418 research outputs found
A novel technique for wide-field polarimetry with a radiotelescope array
We report the use of the Australia Telescope Compact Array (ATCA) to conduct
polarimetric observations of the sky at 5 GHz. The ATCA is normally operated as
an interferometer array, but these observations were conducted in a split array
mode in which the antenna elements were used as single-dishes with their beams
staggered to simultaneously cover a wide area of sky with a resolution of 10
arcmin. The linearly polarized sky radiation was fully characterized from
measurements, made over a range of parallactic angles, of the cross correlated
signals from the orthogonal linear feeds. We describe the technique and present
a polarimetric image of the Vela supernova remnant made as a test of the
method. The development of the techniques was motivated by the need for
wide-field imaging of the foreground contamination of the polarized component
of the cosmic microwave background signal.Comment: 13 pages, 2 figures, accepted for publication in A
B-Mode contamination by synchrotron emission from 3-years WMAP data
We study the contamination of the B-mode of the Cosmic Microwave Background
Polarization (CMBP) by Galactic synchrotron in the lowest emission regions of
the sky. The 22.8-GHz polarization map of the 3-years WMAP data release is used
to identify and analyse such regions. Two areas are selected with
signal-to-noise ratio S/N<2 and S/N<3, covering ~16% and ~26% fraction of the
sky, respectively. The polarization power spectra of these two areas are
dominated by the sky signal on large angular scales (multipoles l < 15), while
the noise prevails on degree scales. Angular extrapolations show that the
synchrotron emission competes with the CMBP B-mode signal for tensor-to-scalar
perturbation power ratio -- at 70-GHz in the 16%
lowest emission sky (S/N<2 area). These values worsen by a factor ~5 in the
S/N<3 region. The novelty is that our estimates regard the whole lowest
emission regions and outline a contamination better than that of the whole high
Galactic latitude sky found by the WMAP team (T/S>0.3). Such regions allow to be measured directly which approximately corresponds to the
limit imposed by using a sky coverage of 15%. This opens interesting
perspectives to investigate the inflationary model space in lowest emission
regions.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
Large Radio Telescopes for Anomalous Microwave Emission Observations
We discuss in this paper the problem of the Anomalous Microwave Emission
(AME) in the light of ongoing or future observations to be performed with the
largest fully steerable radio telescope in the world. High angular resolution
observations of the AME will enable astronomers to drastically improve the
knowledge of the AME mechanisms as well as the interplay between the different
constituents of the interstellar medium in our galaxy. Extragalactic
observations of the AME have started as well, and high resolution is even more
important in this kind of observations. When cross-correlating with IR-dust
emission, high angular resolution is also of fundamental importance in order to
obtain unbiased results. The choice of the observational frequency is also of
key importance in continuum observation. We calculate a merit function that
accounts for the signal-to-noise ratio (SNR) in AME observation given the
current state-of-the-art knowledge and technology. We also include in our merit
functions the frequency dependence in the case of multifrequency observations.
We briefly mention and compare the performance of four of the largest
radiotelescopes in the world and hope the observational programs in each of
them will be as intense as possible.Comment: Review accepted for publication in Advances in Astronom
The synchrotron foreground and CMB temperature-polarization cross correlation power spectrum from the first year WMAP data
We analyse the temperature-polarization cross-correlation in the Galactic
synchrotron template that we have recently developed, and between the template
and CMB temperature maps derived from WMAP data. Since the polarized
synchrotron template itself uses WMAP data, we can estimate residual
synchrotron contamination in the CMB angular spectrum. While
appears to be contamined by synchrotron, no evidence for
contamination is found in the multipole range which is most relevant for the
fit of the cosmological optical depth.Comment: Accepted for pubblication on MNRAS Lette
1.4 GHz polarimetric observations of the two fields imaged by the DASI experiment
We present results of polarization observations at 1.4 GHz of the two fields
imaged by the DASI experiment (, and , ,
respectively). Data were taken with the Australia Telescope Compact Array with
3.4 arcmin resolution and mJy beam sensitivity. The emission
is dominated by point sources and we do not find evidence for diffuse
synchrotron radiation even after source subtraction. This allows to estimate an
upper limit of the diffuse polarized emission. The extrapolation to 30 GHz
suggests that the synchrotron radiation is lower than the polarized signal
measured by the DASI experiment by at least 2 orders of magnitude. This further
supports the conclusions drawn by the DASI team itself about the negligible
Galactic foreground contamination in their data set, improving by a factor
the upper limit estimated by Leitch et al. (2005).
The dominant point source emission allows us to estimate the contamination of
the CMB by extragalactic foregrounds. We computed the power spectrum of their
contribution and its extrapolation to 30 GHz provides a framework where the CMB
signal should dominate. However, our results do not match the conclusions of
the DASI team about the negligibility of point source contamination, suggesting
to take into account a source subtraction from the DASI data.Comment: 7 pages, six figures, submitted to MNRA
Effects of Thermal Fluctuations in the SPOrt Experiment
The role of systematic errors induced by thermal fluctuations is analyzed for
the SPOrt experiment with the aim at estimating their impact on the measurement
of the Cosmic Microwave Background Polarization (CMBP). The transfer functions
of the antenna devices from temperature to data fluctuations are computed, by
writing them in terms of both instrument and thermal environment parameters. In
addition, the corresponding contamination maps are estimated, along with their
polarized power spectra, for different behaviours of the instabilities. The
result is that thermal effects are at a negligible level even for fluctuations
correlated with the Sun illumination provided their frequency is
larger than that of the Sun illumination () by a factor , which defines a requirement for the statistical properties of
the temperature behaviour as well. The analysis with actual SPOrt operative
parameters shows that the instrument is only weakly sensitive to temperature
instabilities, the main contribution coming from the cryogenic stage. The
contamination on the E-mode spectrum does not significantly pollute the CMBP
signal and no specific data cleaning seems to be needed.Comment: 12 pages, 11 figures. Accepted for publication in A&
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