematic residuals and a more complete error
description than the corresponding official products. Foreground
correlations could bias measurements of , and while thermal dust
emission has been argued to be statistically non-zero, no evidence for
synchrotron power has been reported. Unlike the dust-dominated Planck HFI
maps, the majority of the LFI and WMAP polarization maps are instead dominated
by synchrotron emission. Simultaneously constraining and the
polarization miscalibration angle, , of each channel, we find a
best-fit value of with LFI and WMAP data
only. When including the Planck HFI PR4 maps, but fitting separately
for dust-dominated, , and synchrotron-dominated
channels, , we find . This differs from zero with a
statistical significance of , and the main contribution to this
value comes from the LFI 70 GHz channel. While the statistical significances of
these results are low on their own, the measurement derived from the LFI and
WMAP synchrotron-dominated maps agrees with the previously reported
HFI-dominated constraints, despite the very different astrophysical and
instrumental systematics involved in all these experiments.Comment: 10 pages, 7 figures, 2 tables. Submitted to A&
Cosmoglobe DR1 results. I. Improved Wilkinson Microwave Anisotropy Probe maps through Bayesian end-to-end analysis
We present Cosmoglobe Data Release 1, which implements the first joint
analysis of WMAP and Planck LFI time-ordered data, processed within a single
Bayesian end-to-end framework. This framework builds directly on a similar
analysis of the LFI measurements by the BeyondPlanck collaboration, and
approaches the CMB analysis challenge through Gibbs sampling of a global
posterior distribution, simultaneously accounting for calibration, mapmaking,
and component separation. The computational cost of producing one complete
WMAP+LFI Gibbs sample is 812 CPU-hr, of which 603 CPU-hrs are spent on WMAP
low-level processing; this demonstrates that end-to-end Bayesian analysis of
the WMAP data is computationally feasible. We find that our WMAP posterior mean
temperature sky maps and CMB temperature power spectrum are largely consistent
with the official WMAP9 results. Perhaps the most notable difference is that
our CMB dipole amplitude is , which is $11\
\mathrm{\mu K}2.5\ {\sigma}$ higher than
BeyondPlanck; however, it is in perfect agreement with the HFI-dominated Planck
PR4 result. In contrast, our WMAP polarization maps differ more notably from
the WMAP9 results, and in general exhibit significantly lower large-scale
residuals. We attribute this to a better constrained gain and transmission
imbalance model. It is particularly noteworthy that the W-band polarization sky
map, which was excluded from the official WMAP cosmological analysis, for the
first time appears visually consistent with the V-band sky map. Similarly, the
long standing discrepancy between the WMAP K-band and LFI 30 GHz maps is
finally resolved, and the difference between the two maps appears consistent
with instrumental noise at high Galactic latitudes. All maps and the associated
code are made publicly available through the Cosmoglobe web page.Comment: 65 pages, 61 figures. Data available at cosmoglobe.uio.no. Submitted
to A&
BeyondPlanck II. CMB map-making through Gibbs sampling
We present a Gibbs sampling solution to the map-making problem for CMB
measurements, building on existing destriping methodology. Gibbs sampling
breaks the computationally heavy destriping problem into two separate steps;
noise filtering and map binning. Considered as two separate steps, both are
computationally much cheaper than solving the combined problem. This provides a
huge performance benefit as compared to traditional methods, and allows us for
the first time to bring the destriping baseline length to a single sample. We
apply the Gibbs procedure to simulated Planck 30 GHz data. We find that gaps in
the time-ordered data are handled efficiently by filling them with simulated
noise as part of the Gibbs process. The Gibbs procedure yields a chain of map
samples, from which we may compute the posterior mean as a best-estimate map.
The variation in the chain provides information on the correlated residual
noise, without need to construct a full noise covariance matrix. However, if
only a single maximum-likelihood frequency map estimate is required, we find
that traditional conjugate gradient solvers converge much faster than a Gibbs
sampler in terms of total number of iterations. The conceptual advantages of
the Gibbs sampling approach lies in statistically well-defined error
propagation and systematic error correction, and this methodology forms the
conceptual basis for the map-making algorithm employed in the BeyondPlanck
framework, which implements the first end-to-end Bayesian analysis pipeline for
CMB observations.Comment: 11 pages, 10 figures. All BeyondPlanck products and software will be
released publicly at http://beyondplanck.science during the online release
conference (November 18-20, 2020). Connection details will be made available
at the same website. Registration is mandatory for the online tutorial, but
optional for the conferenc
- …