179 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
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&
The QUIET Instrument
The Q/U Imaging ExperimenT (QUIET) is designed to measure polarization in the
Cosmic Microwave Background, targeting the imprint of inflationary
gravitational waves at large angular scales (~ 1 degree). Between 2008 October
and 2010 December, two independent receiver arrays were deployed sequentially
on a 1.4 m side-fed Dragonian telescope. The polarimeters which form the focal
planes use a highly compact design based on High Electron Mobility Transistors
(HEMTs) that provides simultaneous measurements of the Stokes parameters Q, U,
and I in a single module. The 17-element Q-band polarimeter array, with a
central frequency of 43.1 GHz, has the best sensitivity (69 uK sqrt(s)) and the
lowest instrumental systematic errors ever achieved in this band, contributing
to the tensor-to-scalar ratio at r < 0.1. The 84-element W-band polarimeter
array has a sensitivity of 87 uK sqrt(s) at a central frequency of 94.5 GHz. It
has the lowest systematic errors to date, contributing at r < 0.01. The two
arrays together cover multipoles in the range l= 25-975. These are the largest
HEMT-based arrays deployed to date. This article describes the design,
calibration, performance of, and sources of systematic error for the
instrument
First Season QUIET Observations: Measurements of CMB Polarization Power Spectra at 43 GHz in the Multipole Range 25 <= ell <= 475
The Q/U Imaging ExperimenT (QUIET) employs coherent receivers at 43GHz and
95GHz, operating on the Chajnantor plateau in the Atacama Desert in Chile, to
measure the anisotropy in the polarization of the CMB. QUIET primarily targets
the B modes from primordial gravitational waves. The combination of these
frequencies gives sensitivity to foreground contributions from diffuse Galactic
synchrotron radiation. Between 2008 October and 2010 December, >10,000hours of
data were collected, first with the 19-element 43GHz array (3458hours) and then
with the 90-element 95GHz array. Each array observes the same four fields,
selected for low foregrounds, together covering ~1000deg^2. This paper reports
initial results from the 43GHz receiver which has an array sensitivity to CMB
fluctuations of 69uK sqrt(s). The data were extensively studied with a large
suite of null tests before the power spectra, determined with two independent
pipelines, were examined. Analysis choices, including data selection, were
modified until the null tests passed. Cross correlating maps with different
telescope pointings is used to eliminate a bias. This paper reports the EE, BB
and EB power spectra in the multipole range ell=25-475. With the exception of
the lowest multipole bin for one of the fields, where a polarized foreground,
consistent with Galactic synchrotron radiation, is detected with 3sigma
significance, the E-mode spectrum is consistent with the LCDM model, confirming
the only previous detection of the first acoustic peak. The B-mode spectrum is
consistent with zero, leading to a measurement of the tensor-to-scalar ratio of
r=0.35+1.06-0.87. The combination of a new time-stream double-demodulation
technique, Mizuguchi-Dragone optics, natural sky rotation, and frequent
boresight rotation leads to the lowest level of systematic contamination in the
B-mode power so far reported, below the level of r=0.1Comment: 19 pages, 14 figures, higher quality figures are available at
http://quiet.uchicago.edu/results/index.html; Fixed a typo and corrected
statistical error values used as a reference in Figure 14, showing our
systematic uncertainties (unchanged) vs. multipole; Revision to ApJ accepted
version, this paper should be cited as "QUIET Collaboration et al. (2011)
The Q/U Imaging Experiment: Polarization Measurements of Radio Sources at 43 and 95 GHz
We present polarization measurements of extragalactic radio sources observed
during the Cosmic Microwave Background polarization survey of the Q/U Imaging
Experiment (QUIET), operating at 43 GHz (Q-band) and 95 GHz (W-band). We
examine sources selected at 20 GHz from the public, 40 mJy catalog of the
Australia Telescope (AT20G) survey. There are 480 such sources within
QUIET's four low-foreground survey patches, including the nearby radio galaxies
Centaurus A and Pictor A. The median error on our polarized flux density
measurements is 30--40 mJy per Stokes parameter. At S/N significance, we
detect linear polarization for seven sources in Q-band and six in W-band; only
detections per frequency band are expected by chance. For sources
without a detection of polarized emission, we find that half of the sources
have polarization amplitudes below 90 mJy (Q-band) and 106 mJy (W-band), at 95%
confidence. Finally, we compare our polarization measurements to intensity and
polarization measurements of the same sources from the literature. For the four
sources with WMAP and Planck intensity measurements Jy, the polarization
fraction are above 1% in both QUIET bands. At high significance, we compute
polarization fractions as much as 10--20% for some sources, but the effects of
source variability may cut that level in half for contemporaneous comparisons.
Our results indicate that simple models---ones that scale a fixed polarization
fraction with frequency---are inadequate to model the behavior of these sources
and their contributions to polarization maps.Comment: 16 pages, 10 figures. Submitted to Ap
Is long-bout sedentary behaviour associated with long-term glucose levels 3 months after acute ischaemic stroke? A prospective observational cohort study
Background and purpose
Sedentary behaviour is a risk factor for vascular disease and stroke patients are more sedentary than their age-matched peers. The association with glucose levels, as a potential mediator, is unclear, and we have investigated the association between long-bout sedentary behaviour and long-term glucose levels in stroke survivors.
Methods
This study uses data from the Norwegian Cognitive Impairment After Stroke study, a multicentre cohort study. The patients were recruited at hospital admission for acute stroke, and the follow-up was done at the outpatient clinic. Sedentary behaviour—being in a sitting or reclining position—was registered 3 months after stroke using position transition data from the body-worn sensor activPAL attached to the unaffected thigh. A MATLAB script was developed to extract activity data from 08:00 to 10:00 for 4 days and to categorise the data into four bout-length categories. The primary outcome was glycated haemoglobin (HbA1c), analysed at 3 months. Regression models were used to analyse the association between HbA1c and sedentary behaviour in the whole population and stratified based on a diagnosis of diabetes mellitus (DM). Age, body mass index and the use of antidiabetic drugs were added as covariates into the models.
Results
From a total of 815 included patients, 379 patients fulfilled the inclusion criteria for this study. We found no association between time in sedentary behaviour and HbA1c in the whole stroke population. We found time in sedentary behaviour in bouts of ≥90 min to be associated with a higher HbA1c in patients with DM.
Conclusion
Long-bout sedentary time is associated with a higher HbA1c in patients with DM 3 months after ischaemic stroke. Future research should investigate the benefit of breaking up sedentary time as a secondary preventive measure.publishedVersio
Second Season QUIET Observations: Measurements of the CMB Polarization Power Spectrum at 95 GHz
The Q/U Imaging ExperimenT (QUIET) has observed the cosmic microwave
background (CMB) at 43 and 95GHz. The 43-GHz results have been published in
QUIET Collaboration et al. (2011), and here we report the measurement of CMB
polarization power spectra using the 95-GHz data. This data set comprises 5337
hours of observations recorded by an array of 84 polarized coherent receivers
with a total array sensitivity of 87 uK sqrt(s). Four low-foreground fields
were observed, covering a total of ~1000 square degrees with an effective
angular resolution of 12.8', allowing for constraints on primordial
gravitational waves and high-signal-to-noise measurements of the E-modes across
three acoustic peaks. The data reduction was performed using two independent
analysis pipelines, one based on a pseudo-Cl (PCL) cross-correlation approach,
and the other on a maximum-likelihood (ML) approach. All data selection
criteria and filters were modified until a predefined set of null tests had
been satisfied before inspecting any non-null power spectrum. The results
derived by the two pipelines are in good agreement. We characterize the EE, EB
and BB power spectra between l=25 and 975 and find that the EE spectrum is
consistent with LCDM, while the BB power spectrum is consistent with zero.
Based on these measurements, we constrain the tensor-to-scalar ratio to
r=1.1+0.9-0.8 (r<2.8 at 95% C.L.) as derived by the ML pipeline, and
r=1.2+0.9-0.8 (r<2.7 at 95% C.L.) as derived by the PCL pipeline. In one of the
fields, we find a correlation with the dust component of the Planck Sky Model,
though the corresponding excess power is small compared to statistical errors.
Finally, we derive limits on all known systematic errors, and demonstrate that
these correspond to a tensor-to-scalar ratio smaller than r=0.01, the lowest
level yet reported in the literature.Comment: 10 pages, 6 figures, 3 tables, submitted to ApJ, This paper should be
cited as "QUIET Collaboration (2012)." v2: updated to reflect published
versio
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