7 research outputs found
The Primordial Inflation Explorer (PIXIE): A Nulling Polarimeter for Cosmic Microwave Background Observations
The Primordial Inflation Explorer (PIXIE) is an Explorer-class mission to
measure the gravity-wave signature of primordial inflation through its
distinctive imprint on the linear polarization of the cosmic microwave
background. The instrument consists of a polarizing Michelson interferometer
configured as a nulling polarimeter to measure the difference spectrum between
orthogonal linear polarizations from two co-aligned beams. Either input can
view the sky or a temperature-controlled absolute reference blackbody
calibrator. PIXIE will map the absolute intensity and linear polarization
(Stokes I, Q, and U parameters) over the full sky in 400 spectral channels
spanning 2.5 decades in frequency from 30 GHz to 6 THz (1 cm to 50 um
wavelength). Multi-moded optics provide background-limited sensitivity using
only 4 detectors, while the highly symmetric design and multiple signal
modulations provide robust rejection of potential systematic errors. The
principal science goal is the detection and characterization of linear
polarization from an inflationary epoch in the early universe, with
tensor-to-scalar ratio r < 10^{-3} at 5 standard deviations. The rich PIXIE
data set will also constrain physical processes ranging from Big Bang cosmology
to the nature of the first stars to physical conditions within the interstellar
medium of the Galaxy.Comment: 37 pages including 17 figures. Submitted to the Journal of Cosmology
and Astroparticle Physic
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
Divergent human-origin influenza viruses detected in Australian swine populations
Global swine populations infected with influenza A viruses pose a persistent pandemic risk. With the exception of a few countries, our understanding of the genetic diversity of swine influenza viruses is limited, hampering control measures and pandemic risk assessment. Here we report the genomic characteristics and evolutionary history of influenza A viruses isolated in Australia during 2012â2016 from two geographically isolated swine populations in the states of Queensland and Western Australia. Phylogenetic analysis with an expansive human and swine influenza virus dataset comprising >40,000 sequences sampled globally, revealed evidence of the pervasive introduction and long-term establishment of gene segments derived from several human influenza viruses of past seasons, including H1N1/1977, H1N1/1995, H3N2/1968, H3N2/2003, and H1N1pdm09, and a genotype that contained gene segments derived from the past three pandemics (1968, re-emerged 1977 and 2009). Of the six human-derived gene lineages only one comprising two viruses isolated in Queensland during 2012 was closely related to swine viruses detected from other regions, indicating a previously undetected circulation of Australian swine lineages for approximately 3â44 years. Although the date of introduction of these lineages into Australian swine populations could not be accurately ascertained, we found evidence of sustained transmission of two lineages in swine during 2012â2016. The continued detection of human-origin influenza virus lineages in swine over several decades with little or unpredictable antigenic drift indicates that isolated swine populations can act as âantigenic archivesâ of human influenza, raising the risk of re-emergence in humans when sufficient susceptible populations arise.IMPORTANCE We described the evolutionary origins and antigenic properties of influenza A viruses isolated from two separate Australian swine populations during 2012â2016, showing that these viruses were distinct to each other and to those isolated from swine globally. Whole genome sequencing of virus isolates revealed a high genotypic diversity that had been generated exclusively through the introduction and establishment of human influenza viruses that circulated in past seasons. We detected six reassortants with gene segments derived from human H1N1/H1N1pdm09 and various human H3N2 viruses that circulated at various periods since 1968. We also found that these swine viruses were not related to swine viruses collected elsewhere indicating independent circulation. The detection of unique lineages and genotypes in Australia suggests that isolated swine populations that are sufficiently large can sustain influenza for extensive periods, we showed direct evidence of a sustained transmission for at least 4 years between 2012â2016
Performance of a continuously rotating half-wave plate on the POLARBEAR telescope
A continuously rotating half-wave plate (CRHWP) is a promising tool to improve the sensitivity to large angular scales in cosmic microwave background (CMB) polarization measurements. With a CRHWP, single detectors can measure three of the Stokes parameters, I, Q and U, thereby avoiding the set of systematic errors that can be introduced by mismatches in the properties of orthogonal detector pairs. We focus on the implementation of CRHWPs in large aperture telescopes (i.e. the primary mirror is larger than the current maximum half-wave plate diameter of \ue2\u88\ubc0.5 m), where the CRHWP can be placed between the primary mirror and focal plane. In this configuration, one needs to address the intensity to polarization (I\ue2\u86\u92P) leakage of the optics, which becomes a source of 1/f noise and also causes differential gain systematics that arise from CMB temperature fluctuations. In this paper, we present the performance of a CRHWP installed in the \scshape Polarbear experiment, which employs a Gregorian telescope with a 2.5 m primary illumination pattern. The CRHWP is placed near the prime focus between the primary and secondary mirrors. We find that the I\ue2\u86\u92P leakage is larger than the expectation from the physical properties of our primary mirror, resulting in a 1/f knee of 100 mHz. The excess leakage could be due to imperfections in the detector system, i.e. detector non-linearity in the responsivity and time-constant. We demonstrate, however, that by subtracting the leakage correlated with the intensity signal, the 1/f noise knee frequency is reduced to 32 mHz (\ue2\u84\u93 \ue2\u88\ubc 39 for our scan strategy), which is very promising to probe the primordial B-mode signal. We also discuss methods for further noise subtraction in future projects where the precise temperature control of instrumental components and the leakage reduction will play a key role