54 research outputs found
Estimating the tensor-to-scalar ratio and the effect of residual foreground contamination
We consider future balloon-borne and ground-based suborbital experiments
designed to search for inflationary gravitational waves, and investigate the
impact of residual foregrounds that remain in the estimated cosmic microwave
background maps. This is achieved by propagating foreground modelling
uncertainties from the component separation, under the assumption of a
spatially uniform foreground frequency scaling, through to the power spectrum
estimates, and up to measurement of the tensor to scalar ratio in the parameter
estimation step. We characterize the error covariance due to subtracted
foregrounds, and find it to be subdominant compared to instrumental noise and
sample variance in our simulated data analysis. We model the unsubtracted
residual foreground contribution using a two-parameter power law and show that
marginalization over these foreground parameters is effective in accounting for
a bias due to excess foreground power at low . We conclude that, at least
in the suborbital experimental setups we have simulated, foreground errors may
be modeled and propagated up to parameter estimation with only a slight
degradation of the target sensitivity of these experiments derived neglecting
the presence of the foregrounds.Comment: 19 pages, 12 figures, accepted for publication in JCA
SPIDER: Probing the Early Universe with a Suborbital Polarimeter
We evaluate the ability of SPIDER, a balloon-borne polarimeter, to detect a
divergence-free polarization pattern ("B-modes") in the Cosmic Microwave
Background (CMB). In the inflationary scenario, the amplitude of this signal is
proportional to that of the primordial scalar perturbations through the
tensor-to-scalar ratio r. We show that the expected level of systematic error
in the SPIDER instrument is significantly below the amplitude of an interesting
cosmological signal with r=0.03. We present a scanning strategy that enables us
to minimize uncertainty in the reconstruction of the Stokes parameters used to
characterize the CMB, while accessing a relatively wide range of angular
scales. Evaluating the amplitude of the polarized Galactic emission in the
SPIDER field, we conclude that the polarized emission from interstellar dust is
as bright or brighter than the cosmological signal at all SPIDER frequencies
(90 GHz, 150 GHz, and 280 GHz), a situation similar to that found in the
"Southern Hole." We show that two ~20-day flights of the SPIDER instrument can
constrain the amplitude of the B-mode signal to r<0.03 (99% CL) even when
foreground contamination is taken into account. In the absence of foregrounds,
the same limit can be reached after one 20-day flight.Comment: 29 pages, 8 figures, 4 tables; v2: matches published version, flight
schedule updated, two typos fixed in Table 2, references and minor
clarifications added, results unchange
The contribution of DNA ploidy to radiation sensitivity in human tumour cell lines
The contribution of DNA ploidy to radiation sensitivity was investigated in a group of eight human tumour cell lines. As previous studies suggest, while more aneuploid tumours tend to be more radioresistant, there is no significant relationship between ploidy and radiation sensitivity (SF2). The failure to observe a significant effect of ploidy on radiation sensitivity is due to the complex and multifactorial basis of radiation sensitivity. When we determined the relationship between survival and radiation-induced chromosome aberration frequency, a measure independent of most other modifiers of sensitivity, we observed a direct relationship between ploidy and mean lethal aberration frequency. The mean lethal frequency of aberrations increased from about 1 for diploid cells to about 2 for tetraploid cells. The mean lethal frequency of aberrations was independent of DNA repair variations. These observations demonstrate that changes in DNA ploidy are an important contributor to radiation sensitivity variations in human tumour cell lines. Therefore, any battery of predictive assays should include DNA ploidy measurements. © 1999 Cancer Research Campaig
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
Measuring our Peculiar Velocity by "Pre-deboosting" the CMB
It was recently shown that our peculiar velocity \beta with respect to the
CMB induces mixing among multipoles and off-diagonal correlations at all scales
which can be used as a measurement of \beta, which is independent of the
standard measurement using the CMB temperature dipole. The proposed techniques
rely however on a perturbative expansion which breaks down for \ell \gtrsim
1/(\beta) \approx 800. Here we propose a technique which consists of deboosting
the CMB temperature in the time-ordered data and show that it extends the
validity of the perturbation analysis multipoles up to \ell \sim 10000. We also
obtain accurate fitting functions for the mixing between multipoles valid in a
full non-linear treatment. Finally we forecast the achievable precision with
which these correlations can be measured in a number of current and future CMB
missions. We show that Planck could measure the velocity with a precision of
around 60 km/s, ACTPol in 4 years around 40 km/s, while proposed future
experiments could further shrink this error bar by over a factor of around 2.Comment: 14 pages, 7 figures. Revised projections for ACTPol, SPTPol and
ACBAR; included projections for BICEP2; extended conclusions; typos correcte
Spider Optimization II: Optical, Magnetic and Foreground Effects
Spider is a balloon-borne instrument designed to map the polarization of the
cosmic microwave background (CMB) with degree-scale resolution over a large
fraction of the sky. Spider's main goal is to measure the amplitude of
primordial gravitational waves through their imprint on the polarization of the
CMB if the tensor-to-scalar ratio, r, is greater than 0.03. To achieve this
goal, instrumental systematic errors must be controlled with unprecedented
accuracy. Here, we build on previous work to use simulations of Spider
observations to examine the impact of several systematic effects that have been
characterized through testing and modeling of various instrument components. In
particular, we investigate the impact of the non-ideal spectral response of the
half-wave plates, coupling between focal plane components and the Earth's
magnetic field, and beam mismatches and asymmetries. We also present a model of
diffuse polarized foreground emission based on a three-dimensional model of the
Galactic magnetic field and dust, and study the interaction of this foreground
emission with our observation strategy and instrumental effects. We find that
the expected level of foreground and systematic contamination is sufficiently
low for Spider to achieve its science goals.Comment: submitted to APJ, 15 pages, 12 figure
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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