2,935 research outputs found
The Australia Telescope 20GHz (AT20G) Survey: analysis of the extragalactic source sample
The Australia Telescope 20 GHz (AT20G) survey is a blind survey of the whole
Southern sky at 20 GHz with follow-up observations at 4.8, 8.6, and 20 GHz
carried out with the Australia Telescope Compact Array (ATCA). In this paper we
present an analysis of radio spectral properties in total intensity and
polarisation, sizes, optical identifications, and redshifts of the sample of
the 5808 extragalactic sources in the survey catalogue of confirmed sources
over the whole Southern sky excluding the strip at Galactic latitude
|b|<1.5deg. The sample has a flux density limit of 40 mJy. Completeness has
been measured as a function of scan region and flux density. Averaging over the
whole survey area the follow-up survey is 78% complete above 50mJy and 93%
complete above 100mJy. 3332 sources with declination <-15deg have good quality
almost simultaneous observations at 4.8, 8.6, and 20GHz. The spectral analysis
shows that the sample is dominated by flat-spectrum sources. The fraction of
flat-spectrum sources decreases from 81% for 20GHz flux densities S>500mJy, to
60% for S<100mJy. There is also a clear spectral steepening at higher
frequencies with the median spectral index decreasing from -0.16 between 4.8
and 8.6GHz to -0.28 between 8.6 and 20GHz. Simultaneous observations in
polarisation are available for all the sources at all the frequencies. 768
sources have a good quality detection of polarised flux density at 20GHz; 467
of them were also detected in polarisation at 4.8 and/or at 8.6GHz so that it
has been possible to compare the spectral behaviour in total intensity and
polarisation. We have found that the polarised fraction increases slightly with
frequency and decreases with flux density. Cross matches and comparisons have
been made with other catalogues at lower radio frequencies, and in the optical,
X-ray and gamma-ray bands. Redshift estimates are available for 825 sources.Comment: 15 pages, 16 figures, accepted for publication in MNRA
Imaging the first light: experimental challenges and future perspectives in the observation of the Cosmic Microwave Background Anisotropy
Measurements of the cosmic microwave background (CMB) allow high precision
observation of the Last Scattering Surface at redshift 1100. After the
success of the NASA satellite COBE, that in 1992 provided the first detection
of the CMB anisotropy, results from many ground-based and balloon-borne
experiments have showed a remarkable consistency between different results and
provided quantitative estimates of fundamental cosmological properties. During
2003 the team of the NASA WMAP satellite has released the first improved
full-sky maps of the CMB since COBE, leading to a deeper insight into the
origin and evolution of the Universe. The ESA satellite Planck, scheduled for
launch in 2007, is designed to provide the ultimate measurement of the CMB
temperature anisotropy over the full sky, with an accuracy that will be limited
only by astrophysical foregrounds, and robust detection of polarisation
anisotropy. In this paper we review the experimental challenges in high
precision CMB experiments and discuss the future perspectives opened by second
and third generation space missions like WMAP and Planck.Comment: To be published in "Recent Research Developments in Astronomy &
Astrophysics Astrophysiscs" - Vol I
Measuring the tensor to scalar ratio from CMB B-modes in presence of foregrounds
Abreg: We investigate the impact of polarized foreground emission on the
performances of future CMB experiments in measuring the tensor-to-scalar ratio
r. We design a component separation pipeline, based on the Smica method, aimed
at estimating r and the foreground contamination from the data with no prior
assumption on the frequency dependence or spatial distribution of the
foregrounds. We derive error bars accounting for the uncertainty on foreground
contribution. We use the current knowledge of galactic and extra-galactic
foregrounds as implemented in the Planck Sky Model (PSM), to build simulations
of the sky emission for various experimental setups. Our method, permits us to
detect r = 0.1 at more than 3 sigma from B-modes only with Planck data, and r =
0.001 at 6 sigma for the most ambitious designs of the future EPIC probe. We
find that all-sky experiments permit a proper measurement of the reionization
bump despite the large scale foreground emission and are nearly insensitive to
contamination from point sources and lensing if their statistical contribution
can be modelled accurately. Investigating the observation of a small but clean
part of the sky, we show that diffuse foregrounds remain a concern for a
sensitive ground-based experiment with a limited frequency coverage when
measuring r < 0.1, but are dealt with efficiently by a deep field space mission
which is in return quite sensitive to lensing. Our results do not significantly
depend on the overall level and frequency dependence of the diffused foreground
model.Comment: 18 p. submitted to A&
Diffuse source separation in CMB observations
We review issues and methods for diffuse component separation in the context of Cosmic Microwave Background observation
Measuring the parameters of massive black hole binary systems with Pulsar Timing Array observations of gravitational waves
The observation of massive black hole binaries (MBHBs) with Pulsar Timing
Arrays (PTAs) is one of the goals of gravitational wave astronomy in the coming
years. Massive (>10^8 solar masses) and low-redshift (< 1.5) sources are
expected to be individually resolved by up-coming PTAs, and our ability to use
them as astrophysical probes will depend on the accuracy with which their
parameters can be measured. In this paper we estimate the precision of such
measurements using the Fisher-information-matrix formalism. We restrict to
"monochromatic" sources. In this approximation, the system is described by
seven parameters and we determine their expected statistical errors as a
function of the number of pulsars in the array, the array sky coverage, and the
signal-to-noise ratio (SNR) of the signal. At fixed SNR, the gravitational wave
astronomy capability of a PTA is achieved with ~20 pulsars; adding more pulsars
(up to 1000) to the array reduces the source error-box in the sky \Delta\Omega
by a factor ~5 and has negligible consequences on the statistical errors on the
other parameters. \Delta\Omega improves as 1/SNR^2 and the other parameters as
1/SNR. For a fiducial PTA of 100 pulsars uniformly distributed in the sky and a
coherent SNR = 10, we find \Delta\Omega~40 deg^2, a fractional error on the
signal amplitude of ~30% (which constraints only very poorly the chirp mass -
luminosity distance combination M_c^{5/3}/D_L), and the source inclination and
polarization angles are recovered at the ~0.3 rad level. The ongoing Parkes PTA
is particularly sensitive to systems located in the southern hemisphere, where
at SNR = 10 the source position can be determined with \Delta\Omega ~10 deg^2,
but has poorer performance for sources in the northern hemisphere. (Abridged)Comment: 20 pages, 12 figures, 2 color figures, submitted to Phys. Rev.
CMBPol Mission Concept Study: Prospects for polarized foreground removal
In this report we discuss the impact of polarized foregrounds on a future
CMBPol satellite mission. We review our current knowledge of Galactic polarized
emission at microwave frequencies, including synchrotron and thermal dust
emission. We use existing data and our understanding of the physical behavior
of the sources of foreground emission to generate sky templates, and start to
assess how well primordial gravitational wave signals can be separated from
foreground contaminants for a CMBPol mission. At the estimated foreground
minimum of ~100 GHz, the polarized foregrounds are expected to be lower than a
primordial polarization signal with tensor-to-scalar ratio r=0.01, in a small
patch (~1%) of the sky known to have low Galactic emission. Over 75% of the sky
we expect the foreground amplitude to exceed the primordial signal by about a
factor of eight at the foreground minimum and on scales of two degrees. Only on
the largest scales does the polarized foreground amplitude exceed the
primordial signal by a larger factor of about 20. The prospects for detecting
an r=0.01 signal including degree-scale measurements appear promising, with 5
sigma_r ~0.003 forecast from multiple methods. A mission that observes a range
of scales offers better prospects from the foregrounds perspective than one
targeting only the lowest few multipoles. We begin to explore how optimizing
the composition of frequency channels in the focal plane can maximize our
ability to perform component separation, with a range of typically 40 < nu <
300 GHz preferred for ten channels. Foreground cleaning methods are already in
place to tackle a CMBPol mission data set, and further investigation of the
optimization and detectability of the primordial signal will be useful for
mission design.Comment: 42 pages, 14 figures, Foreground Removal Working Group contribution
to the CMBPol Mission Concept Study, v2, matches AIP versio
Reappraising the Greed and Grievance Explanations for Violent Internal Conflict
Two phenomena have been recently utilised to explain conflict onset among rational choice analysts: greed and grievance. The former reflects elite competition over valuable natural resource rents. The latter argues that relative deprivation and the grievance it produces fuels conflict. Central to grievance are concepts of inter-ethnic or horizontal inequality. Identity formation is also crucial to intra-state conflict, as it overcomes the collective action problem. Conflict can rarely be explained by greed alone, yet, the greed versus grievance hypotheses may be complementary explanations for conflict. The greed explanation for conflict duration and secessionist wars works best in cross-country studies, but has to make way for grievance-based arguments in quantitative country-case studies. Grievances and horizontal inequalities may be better at explaining why conflicts begin, but not necessarily why they persist. Neither the presence of greed or grievance is sufficient for the outbreak of violent conflict, something which requires institutional breakdown which we describe as the failure of the social contract. The degradation of the social contract is more likely in the context of poverty and growth failure. The paper provides a synthesis of the greed and grievance hypotheses, ending with comments on post-conflict reconstruction.Conflict, civil war, greed versus grievance, social contract, post-conflict reconstruction
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